@article{bykov_uvarov_slane_ellison_2020, title={Uncovering Magnetic Turbulence in Young Supernova Remnants with Polarized X-Ray Imaging}, volume={899}, ISSN={["1538-4357"]}, DOI={10.3847/1538-4357/aba960}, abstractNote={Abstract}, number={2}, journal={ASTROPHYSICAL JOURNAL}, author={Bykov, Andrei M. and Uvarov, Yury A. and Slane, Patrick and Ellison, Donald C.}, year={2020}, month={Aug} }
 @article{bykov_kalyashova_ellison_osipov_2019, title={High-energy cosmic rays from compact galactic star clusters: Particle fluxes and anisotropy}, volume={64}, ISSN={["1879-1948"]}, DOI={10.1016/j.asr.2019.06.005}, abstractNote={It has been shown that supernova blast waves interacting with winds from massive stars in compact star clusters may be capable of producing cosmic-ray (CR) protons to above 1017 eV. We give a brief description of the colliding-shock-flow mechanism and look at generalizations of the diffusion of ∼100 PeV CRs in the turbulent galactic magnetic field present in the galactic disk. We calculate the temporal evolution of the CR anisotropy from a possible distribution of young compact massive star clusters assuming the sources are intermittent on time scales of a few million years, i.e., comparable to their residence time in the Milky Way. Within the confines of our model, we determine the galactic/extra-galactic fraction of high-energy CRs resulting in anisotropies consistent with observed values. We find that galactic star clusters may contribute a substantial fraction of ∼100 PeV CRs without producing anisotropies above observed limits.}, number={12}, journal={ADVANCES IN SPACE RESEARCH}, author={Bykov, A. M. and Kalyashova, M. E. and Ellison, D. C. and Osipov, S. M.}, year={2019}, month={Dec}, pages={2439–2444} }
 @article{bykov_ellison_kalyashova_osipov_2019, title={High-energy cosmic rays from supernovae in young clusters of massive stars}, volume={30}, ISSN={["1720-0776"]}, DOI={10.1007/s12210-019-00788-7}, abstractNote={While the bulk of GeV–100 TeV cosmic rays (CRs) can be accelerated at the shock of the most common supernova remnants the problem of CR acceleration to and above the PeV regime is still an open issue. We give a brief discussion of the production of 100 PeV regime cosmic rays by supernovae in young compact clusters of massive stars. This implies a spatial and temporal correlation of the SNe in the cluster over $$\sim 10^7\,\hbox {year}$$ which may affect the high-energy CR anisotropy. Here, we discuss the temporal evolution of the dipole angular anisotropy of 300 PeV CR protons accelerated within young massive star clusters.}, number={SUPPL 1}, journal={RENDICONTI LINCEI-SCIENZE FISICHE E NATURALI}, author={Bykov, A. M. and Ellison, D. C. and Kalyashova, M. E. and Osipov, S. M.}, year={2019}, month={Dec}, pages={155–158} }
 @article{osipov_bykov_ellison_2019, title={Monte Carlo modelling of particle acceleration in collisionless shocks with effective mean electric field}, volume={1400}, ISSN={["1742-6596"]}, DOI={10.1088/1742-6596/1400/2/022004}, abstractNote={Abstract}, journal={INTERNATIONAL CONFERENCE PHYSICA.SPB/2019}, author={Osipov, S. M. and Bykov, A. M. and Ellison, D. C.}, year={2019} }
 @article{kalyashova_bykov_osipov_ellison_badmaev_2019, title={Wolf-Rayet stars in young massive star clusters as potential sources of Galactic cosmic rays}, volume={1400}, ISSN={["1742-6596"]}, DOI={10.1088/1742-6596/1400/2/022011}, abstractNote={Abstract}, journal={INTERNATIONAL CONFERENCE PHYSICA.SPB/2019}, author={Kalyashova, M. E. and Bykov, A. M. and Osipov, S. M. and Ellison, D. C. and Badmaev, D. V.}, year={2019} }
 @misc{bykov_ellison_marcowith_osipov_2018, title={Cosmic Ray Production in Supernovae}, volume={214}, ISSN={["1572-9672"]}, DOI={10.1007/s11214-018-0479-4}, abstractNote={We give a brief review of the origin and acceleration of cosmic rays (CRs), emphasizing the production of CRs at different stages of supernova evolution by the first-order Fermi shock acceleration mechanism. We suggest that supernovae with trans-relativistic outflows, despite being rather rare, may accelerate CRs to energies above $10^{18}\mbox{ eV}$ over the first year of their evolution. Supernovae in young compact clusters of massive stars, and interaction powered superluminous supernovae, may accelerate CRs well above the PeV regime. We discuss the acceleration of the bulk of the galactic CRs in isolated supernova remnants and re-acceleration of escaped CRs by the multiple shocks present in superbubbles produced by associations of OB stars. The effects of magnetic field amplification by CR driven instabilities, as well as superdiffusive CR transport, are discussed for nonthermal radiation produced by nonlinear shocks of all speeds including trans-relativistic ones.}, number={1}, journal={SPACE SCIENCE REVIEWS}, author={Bykov, A. M. and Ellison, D. C. and Marcowith, A. and Osipov, S. M.}, year={2018}, month={Feb} }
 @article{bykov_ellison_gladilin_osipov_2018, title={Supernovae in compact star clusters as sources of high-energy cosmic rays and neutrinos}, volume={62}, ISSN={["1879-1948"]}, DOI={10.1016/j.asr.2017.05.043}, abstractNote={We discuss a specific population of galactic PeVatrons which may be the source of the galactic CR component well above PeV energies. Supernovae in compact clusters of massive stars are proposed as powerful sources of CRs, neutrinos, and gamma-ray emission. Numerical simulations of non-linear CR acceleration at converging shock flows have revealed that these accelerators can provide very hard spectra of protons up to 1016–1017 eV which is well above the “knee” in the all-particle CR spectrum at about 3×1015 eV. We suggest that known supernova remnants interacting with stellar winds in the compact clusters of young massive stars Westerlund I and Cl*1806-20 can be associated with the sources of the TeV gamma-emission detected by H.E.S.S. and may be responsible for a fraction of the high-energy neutrinos detected with the IceCube observatory. A recent cosmic ray (CR) composition measurement with the LOFAR array has revealed a light-mass component possibly dominating the all-particle spectrum at energies around 1017 eV. Such a strong light component (mainly protons and helium) may require specific galactic CR sources such as supernovae interacting with compact clusters of massive stars in addition to isolated supernova remnants.}, number={10}, journal={ADVANCES IN SPACE RESEARCH}, author={Bykov, A. M. and Ellison, D. C. and Gladilin, P. E. and Osipov, S. M.}, year={2018}, month={Nov}, pages={2764–2772} }
 @article{bykov_ellison_gladilin_osipov_2017, title={High energy neutrinos and gamma-ray emission from supernovae in compact star clusters}, volume={1792}, ISSN={["0094-243X"]}, DOI={10.1063/1.4968888}, abstractNote={Compact clusters of young massive stars are observed in the Milky Way and in starburst galaxies. The compact clusters with multiple powerful winds of young massive stars and supernova shocks are favorable sites for high-energy particle acceleration. We argue that expanding young supernova (SN) shells in compact stellar clusters can be very efficient PeV CR accelerators. At a stage when a supernova shock is colliding with collective fast winds from massive stars in a compact cluster the Fermi mechanism allows particle acceleration to energies well above the standard limits of diffusive shock acceleration in an isolated SNR. The energy spectrum of protons in such an accelerator is a hard power-law with a broad spectral upturn above TeV before a break at multi-PeV energies, providing a large energy flux in the high-energy end of the spectrum. The acceleration stage in the colliding shock flow lasts for a few hundred years after the supernova explosion producing high-energy CRs that escape the accelerator and...}, journal={HIGH ENERGY GAMMA-RAY ASTRONOMY}, author={Bykov, A. M. and Ellison, D. C. and Gladilin, P. E. and Osipov, S. M.}, year={2017} }
 @article{bykov_ellison_osipov_2017, title={Nonlinear Monte Carlo model of superdiffusive shock acceleration with magnetic field amplification}, volume={95}, ISSN={["2470-0053"]}, DOI={10.1103/physreve.95.033207}, abstractNote={Fast collisionless shocks in cosmic plasmas convert their kinetic energy flow into the hot downstream thermal plasma with a substantial fraction of energy going into a broad spectrum of superthermal charged particles and magnetic fluctuations. The superthermal particles can penetrate into the shock upstream region producing an extended shock precursor. The cold upstream plasma flow is decelerated by the force provided by the superthermal particle pressure gradient. In high Mach number collisionless shocks, efficient particle acceleration is likely coupled with turbulent magnetic field amplification (MFA) generated by the anisotropic distribution of accelerated particles. This anisotropy is determined by fast particle transport, making the problem strongly nonlinear and multiscale. Here, we present a nonlinear Monte Carlo model of collisionless shock structure with superdiffusive propagation of high-energy Fermi accelerated particles coupled to particle acceleration and MFA, which affords a consistent description of strong shocks. A distinctive feature of the Monte Carlo technique is that it includes the full angular anisotropy of the particle distribution at all precursor positions. The model reveals that the superdiffusive transport of energetic particles (i.e., Lévy-walk propagation) generates a strong quadruple anisotropy in the precursor particle distribution. The resultant pressure anisotropy of the high-energy particles produces a nonresonant mirror-type instability that amplifies compressible wave modes with wavelengths longer than the gyroradii of the highest-energy protons produced by the shock.}, number={3}, journal={PHYSICAL REVIEW E}, author={Bykov, Andrei M. and Ellison, Donald C. and Osipov, Sergei M.}, year={2017}, month={Mar} }
 @article{warren_ellison_barkov_nagataki_2017, title={Nonlinear Particle Acceleration and Thermal Particles in GRB Afterglows}, volume={835}, ISSN={["1538-4357"]}, DOI={10.3847/1538-4357/aa56c3}, abstractNote={Abstract}, number={2}, journal={ASTROPHYSICAL JOURNAL}, author={Warren, Donald C. and Ellison, Donald C. and Barkov, Maxim V. and Nagataki, Shigehiro}, year={2017}, month={Feb} }
 @article{patnaude_lee_slane_badenes_nagataki_ellison_milisavljevic_2017, title={The Impact of Progenitor Mass Loss on the Dynamical and Spectral Evolution of Supernova Remnants}, volume={849}, ISSN={["1538-4357"]}, DOI={10.3847/1538-4357/aa9189}, abstractNote={Abstract}, number={2}, journal={ASTROPHYSICAL JOURNAL}, author={Patnaude, Daniel J. and Lee, Shiu-Hang and Slane, Patrick O. and Badenes, Carles and Nagataki, Shigehiro and Ellison, Donald C. and Milisavljevic, Dan}, year={2017}, month={Nov} }
 @misc{pelletier_bykov_ellison_lemoine_2017, title={Towards Understanding the Physics of Collisionless Relativistic Shocks Relativistic Collisionless Shocks}, volume={207}, ISSN={["1572-9672"]}, DOI={10.1007/s11214-017-0364-6}, abstractNote={Relativistic astrophysical collisionless shocks represent outstanding dissipation agents of the huge power of relativistic outflows produced by accreting black holes, core collapsed supernovae and other objects into multi-messenger radiation (cosmic rays, neutrinos, electromagnetic radiation). This article provides a theoretical discussion of the fundamental physical ingredients of these extreme phenomena. In the context of weakly magnetized shocks, in particular, it is shown how the filamentation type instabilities, which develop in the precursor of pair dominated or electron-ion shocks, provide the seeds for the scattering of high energy particles as well as the agent which preheats and slows down the incoming precursor plasma. This analytical discussion is completed with a mesoscopic, non-linear model of particle acceleration in relativistic shocks based on Monte Carlo techniques. This Monte Carlo model uses a semi-phenomenological description of particle scattering which allows it to calculate the back-reaction of accelerated particles on the shock structure on length and momentum scales which are currently beyond the range of microscopic particle-in-cell (PIC) simulations.}, number={1-4}, journal={SPACE SCIENCE REVIEWS}, author={Pelletier, Guy and Bykov, Andrei and Ellison, Don and Lemoine, Martin}, year={2017}, month={Jul}, pages={319–360} }
 @article{ellison_warren_bykov_2016, title={Particle spectra and efficiency in nonlinear relativistic shock acceleration - survey of scattering models}, volume={456}, ISSN={["1365-2966"]}, DOI={10.1093/mnras/stv2912}, abstractNote={We include a general form for the scattering mean free path in a nonlinear Monte Carlo model of relativistic shock formation and Fermi acceleration. Particle-in-cell (PIC) simulations, as well as analytic work, suggest that relativistic shocks tend to produce short-scale, self-generated magnetic turbulence that leads to a scattering mean free path (mfp) with a stronger momentum dependence than the mfp ~ p dependence for Bohm diffusion. In unmagnetized shocks, this turbulence is strong enough to dominate the background magnetic field so the shock can be treated as parallel regardless of the initial magnetic field orientation, making application to gamma-ray bursts (GRBs), pulsar winds, Type Ibc supernovae, and extra-galactic radio sources more straightforward and realistic. In addition to changing the scale of the shock precursor, we show that, when nonlinear effects from efficient Fermi acceleration are taken into account, the momentum dependence of the mfp has an important influence on the efficiency of cosmic-ray production as well as the accelerated particle spectral shape. These effects are absent in nonrelativistic shocks and do not appear in relativistic shock models unless nonlinear effects are self-consistently described. We show, for limited examples, how the changes in Fermi acceleration translate to changes in the intensity and spectral shape of gamma-ray emission from proton-proton interactions and pion-decay radiation.}, number={3}, journal={MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY}, author={Ellison, Donald C. and Warren, Donald C. and Bykov, Andrei M.}, year={2016}, month={Mar}, pages={3090–3099} }
 @article{krawczynski_stern_harrison_kislat_zajczyk_beilicke_hoormann_guo_endsley_ingram_et al._2016, title={X-ray polarimetry with the Polarization Spectroscopic Telescope Array (PolSTAR)}, volume={75}, journal={Astroparticle Physics}, author={Krawczynski, H. S. and Stern, D. and Harrison, F. A. and Kislat, F. F. and Zajczyk, A. and Beilicke, M. and Hoormann, J. and Guo, Q. and Endsley, R. and Ingram, A. R. and et al.}, year={2016}, pages={8–28} }
 @article{patnaude_lee_slane_badenes_heger_ellison_nagataki_2015, title={ARE MODELS FOR CORE-COLLAPSE SUPERNOVA PROGENITORS CONSISTENT WITH THE PROPERTIES OF SUPERNOVA REMNANTS?}, volume={803}, ISSN={["1538-4357"]}, DOI={10.1088/0004-637x/803/2/101}, abstractNote={The recent discovery that the Fe–K line luminosities and energy centroids observed in nearby supernova remnants are a strong discriminant of both progenitor type and circumstellar environment has implications for our understanding of supernova progenitor evolution. Using models for the chemical composition of core-collapse supernova (CCSN) ejecta, we model the dynamics and thermal X-ray emission from shocked ejecta and circumstellar material, modeled as an r − 2 ?> wind, to ages of 3000 yr. We compare the X-ray spectra expected from these models to observations made with the Suzaku satellite. We also model the dynamics and X-ray emission from Type Ia progenitor models. We find a clear distinction in Fe–K line energy centroid between core-collapse and Type Ia models. The CCSN models predict higher Fe–K line centroid energies than the Type Ia models, in agreement with observations. We argue that the higher line centroids are a consequence of the increased densities found in the circumstellar environment created by the expansion of the slow-moving wind from the massive progenitors.}, number={2}, journal={ASTROPHYSICAL JOURNAL}, author={Patnaude, Daniel J. and Lee, Shiu-Hang and Slane, Patrick O. and Badenes, Carles and Heger, Alexander and Ellison, Donald C. and Nagataki, Shigehiro}, year={2015}, month={Apr} }
 @article{warren_ellison_bykov_lee_2015, title={Electron and ion acceleration in relativistic shocks with applications to GRB afterglows}, volume={452}, ISSN={["1365-2966"]}, DOI={10.1093/mnras/stv1304}, abstractNote={We have modeled the simultaneous first-order Fermi shock acceleration of protons, electrons, and helium nuclei by relativistic shocks. By parameterizing the particle diffusion, our steady-state Monte Carlo simulation allows us to follow particles from particle injection at nonthermal thermal energies to above PeV energies, including the nonlinear smoothing of the shock structure due to cosmic-ray (CR) backpressure. We observe the mass-to-charge (A/Z) enhancement effect believed to occur in efficient Fermi acceleration in non-relativistic shocks and we parameterize the transfer of ion energy to electrons seen in particle-in-cell (PIC) simulations. For a given set of environmental and model parameters, the Monte Carlo simulation determines the absolute normalization of the particle distributions and the resulting synchrotron, inverse-Compton, and pion-decay emission in a largely self-consistent manner. The simulation is flexible and can be readily used with a wide range of parameters typical of gamma-ray burst (GRB) afterglows. We describe some preliminary results for photon emission from shocks of different Lorentz factors and outline how the Monte Carlo simulation can be generalized and coupled to hydrodynamic simulations of GRB blast waves. We assume Bohm diffusion for simplicity but emphasize that the nonlinear effects we describe stem mainly from an extended shock precursor where higher energy particles diffuse further upstream. Quantitative differences will occur with different diffusion models, particularly for the maximum CR energy and photon emission, but these nonlinear effects should be qualitatively similar as long as the scattering mean free path is an increasing function of momentum.}, number={1}, journal={MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY}, author={Warren, Donald C. and Ellison, Donald C. and Bykov, Andrei M. and Lee, Shiu-Hang}, year={2015}, month={Sep}, pages={431–443} }
 @article{lee_patnaude_raymond_nagataki_slane_ellison_2015, title={MODELING BRIGHT gamma-RAY AND RADIO EMISSION AT FAST CLOUD SHOCKS}, volume={806}, ISSN={["1538-4357"]}, DOI={10.1088/0004-637x/806/1/71}, abstractNote={Recent observations by the Large Area Telescope on board the Fermi satellite have revealed bright γ-ray emission from middle-aged supernova remnants (SNRs) inside our Galaxy. These remnants, which also possess bright non-thermal radio shells, are often found to be interacting directly with surrounding gas clouds. We explore the non-thermal emission mechanism at these dynamically evolved SNRs by constructing a hydrodynamical model. Two scenarios of particle acceleration, either a re-acceleration of Galactic cosmic rays or an efficient nonlinear diffusive shock acceleration (NLDSA) of particles injected from downstream, are considered. Using parameters inferred from observations, our models are contrasted with the observed spectra of SNR W44. For the re-acceleration case, we predict a significant enhancement of radio and GeV emission as the SNR undergoes a transition into the radiative phase. If sufficiently strong magnetic turbulence is present in the molecular cloud, the re-acceleration scenario can explain the observed broadband spectral properties. The NLDSA scenario also succeeds in explaining the γ-ray spectrum but fails to reproduce the radio spectral index. Efficient NLDSA also results in a significant post-shock non-thermal pressure that limits the compression during cooling and prevents the formation of a prominent dense shell. Some other interesting differences between the two models in hydrodynamical behavior and resulting spectral features are illustrated.}, number={1}, journal={ASTROPHYSICAL JOURNAL}, author={Lee, Shiu-Hang and Patnaude, Daniel J. and Raymond, John C. and Nagataki, Shigehiro and Slane, Patrick O. and Ellison, Donald C.}, year={2015}, month={Jun} }
 @misc{slane_bykov_ellison_dubner_castro_2015, title={Supernova Remnants Interacting with Molecular Clouds: X-Ray and Gamma-Ray Signatures}, volume={188}, ISSN={["1572-9672"]}, DOI={10.1007/s11214-014-0062-6}, abstractNote={The giant molecular clouds (MCs) found in the Milky Way and similar galaxies play a crucial role in the evolution of these systems. The supernova explosions that mark the death of massive stars in these regions often lead to interactions between the supernova remnants (SNRs) and the clouds. These interactions have a profound effect on our understanding of SNRs. Shocks in SNRs should be capable of accelerating particles to cosmic ray (CR) energies with efficiencies high enough to power Galactic CRs. X-ray and gamma-ray studies have established the presence of relativistic electrons and protons is some SNRs and provided strong evidence for diffusive shock acceleration as the primary acceleration mechanism, including strongly amplified magnetic fields, temperature and ionization effects on the shock-heated plasmas, and modifications to the dynamical evolution of some systems. Because protons dominate the overall energetics of the CRs, it is crucial to understand this hadronic component even though electrons are much more efficient radiators and it can be difficult to identify the hadronic component. However, near MCs the densities are sufficiently high to allow the gamma-ray emission to be dominated by protons. Thus, these interaction sites provide some of our best opportunities to constrain the overall energetics of these particle accelerators. Here we summarize some key properties of interactions between SNRs and MCs, with an emphasis on recent X-ray and gamma-ray studies that are providing important constraints on our understanding of cosmic rays in our Galaxy.}, number={1-4}, journal={SPACE SCIENCE REVIEWS}, author={Slane, Patrick and Bykov, Andrei and Ellison, Donald C. and Dubner, Gloria and Castro, Daniel}, year={2015}, month={May}, pages={187–210} }
 @article{bykov_ellison_gladilin_osipov_2015, title={Ultrahard spectra of PeV neutrinos from supernovae in compact star clusters}, volume={453}, ISSN={["1365-2966"]}, DOI={10.1093/mnras/stv1606}, abstractNote={Starburst regions with multiple powerful winds of young massive stars and supernova remnants (SNRs) are favorable sites for high-energy cosmic ray (CR) acceleration. A supernova (SN) shock colliding with a fast wind from a compact cluster of young stars allows the acceleration of protons to energies well above the standard limits of diusive shock acceleration in an isolated SN. The proton spectrum in such a wind-SN pevatron accelerator is hard with a large ux in the high-energy-end of the spectrum producing copious -rays and neutrinos in inelastic nuclear collisions. We argue that SN shocks in the Westerlund 1 (Wd1) cluster in the Milky Way may accelerate protons to > 40 PeV. Once accelerated, these CRs will diuse into surrounding dense clouds and produce neutrinos with uxes sucient to explain a fraction of the events detected by IceCube from the inner Galaxy.}, number={1}, journal={MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY}, author={Bykov, A. M. and Ellison, D. C. and Gladilin, P. E. and Osipov, S. M.}, year={2015}, month={Oct}, pages={113–121} }
 @article{slane_lee_ellison_patnaude_hughes_eriksen_castro_nagataki_2014, title={A CR-HYDRO-NEI MODEL OF THE STRUCTURE AND BROADBAND EMISSION FROM TYCHO'S SUPERNOVA REMNANT}, volume={783}, ISSN={["1538-4357"]}, DOI={10.1088/0004-637x/783/1/33}, abstractNote={Tycho's supernova remnant (SNR) is well-established as a source of particle acceleration to very high energies. Constraints from numerous studies indicate that the observed γ-ray emission results primarily from hadronic processes, providing direct evidence of highly relativistic ions that have been accelerated by the SNR. Here we present an investigation of the dynamical and spectral evolution of Tycho's SNR by carrying out hydrodynamical simulations that include diffusive shock acceleration of particles in the amplified magnetic field at the forward shock of the SNR. Our simulations provide a consistent view of the shock positions, the nonthermal emission, the thermal X-ray emission from the forward shock, and the brightness profiles of the radio and X-ray emission. We compare these with the observed properties of Tycho to determine the density of the ambient material, the particle acceleration efficiency and maximum energy, the accelerated electron-to-proton ratio, and the properties of the shocked gas downstream of the expanding SNR shell. We find that evolution of a typical Type Ia supernova in a low ambient density (n 0 ∼ 0.3 cm−3), with an upstream magnetic field of ∼5 μG, and with ∼16% of the SNR kinetic energy being converted into relativistic electrons and ions through diffusive shock acceleration, reproduces the observed properties of Tycho. Under such a scenario, the bulk of observed γ-ray emission at high energies is produced by π0-decay resulting from the collisions of energetic hadrons, while inverse-Compton emission is significant at lower energies, comprising roughly half of the flux between 1 and 10 GeV.}, number={1}, journal={ASTROPHYSICAL JOURNAL}, author={Slane, P. and Lee, S. -H. and Ellison, D. C. and Patnaude, D. J. and Hughes, J. P. and Eriksen, K. A. and Castro, D. and Nagataki, S.}, year={2014}, month={Mar} }
 @article{bykov_ellison_osipov_vladimirov_2014, title={MAGNETIC FIELD AMPLIFICATION IN NONLINEAR DIFFUSIVE SHOCK ACCELERATION INCLUDING RESONANT AND NON-RESONANT COSMIC-RAY DRIVEN INSTABILITIES}, volume={789}, ISSN={["1538-4357"]}, DOI={10.1088/0004-637x/789/2/137}, abstractNote={We present a nonlinear Monte Carlo model of efficient diffusive shock acceleration where the magnetic turbulence responsible for particle diffusion is calculated self-consistently from the resonant cosmic-ray (CR) streaming instability, together with non-resonant short- and long-wavelength CR-current-driven instabilities. We include the backpressure from CRs interacting with the strongly amplified magnetic turbulence which decelerates and heats the super-Alfvénic flow in the extended shock precursor. Uniquely, in our plane-parallel, steady-state, multi-scale model, the full range of particles, from thermal (∼eV) injected at the viscous subshock to the escape of the highest energy CRs (∼PeV) from the shock precursor, are calculated consistently with the shock structure, precursor heating, magnetic field amplification, and scattering center drift relative to the background plasma. In addition, we show how the cascade of turbulence to shorter wavelengths influences the total shock compression, the downstream proton temperature, the magnetic fluctuation spectra, and accelerated particle spectra. A parameter survey is included where we vary shock parameters, the mode of magnetic turbulence generation, and turbulence cascading. From our survey results, we obtain scaling relations for the maximum particle momentum and amplified magnetic field as functions of shock speed, ambient density, and shock size.}, number={2}, journal={ASTROPHYSICAL JOURNAL}, author={Bykov, Andrei M. and Ellison, Donald C. and Osipov, Sergei M. and Vladimirov, Andrey E.}, year={2014}, month={Jul} }
 @article{lee_patnaude_ellison_nagataki_slane_2014, title={REVERSE AND FORWARD SHOCK X-RAY EMISSION IN AN EVOLUTIONARY MODEL OF SUPERNOVA REMNANTS UNDERGOING EFFICIENT DIFFUSIVE SHOCK ACCELERATION}, volume={791}, ISSN={["1538-4357"]}, DOI={10.1088/0004-637x/791/2/97}, abstractNote={We present new models for the forward and reverse shock thermal X-ray emission from core-collapse and Type Ia supernova remnants (SNRs) that include the efficient production of cosmic rays (CR) via nonlinear diffusive shock acceleration (DSA). Our CR-hydro-NEI code takes into account non-equilibrium ionization, hydrodynamic effects of efficient CR production on the SNR evolution, and collisional temperature equilibration among heavy ions and electrons in both the shocked supernova (SN) ejecta and the shocked circumstellar material. While X-ray emission is emphasized here, our code self-consistently determines both thermal and non-thermal broadband emission from radio to TeV energies. We include Doppler broadening of the spectral lines by thermal motions of the ions and by the remnant expansion. We study, in general terms, the roles that the ambient environment, progenitor models, temperature equilibration, and processes related to DSA have on the thermal and non-thermal spectra. The study of X-ray line emission from young SNRs is a powerful tool for determining specific SN elemental contributions and for providing critical information that helps to understand the type and energetics of the explosion, the composition of the ambient medium in which the SN exploded, and the ionization and dynamics of the hot plasma in the shocked SN ejecta and interstellar medium. With the approaching launch of the next-generation X-ray satellite Astro-H, observations of spectral lines with unprecedented high resolution will become a reality. Our self-consistent calculations of the X-ray spectra from various progenitors will help interpret future observations of SNRs.}, number={2}, journal={ASTROPHYSICAL JOURNAL}, author={Lee, Shiu-Hang and Patnaude, Daniel J. and Ellison, Donald C. and Nagataki, Shigehiro and Slane, Patrick O.}, year={2014}, month={Aug} }
 @article{lee_slane_ellison_nagataki_patnaude_2013, title={A CR-hydro-NEI model of multi-wavelength emission from the Vela Jr. Supernova Remnant (SNR RX J0852.0-4622)}, volume={767}, number={1}, journal={Astrophysical Journal}, author={Lee, S. H. and Slane, P. O. and Ellison, D. C. and Nagataki, S. and Patnaude, D. J.}, year={2013} }
 @article{ellison_2013, title={Beyond the Test-Particle Power Law: Particle Spectra in Diffusive Shock Acceleration}, volume={1516}, ISSN={["0094-243X"]}, DOI={10.1063/1.4792567}, abstractNote={Collisionless shocks are common in astrophysics and the diffusive shock acceleration (DSA) mechanism is believed to accelerate cosmic rays efficiently in these shocks. When this mechanism was introduced thirty-five years ago, the fact that the test-particle theory produced a power law in momentum reasonably consistent with that observed in cosmic rays, lead to the quick acceptance of DSA as the mechanism of choice for shocks throughout the universe. There are, however, a number of factors that can modify the spectrum produced by DSA and caution must be used in applying the test-particle result. I will briefly describe how DSA can produce spectra that vary considerably from the canonical test-particle power law.}, journal={CENTENARY SYMPOSIUM 2012: DISCOVERY OF COSMIC RAYS}, author={Ellison, Donald C.}, year={2013}, pages={195–200} }
 @article{lee_ellison_nagataki_2012, title={A GENERALIZED MODEL OF NONLINEAR DIFFUSIVE SHOCK ACCELERATION COUPLED TO AN EVOLVING SUPERNOVA REMNANT}, volume={750}, ISSN={["1538-4357"]}, DOI={10.1088/0004-637x/750/2/156}, abstractNote={To better model the efficient production of cosmic rays (CRs) in supernova remnants (SNRs) with the associated coupling between CR production and SNR dynamics, we have generalized an existing cr-hydro-NEI code to include the following processes: (1) an explicit calculation of the upstream precursor structure including the position-dependent flow speed, density, temperature, and magnetic field strength; (2) a momentum- and space-dependent CR diffusion coefficient; (3) an explicit calculation of magnetic field amplification; (4) calculation of the maximum CR momentum using the amplified magnetic field; (5) a finite Alfvén speed for the particle scattering centers; and (6) the ability to accelerate a superthermal seed population of CRs, as well as the ambient thermal plasma. While a great deal of work has been done modeling SNRs, most work has concentrated on either the continuum emission from relativistic electrons or ions or the thermal emission from the shock heated plasma. Our generalized code combines these elements and describes the interplay between CR production and SNR evolution, including the nonlinear coupling of efficient diffusive shock acceleration, based mainly on the work of P. Blasi and coworkers, and a non-equilibrium ionization (NEI) calculation of thermal X-ray line emission. We believe that our generalized model will provide a consistent modeling platform for SNRs, including those interacting with molecular clouds, and improve the interpretation of current and future observations, including the high-quality spectra expected from Astro-H. SNR RX J1713.7−3946 is modeled as an example.}, number={2}, journal={ASTROPHYSICAL JOURNAL}, author={Lee, Shiu-Hang and Ellison, Donald C. and Nagataki, Shigehiro}, year={2012}, month={May} }
 @article{ellison_slane_patnaude_bykov_2012, title={CORE-COLLAPSE MODEL OF BROADBAND EMISSION FROM SNR RX J1713.7-3946 WITH THERMAL X-RAYS AND GAMMA RAYS FROM ESCAPING COSMIC RAYS}, volume={744}, ISSN={["1538-4357"]}, DOI={10.1088/0004-637x/744/1/39}, abstractNote={We present a spherically symmetric, core-collapse model of SNR RX J1713.7−3946 that includes a hydrodynamic simulation of the remnant evolution coupled to the efficient production of cosmic rays (CRs) by nonlinear diffusive shock acceleration. High-energy CRs that escape from the forward shock (FS) are propagated in surrounding dense material that simulates either a swept-up, pre-supernova shell or a nearby molecular cloud. The continuum emission from trapped and escaping CRs, along with the thermal X-ray emission from the shocked heated interstellar medium behind the FS, integrated over the remnant, is compared against broadband observations. Our results show conclusively that, overall, the GeV–TeV emission is dominated by inverse-Compton from CR electrons if the supernova is isolated regardless of its type, i.e., not interacting with a ≫100 M☉ shell or cloud. If the supernova remnant is interacting with a much larger mass ≳ 104 M☉, pion decay from the escaping CRs may dominate the TeV emission, although a precise fit at high energy will depend on the still uncertain details of how the highest energy CRs are accelerated by, and escape from, the FS. Based on morphological and other constraints, we consider the 104 M☉ pion-decay scenario highly unlikely for SNR RX J1713.7−3946 regardless of the details of CR escape. Importantly, even though CR electrons dominate the GeV–TeV emission, the efficient production of CR ions is an essential part of our leptonic model.}, number={1}, journal={ASTROPHYSICAL JOURNAL}, author={Ellison, Donald C. and Slane, Patrick and Patnaude, Daniel J. and Bykov, Andrei M.}, year={2012}, month={Jan} }
 @article{castro_slane_ellison_patnaude_2012, title={FERMI-LAT OBSERVATIONS AND A BROADBAND STUDY OF SUPERNOVA REMNANT CTB 109}, volume={756}, ISSN={["0004-637X"]}, DOI={10.1088/0004-637x/756/1/88}, abstractNote={CTB 109 (G109.1–1.0) is a Galactic supernova remnant (SNR) with a hemispherical shell morphology in X-rays and in the radio band. In this work, we report the detection of γ-ray emission coincident with CTB 109, using 37 months of data from the Large Area Telescope on board the Fermi Gamma-ray Space Telescope. We study the broadband characteristics of the remnant using a model that includes hydrodynamics, efficient cosmic-ray (CR) acceleration, nonthermal emission, and a self-consistent calculation of the X-ray thermal emission. We find that the observations can be successfully fit with two distinct parameter sets, one where the γ-ray emission is produced primarily by leptons accelerated at the SNR forward shock and the other where γ-rays produced by forward shock accelerated CR ions dominate the high-energy emission. Consideration of thermal X-ray emission introduces a novel element to the broadband fitting process, and while it does not rule out either the leptonic or the hadronic scenarios, it constrains the parameter sets required by the model to fit the observations. Moreover, the model that best fits the thermal and nonthermal emission observations is an intermediate case, where both radiation from accelerated electrons and hadrons contribute almost equally to the γ-ray flux observed.}, number={1}, journal={ASTROPHYSICAL JOURNAL}, author={Castro, Daniel and Slane, Patrick and Ellison, Donald C. and Patnaude, Daniel J.}, year={2012}, month={Sep} }
 @misc{bykov_ellison_renaud_2012, title={Magnetic Fields in Cosmic Particle Acceleration Sources}, volume={166}, ISSN={["1572-9672"]}, DOI={10.1007/s11214-011-9761-4}, abstractNote={We review here some magnetic phenomena in astrophysical particle accelerators associated with collisionless shocks in supernova remnants, radio galaxies and clusters of galaxies. A specific feature is that the accelerated particles can play an important role in magnetic field evolution in the objects. We discuss a number of CR-driven, magnetic field amplification processes that are likely to operate when diffusive shock acceleration (DSA) becomes efficient and nonlinear. The turbulent magnetic fields produced by these processes determine the maximum energies of accelerated particles and result in specific features in the observed photon radiation of the sources. Equally important, magnetic field amplification by the CR currents and pressure anisotropies may affect the shocked gas temperatures and compression, both in the shock precursor and in the downstream flow, if the shock is an efficient CR accelerator. Strong fluctuations of the magnetic field on scales above the radiation formation length in the shock vicinity result in intermittent structures observable in synchrotron emission images. Resonant and non-resonant CR streaming instabilities in the shock precursor can generate mesoscale magnetic fields with scale-sizes comparable to supernova remnants and even superbubbles. This opens the possibility that magnetic fields in the earliest galaxies were produced by the first generation Population III supernova remnants and by clustered supernovae in star forming regions.}, number={1-4}, journal={SPACE SCIENCE REVIEWS}, author={Bykov, Andrei M. and Ellison, Donald C. and Renaud, Matthieu}, year={2012}, month={May}, pages={71–95} }
 @article{bykov_osipov_ellison_2011, title={Cosmic ray current driven turbulence in shocks with efficient particle acceleration: the oblique, long-wavelength mode instability}, volume={410}, ISSN={["1365-2966"]}, DOI={10.1111/j.1365-2966.2010.17421.x}, abstractNote={In order for diffusive shock acceleration (DSA) to accelerate particles to high energies, the energetic particles must be able to interact with magnetic turbulence over a broad wavelength range. The weakly anisotropic distribution of accelerated particles, i.e. cosmic rays (CRs), is believed capable of producing this turbulence in a symbiotic relationship where the magnetic turbulence required to accelerate the CRs is created by the accelerated CRs themselves. In efficient DSA, this wave-particle interaction can be strongly non-linear where CRs modify the plasma flow and the specific mechanisms of magnetic field amplification. Resonant interactions have long been known to amplify magnetic fluctuations on the scale of the CR gyroradius, and Bell showed that the CR current can efficiently amplify magnetic fluctuations with scales smaller than the CR gyroradius. Here, we show with a multiscale, quasi-linear analysis that the presence of turbulence with scales shorter than the CR gyroradius enhances the growth of modes with scales longer than the gyroradius, at least for particular polarizations. We use a mean-field approach to average the equation of motion and the induction equation over the ensemble of magnetic field oscillations accounting for the anisotropy of relativistic particles on the background plasma. We derive the response of the magnetized CR current on magnetic field fluctuations and show that, in the presence of short-scale, Bell-type turbulence, long-wavelength modes are amplified. The polarization, helicity and angular dependence of the growth rates are calculated for obliquely propagating modes for wavelengths both below and above the CR mean free path. The long-wavelength growth rates we estimate for typical supernova remnant parameters are sufficiently fast to suggest a fundamental increase in the maximum CR energy that a given shock can produce.}, number={1}, journal={MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY}, author={Bykov, A. M. and Osipov, S. M. and Ellison, D. C.}, year={2011}, month={Jan}, pages={39–52} }
 @article{ellison_bykov_2011, title={GAMMA-RAY EMISSION OF ACCELERATED PARTICLES ESCAPING A SUPERNOVA REMNANT IN A MOLECULAR CLOUD}, volume={731}, ISSN={["1538-4357"]}, DOI={10.1088/0004-637x/731/2/87}, abstractNote={We present a model of gamma-ray emission from core-collapse supernovae (SNe) originating from the explosions of massive young stars. The fast forward shock of the supernova remnant (SNR) can accelerate particles by diffusive shock acceleration (DSA) in a cavern blown by a strong, pre-SN stellar wind. As a fundamental part of nonlinear DSA, some fraction of the accelerated particles escape the shock and interact with a surrounding massive dense shell producing hard photon emission. To calculate this emission, we have developed a new Monte Carlo technique for propagating the cosmic rays (CRs) produced by the forward shock of the SNR, into the dense, external material. This technique is incorporated in a hydrodynamic model of an evolving SNR which includes the nonlinear feedback of CRs on the SNR evolution, the production of escaping CRs along with those that remain trapped within the remnant, and the broadband emission of radiation from trapped and escaping CRs. While our combined CR-hydro-escape model is quite general and applies to both core collapse and thermonuclear SNe, the parameters we choose for our discussion here are more typical of SNRs from very massive stars whose emission spectra differ somewhat from those produced by lower mass progenitors directly interacting with a molecular cloud.}, number={2}, journal={ASTROPHYSICAL JOURNAL}, author={Ellison, Donald C. and Bykov, Andrei M.}, year={2011}, month={Apr} }
 @article{castro_slane_patnaude_ellison_2011, title={THE IMPACT OF EFFICIENT PARTICLE ACCELERATION ON THE EVOLUTION OF SUPERNOVA REMNANTS IN THE SEDOV-TAYLOR PHASE}, volume={734}, ISSN={["0004-637X"]}, DOI={10.1088/0004-637x/734/2/85}, abstractNote={We investigate the effects of the efficient production of cosmic rays (CRs) on the evolution of supernova remnants (SNRs) in the adiabatic Sedov–Taylor phase. We model the SNR by coupling the hydrodynamic evolution with nonlinear diffusive shock acceleration (DSA) and track self-consistently the ionization state of the shock-heated plasma. Using a plasma emissivity code and the results of the model, we predict the thermal X-ray emission and combine it with the non-thermal component in order to obtain the complete spectrum in this energy range. Hence, we study how the interpretation of thermal X-ray observations is affected by the efficiency of the DSA process, and find that, compared to test particle cases, the efficient DSA example yields a smaller shock radius and speed, a larger compression ratio, and lower intensity X-ray thermal emission. We also find that a model where the shock is not assumed to produce CRs can fit the X-ray observational properties of an example with efficient particle acceleration, with a different set of input parameters, and in particular a much lower explosion energy. Additionally, we model the broadband non-thermal emission and investigate what signatures result from the acceleration of particles.}, number={2}, journal={ASTROPHYSICAL JOURNAL}, author={Castro, Daniel and Slane, Patrick and Patnaude, Daniel J. and Ellison, Donald C.}, year={2011}, month={Jun} }
 @article{bykov_ellison_osipov_pavlov_uvarov_2011, title={X-RAY STRIPES IN TYCHO'S SUPERNOVA REMNANT: SYNCHROTRON FOOTPRINTS OF A NONLINEAR COSMIC-RAY-DRIVEN INSTABILITY}, volume={735}, ISSN={["2041-8205"]}, DOI={10.1088/2041-8205/735/2/l40}, abstractNote={High-resolution Chandra observations of Tycho's supernova remnant (SNR) have revealed several sets of quasi-steady, high-emissivity, nearly parallel X-ray stripes in some localized regions of the SNR. These stripes are most likely the result of cosmic-ray (CR) generated magnetic turbulence at the SNR blast wave. However, for the amazingly regular pattern of these stripes to appear, simultaneous action of a number of shock-plasma phenomena is required, which is not predicted by most models of magnetic field amplification. A consistent explanation of these stripes yields information on the complex nonlinear plasma processes connecting efficient CR acceleration and magnetic field fluctuations in strong collisionless shocks. The nonlinear diffusive shock acceleration (NL-DSA) model described here, which includes magnetic field amplification from a CR-current-driven instability, does predict stripes consistent with the synchrotron observations of Tycho's SNR. We argue that the local ambient mean magnetic field geometry determines the orientation of the stripes and therefore it can be reconstructed with the high-resolution X-ray imaging. The estimated maximum energy of the CR protons responsible for the stripes is ∼1015 eV. Furthermore, the model predicts that a specific X-ray polarization pattern, with a polarized fraction ∼50%, accompanies the stripes, which can be tested with future X-ray polarimeter missions.}, number={2}, journal={ASTROPHYSICAL JOURNAL LETTERS}, author={Bykov, Andrei M. and Ellison, Donald C. and Osipov, Sergei M. and Pavlov, George G. and Uvarov, Yury A.}, year={2011}, month={Jul} }
 @article{ellison_patnaude_slane_raymond_2010, title={EFFICIENT COSMIC RAY ACCELERATION, HYDRODYNAMICS, AND SELF-CONSISTENT THERMAL X-RAY EMISSION APPLIED TO SUPERNOVA REMNANT RX J1713.7-3946}, volume={712}, ISSN={["0004-637X"]}, DOI={10.1088/0004-637x/712/1/287}, abstractNote={We model the broadband emission from supernova remnant (SNR) RX J1713.7–3946 including, for the first time, a consistent calculation of thermal X-ray emission together with non-thermal emission in a nonlinear diffusive shock acceleration model. Our model tracks the evolution of the SNR including the plasma ionization state between the forward shock and the contact discontinuity. We use a plasma emissivity code to predict the thermal X-ray emission spectrum assuming the initially cold electrons are heated either by Coulomb collisions with the shock-heated protons (the slowest possible heating), or come into instant equilibration with the protons. For either electron heating model, electrons reach ≳107 K rapidly and the X-ray line emission near 1 keV is more than 10 times as luminous as the underlying thermal bremsstrahlung continuum. Since recent Suzaku observations show no detectable line emission, this places strong constraints on the unshocked ambient medium density and on the relativistic electron-to-proton ratio. For the uniform circumstellar medium (CSM) models that we consider, the low densities and high relativistic electron-to-proton ratios required to match the Suzaku X-ray observations definitively rule out pion decay as the emission process producing GeV–TeV photons. We show that leptonic models, where inverse-Compton scattering against the cosmic background radiation dominates the GeV–TeV emission, produce better fits to the broadband thermal and non-thermal observations in a uniform CSM.}, number={1}, journal={ASTROPHYSICAL JOURNAL}, author={Ellison, Donald C. and Patnaude, Daniel J. and Slane, Patrick and Raymond, John}, year={2010}, month={Mar}, pages={287–293} }
 @article{patnaude_slane_raymond_ellison_2010, title={THE ROLE OF DIFFUSIVE SHOCK ACCELERATION ON NONEQUILIBRIUM IONIZATION IN SUPERNOVA REMNANT SHOCKS. II. EMITTED SPECTRA}, volume={725}, ISSN={["0004-637X"]}, DOI={10.1088/0004-637x/725/2/1476}, abstractNote={We present a grid of nonequilibrium ionization models for the X-ray spectra from supernova remnants undergoing efficient diffusive shock acceleration. The calculation follows the hydrodynamics of the blast wave as well as the time-dependent ionization of the plasma behind the shock. The ionization state is passed to a plasma emissivity code to compute the thermal X-ray emission, which is combined with the emission from nonthermal synchrotron emission to produce a self-consistent model for the thermal and nonthermal emission from cosmic-ray dominated shocks. We show how plasma diagnostics such as the G′-ratio of He-like ions, defined as the ratio of the sum of the intercombination, forbidden, and satellite lines to the resonance line, can vary with acceleration efficiency, and discuss how the thermal X-ray emission, when the time-dependent ionization is not calculated self-consistently with the hydrodynamics, can differ from the thermal X-ray emission from models which do account for the hydrodynamics. Finally, we compare the thermal X-ray emission from models which show moderate acceleration (∼35%) to the thermal X-ray emission from test-particle models.}, number={2}, journal={ASTROPHYSICAL JOURNAL}, author={Patnaude, Daniel J. and Slane, Patrick and Raymond, John C. and Ellison, Donald C.}, year={2010}, month={Dec}, pages={1476–1484} }
 @article{vladimirov_bykov_ellison_2009, title={SPECTRA OF MAGNETIC FLUCTUATIONS AND RELATIVISTIC PARTICLES PRODUCED BY A NONRESONANT WAVE INSTABILITY IN SUPERNOVA REMNANT SHOCKS}, volume={703}, ISSN={["2041-8213"]}, DOI={10.1088/0004-637X/703/1/L29}, abstractNote={We model strong forward shocks in young supernova remnants with efficient particle acceleration where a nonresonant instability driven by the cosmic ray current amplifies magnetic turbulence in the shock precursor. Particle injection, magnetic field amplification (MFA), and the nonlinear feedback of particles and fields on the bulk flow are derived consistently. The shock structure depends critically on the efficiency of turbulence cascading. If cascading is suppressed, MFA is strong, the shock precursor is stratified, and the turbulence spectrum contains several discrete peaks. These peaks, as well as the amount of MFA, should influence synchrotron X-rays, allowing observational tests of cascading and other assumptions intrinsic to the nonlinear model of nonresonant wave growth.}, number={1}, journal={ASTROPHYSICAL JOURNAL LETTERS}, author={Vladimirov, Andrey E. and Bykov, Andrei M. and Ellison, Donald C.}, year={2009}, month={Sep}, pages={L29–L32} }
 @article{patnaude_ellison_slane_2009, title={THE ROLE OF DIFFUSIVE SHOCK ACCELERATION ON NONEQUILIBRIUM IONIZATION IN SUPERNOVA REMNANTS}, volume={696}, ISSN={["1538-4357"]}, DOI={10.1088/0004-637X/696/2/1956}, abstractNote={We present results of semianalytic calculations which show clear evidence for changes in the nonequilibrium ionization behind a supernova remnant forward shock undergoing efficient diffusive shock acceleration (DSA). The efficient acceleration of particles (i.e., cosmic rays (CRs)) lowers the shock temperature and raises the density of the shocked gas, thus altering the ionization state of the plasma in comparison to the test-particle (TP) approximation where CRs gain an insignificant fraction of the shock energy. The differences between the TP and efficient acceleration cases are substantial and occur for both slow and fast temperature equilibration rates: in cases of higher acceleration efficiency, particular ion states are more populated at lower electron temperatures. We also present results which show that, in the efficient shock acceleration case, higher ionization fractions are reached noticeably closer to the shock front than in the TP case, clearly indicating that DSA may enhance thermal X-ray production. We attribute this to the higher postshock densities which lead to faster electron temperature equilibration and higher ionization rates. These spatial differences should be resolvable with current and future X-ray missions, and can be used as diagnostics in estimating the acceleration efficiency in CR-modified shocks.}, number={2}, journal={ASTROPHYSICAL JOURNAL}, author={Patnaude, Daniel J. and Ellison, Donald C. and Slane, Patrick}, year={2009}, month={May}, pages={1956–1963} }
 @article{bykov_uvarov_ellison_2008, title={DOTS, CLUMPS, AND FILAMENTS: THE INTERMITTENT IMAGES OF SYNCHROTRON EMISSION IN RANDOM MAGNETIC FIELDS OF YOUNG SUPERNOVA REMNANTS}, volume={689}, ISSN={["2041-8213"]}, DOI={10.1086/595868}, abstractNote={Nonthermal X-ray emission in some supernova remnants originates from synchrotron radiation of ultrarelativistic particles in turbulent magnetic fields. We address the effect of a random magnetic field on synchrotron emission images and spectra. A random magnetic field is simulated to construct synchrotron emission maps of a source with a steady distribution of ultrarelativistic electrons. Nonsteady localized structures (dots, clumps, and filaments), in which the magnetic field reaches exceptionally high values, typically arise in the random field sample. These magnetic field concentrations dominate the synchrotron emission (integrated along the line of sight) from the highest energy electrons in the cutoff regime of the distribution, resulting in an evolving, intermittent, clumpy appearance. The simulated structures resemble those observed in X-ray images of some young supernova remnants. The lifetime of X-ray clumps can be short enough to be consistent with that observed even in the case of a steady particle distribution. The efficiency of synchrotron radiation from the cutoff regime in the electron spectrum is strongly enhanced in a turbulent field compared to emission from a uniform field of the same magnitude.}, number={2}, journal={ASTROPHYSICAL JOURNAL LETTERS}, author={Bykov, Andrei M. and Uvarov, Yury A. and Ellison, Donald C.}, year={2008}, month={Dec}, pages={L133–L136} }
 @article{ellison_vladimirov_2008, title={Magnetic field amplification and rapid time variations in SNR RX J1713.7-3946}, volume={673}, ISSN={["2041-8213"]}, DOI={10.1086/527359}, abstractNote={Evidence is accumulating suggesting that collisionless shocks in supernova remnants (SNRs) can amplify the interstellar magnetic field to hundreds of microgauss or even milligauss levels, as recently claimed for SNR RX J1713.7–3946. If these fields exist, they are almost certainly created by magnetic field amplification (MFA) associated with the efficient production of cosmic rays by diffusive shock acceleration (DSA) and their existence strengthens the case for SNRs being the primary source of Galactic cosmic-ray ions to the “knee” and beyond. However, the high magnetic field values in SNRs are obtained exclusively from the interpretation of observations of radiation from relativistic electrons, and if MFA via nonlinear DSA produces these fields, the magnetic field that determines the maximum ion energy will be substantially less than the field that determines the maximum electron energy. We use results of a steady-state Monte Carlo simulation to show how nonlinear effects from efficient cosmic-ray production and MFA reduce the maximum energy of protons relative to what would be expected from test-particle acceleration.}, number={1}, journal={ASTROPHYSICAL JOURNAL LETTERS}, author={Ellison, Donald C. and Vladimirov, Andrey}, year={2008}, month={Jan}, pages={L47–L50} }
 @article{vladimirov_bykov_ellison_2008, title={TURBULENCE DISSIPATION AND PARTICLE INJECTION IN NONLINEAR DIFFUSIVE SHOCK ACCELERATION WITH MAGNETIC FIELD AMPLIFICATION}, volume={688}, ISSN={["1538-4357"]}, DOI={10.1086/592240}, abstractNote={The highly amplified magnetic fields suggested by observations of some supernova remnant shells are most likely an intrinsic part of efficient particle acceleration by shocks. This strong turbulence, which may result from cosmic-ray-driven instabilities, both resonant and nonresonant, in the shock precursor, is certain to play a critical role in self-consistent, nonlinear models of strong, cosmic-ray-modified shocks. Here we present a Monte Carlo model of nonlinear diffusive shock acceleration (DSA) accounting for magnetic field amplification through resonant instabilities induced by accelerated particles, and including the effects of dissipation of turbulence upstream of a shock and the subsequent precursor plasma heating. Feedback effects between the plasma heating due to turbulence dissipation and particle injection are strong, adding to the nonlinear nature of efficient DSA. Describing the turbulence damping in a parameterized way, we reach two important results: first, for conditions typical of supernova remnant shocks, even a small amount of dissipated turbulence energy (~10%) is sufficient to significantly heat the precursor plasma; and second, the heating upstream of the shock leads to an increase in the injection of thermal particles at the subshock by a factor of several. In our results, the response of the nonlinear shock structure to the boost in particle injection prevented the efficiency of particle acceleration and magnetic field amplification from increasing. We argue, however, that more advanced (possibly nonresonant) models of turbulence generation and dissipation may lead to a scenario in which particle injection boost due to turbulence dissipation results in more efficient acceleration and even stronger amplified magnetic fields than without the dissipation.}, number={2}, journal={ASTROPHYSICAL JOURNAL}, author={Vladimirov, Andrey E. and Bykov, Andrei M. and Ellison, Donald C.}, year={2008}, month={Dec}, pages={1084–1101} }
 @article{lee_kamae_ellison_2008, title={Three-dimensional model of broadband emission from supernova remnants undergoing nonlinear diffusive shock acceleration}, volume={686}, ISSN={["1538-4357"]}, DOI={10.1086/591308}, abstractNote={We present a three-dimensional model of supernova remnants (SNRs) in which the hydrodynamical evolution of the remnant is modeled consistently with nonlinear diffusive shock acceleration occurring at the outer blast wave. The model includes particle escape and diffusion outside of the forward shock and particle interactions with arbitrary distributions of external ambient material, such as molecular clouds. We include synchrotron emission and cooling, bremsstrahlung radiation, neutral pion production, and inverse Compton (IC) and Coulomb energy loss. Broadband spectra have been calculated for typical parameters, including dense regions of gas external to a 1000 yr old SNR. In this paper, we describe the details of our model, but do not attempt a detailed fit to any specific remnant. We also do not include magnetic field amplification (MFA), even though this effect may be important in some young remnants. Our aim is to develop a flexible platform that can be generalized to include effects such as MFA, and that can be easily adapted to various SNR environments, including Type Ia SNRs, which explode in a constant-density medium, and Type II SNRs, which explode in a presupernova wind. When applied to a specific SNR, our model will predict cosmic-ray spectra and multiwavelength morphology in projected images for instruments with varying spatial and spectral resolutions. We show examples of these spectra and images and emphasize the importance of measurements in the hard X-ray, GeV, and TeV gamma-ray bands for investigating key ingredients in the acceleration mechanism, and for deducing whether or not TeV emission is produced by IC from electrons or pion decay from protons.}, number={1}, journal={ASTROPHYSICAL JOURNAL}, author={Lee, Shiu-Hang and Kamae, Tuneyoshi and Ellison, Donald C.}, year={2008}, month={Oct}, pages={325–336} }
 @article{ellison_patnaude_slane_blasi_gabici_2007, title={Particle acceleration in supernova remnants and the production of thermal and nonthermal radiation}, volume={661}, ISSN={["0004-637X"]}, DOI={10.1086/517518}, abstractNote={Efficient cosmic-ray production can have a significant effect on the evolution and X-ray emission from SNRs. Using hydrodynamic simulations coupled to diffusive shock acceleration, we produce thermal and nonthermal forward-shock photon spectra. For a given ambient density and explosion energy, we find that the position of the forward shock at a given age is a strong function of the acceleration efficiency, providing a signature of cosmic-ray production. Using an approximate treatment for the ionization state of the plasma, we investigate the effects of slow versus rapid heating of the postshock electrons on the ratio of thermal to nonthermal X-ray emission at the forward shock. We also investigate the effects of magnetic field strength on the observed spectrum for efficient cosmic-ray acceleration. The primary effect of a large field is a flattening of the nonthermal spectrum in the soft X-ray band. X-ray spectral index measurements may thus be indicators of the postshock magnetic field strength. The predicted gamma-ray flux from inverse Compton (IC) scattering and neutral pion decay is strongly affected by the ambient conditions, and for the parameters used in our examples, the IC emission at E ~ 1 TeV exceeds that from pion decay, although at both lower and higher energies this trend is reversed for cases of high ambient density. More importantly, high magnetic fields produce a steepening of the electron spectrum over a wide energy range, which may make it more difficult to differentiate between IC and pion-decay emission solely by spectral shape.}, number={2}, journal={ASTROPHYSICAL JOURNAL}, author={Ellison, Donald C. and Patnaude, Daniel J. and Slane, Patrick and Blasi, Pasquale and Gabici, Stefano}, year={2007}, month={Jun}, pages={879–891} }
 @article{ellison_2006, title={Astroparticle physics - Cosmic-ray acceleration at the limit}, volume={2}, ISSN={["1745-2473"]}, DOI={10.1038/nphys401}, number={9}, journal={NATURE PHYSICS}, author={Ellison, Don}, year={2006}, month={Sep}, pages={589–590} }
 @article{vladimirov_ellison_bykov_2006, title={Nonlinear diffusive shock acceleration with magnetic field amplification}, volume={652}, ISSN={["1538-4357"]}, DOI={10.1086/508154}, abstractNote={We introduce a Monte Carlo model of nonlinear diffusive shock acceleration that allows for the generation of large-amplitude magnetic turbulence, i.e., ΔB ≫ B0, where B0 is the ambient magnetic field. The model is the first to include strong wave generation, efficient particle acceleration to relativistic energies in nonrelativistic shocks, and thermal particle injection in an internally self-consistent manner. We find that the upstream magnetic field B0 can be amplified by large factors and show that this amplification depends strongly on the ambient Alfvén Mach number. We also show that, in the nonlinear model, large increases in B do not necessarily translate into a large increase in the maximum particle momentum a particular shock can produce, a consequence of high-momentum particles diffusing in the shock precursor where the large amplified field converges to the low ambient value. To deal with the field growth rate in the regime of strong fluctuations, we extend to strong turbulence a parameterization that is consistent with the resonant quasi-linear growth rate in the weak turbulence limit. We believe our parameterization spans the maximum and minimum range of the fluctuation growth, and within these limits we show that the nonlinear shock structure, acceleration efficiency, and thermal particle injection rates depend strongly on the yet to be determined details of wave growth in strongly turbulent fields. The most direct application of our results will be to estimate magnetic fields amplified by strong cosmic-ray modified shocks in supernova remnants.}, number={2}, journal={ASTROPHYSICAL JOURNAL}, author={Vladimirov, Andrey and Ellison, Donald C. and Bykov, Andrei}, year={2006}, month={Dec}, pages={1246–1258} }
 @article{cassam-chenai_decourchelle_ballet_ellison_2005, title={Morphology of synchrotron emission in young supernova remnants}, volume={443}, ISSN={["1432-0746"]}, DOI={10.1051/0004-6361:20052853}, abstractNote={In the framework of test-particle and cosmic-ray modified hydrodynamics, we calculate synchrotron emission radial profiles in young ejecta-dominated supernova remnants (SNRs) evolving in an ambient medium which is uniform in density and magnetic field. We find that, even without any magnetic field amplification by Raleigh-Taylor instabilities, the radio synchrotron emission peaks at the contact discontinuity because the magnetic field is compressed and is larger there than at the forward shock. The X-ray synchrotron emission sharply drops behind the forward shock as the highest energy electrons suffer severe radiative losses.}, number={3}, journal={ASTRONOMY & ASTROPHYSICS}, author={Cassam-Chenai, G and Decourchelle, A and Ballet, J and Ellison, DC}, year={2005}, month={Dec}, pages={955–U86} }
 @article{baring_ellison_slane_2005, title={Multiwavelength spectral models for SNR G347.3-0.5 from non-linear shock acceleration}, volume={35}, ISBN={["*************"]}, ISSN={["0273-1177"]}, DOI={10.1016/j.asr.2005.02.067}, abstractNote={The remnant G347.3-0.5 exhibits strong shell emission in the radio and X-ray bands, and has a purported detection in the TeV gamma-ray band by the CANGAROO-II telescope. The CANGAROO results were touted as evidence for the production of cosmic ray ions, a claim that has proven controversial due to constraining fluxes associated with a proximate unidentified EGRET source 3EG J1714-3857. HESS has now seen this source in the TeV band. The complex environment of the remnant renders modeling of its broadband spectrum sensitive to assumptions concerning the nature and parameters of the circumremnant medium. This paper explores a sampling of reasonable possibilities for multiwavelength spectral predictions from this source, using a non-linear model of diffusive particle acceleration at the shocked shell. The magnetic field strength, shell size and degree of particle cross-field diffusion act as variables to which the radio to X-ray to gamma-ray signal is sensitive. The modeling of the extant data constrains these variables, and the potential impact of the recent HESS detection on such parameters is addressed. Putative pion decay signals in hard gamma-rays resulting from hadronic interactions in dense molecular clouds are briefly discussed; the requisite suppression of the GeV component needed to accommodate the 3EG J1714-3857 EGRET data provides potential bounds on the diffusive distance from the shell to the proximate clouds.}, number={6}, journal={YOUNG NEUTRON STARS AND SUPERNOVA REMNANTS}, author={Baring, MG and Ellison, DC and Slane, PO}, year={2005}, pages={1041–1046} }
 @article{ellison_decourchelle_ballet_2005, title={Nonlinear particle acceleration at reverse shocks in supernova remnants}, volume={429}, ISSN={["0004-6361"]}, DOI={10.1051/0004-6361:20041668}, abstractNote={Without amplification, magnetic fields in expanding ejecta of young supernova remnants (SNRs) will be orders of magnitude below those required to shock accelerate thermal electrons, or ions, to relativistic energies or to produce radio synchrotron emission at the reverse shock. The reported observations of such emission give support to the idea that diffusive shock acceleration (DSA) can amplify magnetic fields by large factors. Furthermore, the uncertain character of the amplification process leaves open the possibility that ejecta fields, while large enough to support radio emission and DSA, may be much lower than typical interstellar medium values. We show that DSA in such low reverse shock fields is extremely nonlinear and efficient in the production of cosmic-ray (CR) ions, although CRs greatly in excess of mc 2 are not produced. These nonlinear effects, which occur at the forward shock as well, are manifested most importantly in shock compression ratios � 4 and cause the interaction region between the forward and reverse shocks to become narrower, denser, and cooler than would be the case if efficient cosmic-ray production did not occur. The changes in the SNR structure and evolution should be clearly observable, if present, and they convey important information on the nature of DSA and magnetic field amplification with broad astrophysical implications.}, number={2}, journal={ASTRONOMY & ASTROPHYSICS}, author={Ellison, DC and Decourchelle, A and Ballet, J}, year={2005}, month={Jan}, pages={569–580} }
 @article{ellison_cassam-chenai_2005, title={Radio and X-ray profiles in supernova remnants undergoing efficient cosmic-ray production}, volume={632}, ISSN={["1538-4357"]}, DOI={10.1086/444449}, abstractNote={The strong shocks in young supernova remnants (SNRs) should accelerate cosmic rays (CRs), and there is no doubt that relativistic electrons are produced in SNRs. However, direct and convincing evidence that SNRs produce CR nuclei has not yet been obtained and may, in fact, be long in coming if current γ-ray observatories do not see an unambiguous pion-decay feature. Nevertheless, the lack of an observed pion-decay feature does not necessarily mean that CR ions are not abundantly produced since ions do not radiate efficiently. If CR ions are produced efficiently by diffusive shock acceleration (DSA), their presence will modify the hydrodynamics of the SNR and produce morphological effects that can be clearly seen in radiation produced by electrons. We describe in some detail our CR-hydro model, which couples DSA with the remnant hydrodynamics, and the synchrotron emission expected for two distinct parameter sets representing Type Ia and Type II supernovae. Several morphological features emerge in radial profiles, including the forward shock precursor, which are observable with current X-ray observatories and should definitively show whether young SNRs produce CR ions efficiently or not. For the specific case of SN 1006 we conclude, as have others, that the extremely short X-ray scale heights observed near the outer shock argue convincingly for the efficient production of CR ions.}, number={2}, journal={ASTROPHYSICAL JOURNAL}, author={Ellison, DC and Cassam-Chenai, G}, year={2005}, month={Oct}, pages={920–931} }
 @article{ellison_double_2004, title={Diffusive shock acceleration in unmodified relativistic, oblique shocks}, volume={22}, ISSN={["1873-2852"]}, DOI={10.1016/j.astropartphys.2004.08.005}, abstractNote={We present results from a fully relativistic Monte Carlo simulation of diffusive shock acceleration (DSA) in unmodified shocks. The computer code uses a single algorithmic sequence to smoothly span the range from non-relativistic speeds to fully relativistic shocks of arbitrary obliquity, providing a powerful consistency check. While known results are obtained for non-relativistic and ultra-relativistic parallel shocks, new results are presented for the less explored trans-relativistic regime and for oblique, fully relativistic shocks. We find, for a wide trans-relativistic range extending to shock Lorentz factors >30, that the particle spectrum produced by DSA varies strongly from the canonical f(p) ∝ p−4.23 spectrum known to result in ultra-relativistic shocks. Trans-relativistic shocks may play an important role in γ-ray bursts and other sources and most relativistic shocks will be highly oblique.}, number={3-4}, journal={ASTROPARTICLE PHYSICS}, author={Ellison, DC and Double, GP}, year={2004}, month={Nov}, pages={323–338} }
 @article{ellison_decourchelle_ballet_2004, title={Hydrodynamic simulation of supernova remnants including efficient particle acceleration}, volume={413}, ISSN={["0004-6361"]}, DOI={10.1051/0004-6361:20034073}, abstractNote={A number of supernova remnants (SNRs) show nonthermal X-rays assumed to be synchrotron emission from shock accelerated TeV electrons. The existence of these TeV electrons strongly suggests that the shocks in SNRs are sources of galactic cosmic rays (CRs). In addition, there is convincing evidence from broad-band studies of individual SNRs and elsewhere that the particle acceleration process in SNRs can be efficient and nonlinear. If SNR shocks are efficient particle accelerators, the production of CRs impacts the thermal properties of the shock heated, X-ray emitting gas and the SNR evolution. We report on a technique that couples nonlinear diffusive shock acceleration, including the backreaction of the accelerated particles on the structure of the forward and reverse shocks, with a hydrodynamic simulation of SNR evolution. Compared to models which ignore CRs, the most important hydrodynamical effects of placing a significant fraction of shock energy into CRs are larger shock compression ratios and lower temperatures in the shocked gas. We compare our results, which use an approximate description of the acceleration process, with a more complete model where the full CR transport equations are solved [CITE][]BKV2002, and find excellent agreement for the CR spectrum summed over the SNR lifetime and the evolving shock compression ratio. The importance of the coupling between particle acceleration and SNR dynamics for the interpretation of broad-band continuum and thermal X-ray observations is discussed.}, number={1}, journal={ASTRONOMY & ASTROPHYSICS}, author={Ellison, DC and Decourchelle, A and Ballet, J}, year={2004}, month={Jan}, pages={189–201} }
 @article{double_baring_jones_ellison_2004, title={Magnetohydrodynamic jump conditions for oblique relativistic shocks with gyrotropic pressure}, volume={600}, ISSN={["0004-637X"]}, DOI={10.1086/379702}, abstractNote={Shock jump conditions, i.e., the specification of the downstream parameters of the gas in terms of the upstream parameters, are obtained for steady state, plane shocks with oblique magnetic fields and arbitrary flow speeds. This is done by combining the continuity of particle number flux and the electromagnetic boundary conditions at the shock with the magnetohydrodynamic conservation laws derived from the stress-energy tensor. For ultrarelativistic and nonrelativistic shocks, the jump conditions may be solved analytically. For mildly relativistic shocks, analytic solutions are obtained for isotropic pressure using an approximation for the adiabatic index that is valid in high sonic Mach number cases. Examples assuming isotropic pressure illustrate how the shock compression ratio depends on the shock speed and obliquity. In the more general case of gyrotropic pressure, the jump conditions cannot be solved analytically without additional assumptions, and the effects of gyrotropic pressure are investigated by parameterizing the distribution of pressure parallel and perpendicular to the magnetic field. Our numerical solutions reveal that relatively small departures from isotropy (e.g., ~20%) produce significant changes in the shock compression ratio, r, at all shock Lorentz factors, including ultrarelativistic ones, where an analytic solution with gyrotropic pressure is obtained. In particular, either dynamically important fields or significant pressure anisotropies can incur marked departures from the canonical gas dynamic value of r = 3 for a shocked ultrarelativistic flow, and this may impact models of particle acceleration in gamma-ray bursts and other environments in which relativistic shocks are inferred. The jump conditions presented apply directly to test-particle acceleration and will facilitate future self-consistent numerical modeling of particle acceleration at oblique, relativistic shocks; such models include the modification of the fluid velocity profile due to the contribution of energetic particles to the momentum and energy fluxes.}, number={1}, journal={ASTROPHYSICAL JOURNAL}, author={Double, GP and Baring, MG and Jones, FC and Ellison, DC}, year={2004}, month={Jan}, pages={485–500} }
 @article{ellison_2002, title={Efficient cosmic-ray production in supernova remnant G347.3-0.5}, volume={46}, ISSN={["1387-6473"]}, DOI={10.1016/S1387-6473(02)00191-4}, abstractNote={Broad-band models of the bright NW limb of G347.3–0.5 give convincing evidence that the forward shock of this supernova remnant is accelerating cosmic rays efficiently, placing >25% of the shock kinetic energy flux into relativistic ions. Despite this high efficiency, the maximum electron and proton energies are well below the observed ‘knee’ at ∼1015 eV in the Galactic cosmic-ray spectrum.}, number={8-10}, journal={NEW ASTRONOMY REVIEWS}, author={Ellison, DC}, year={2002}, month={Jul}, pages={503–506} }
 @article{ellison_double_2002, title={Nonlinear particle acceleration in relativistic shocks}, volume={18}, ISSN={["0927-6505"]}, DOI={10.1016/S0927-6505(02)00142-1}, abstractNote={Monte Carlo techniques are used to model nonlinear particle acceleration in parallel collisionless shocks of various speeds, including mildly relativistic ones. When the acceleration is efficient, the backreaction of accelerated particles modifies the shock structure and causes the compression ratio, r, to increase above test-particle values. Modified shocks with Lorentz factors less than about 3 can have compression ratios considerably greater than 3 and the momentum distribution of energetic particles no longer follows a power law relation. These results may be important for the interpretation of gamma-ray bursts if mildly relativistic internal and/or afterglow shocks play an important role accelerating particles that produce the observed radiation. For shock Lorentz factors greater than about 10, r approaches 3 and the so-called `universal' test-particle result of N(E) proportional to E^{-2.3} is obtained for sufficiently energetic particles. In all cases, the absolute normalization of the particle distribution follows directly from our model assumptions and is explicitly determined.}, number={3}, journal={ASTROPARTICLE PHYSICS}, author={Ellison, DC and Double, GP}, year={2002}, month={Dec}, pages={213–228} }
 @article{ellison_slane_gaensler_2001, title={Broadband observations and modeling of the shell-type supernova remnant G347.3-0.5}, volume={563}, ISSN={["0004-637X"]}, DOI={10.1086/323687}, abstractNote={The supernova remnant G347.3-0.5 emits a featureless power law in X-rays, thought to indicate shock acceleration of electrons to high energies. We here produce a broadband spectrum of the bright northwest limb of this source by combining radio observations from the Australia Telescope Compact Array (ATCA), X-ray observations from the Advanced Satellite for Cosmology and Astrophysics (ASCA), and TeV γ-ray observations from the CANGAROO imaging Cerenkov telescope. We assume that this emission is produced by an electron population generated by diffusive shock acceleration at the remnant forward shock. The nonlinear aspects of the particle acceleration force a connection between the widely different wavelength bands and between the electrons and the unseen ions, presumably accelerated simultaneously with the electrons. This allows us to infer the relativistic proton spectrum and estimate ambient parameters such as the supernova explosion energy, magnetic field, matter density in the emission region, and efficiency of the shock acceleration process. We find convincing evidence that the shock acceleration is efficient, placing greater than 25% of the shock kinetic energy flux into relativistic ions. Despite this high efficiency, the maximum electron and proton energies, while depending somewhat on assumptions for the compression of the magnetic field in the shock, are well below the observed "knee" at ~1015 eV in the Galactic cosmic-ray spectrum.}, number={1}, journal={ASTROPHYSICAL JOURNAL}, author={Ellison, DC and Slane, P and Gaensler, BM}, year={2001}, month={Dec}, pages={191–201} }
 @misc{ellison_2001, title={Nonlinear shock acceleration and cosmic-ray production in young supernova remnants}, volume={99}, ISSN={["1572-9672"]}, DOI={10.1023/A:1013865619908}, number={1-4}, journal={SPACE SCIENCE REVIEWS}, author={Ellison, DC}, year={2001}, month={Oct}, pages={305–315} }
 @article{ellison_2001, title={Nonlinear shock acceleration with an application to SNR G347.3-0.5}, DOI={10.1143/ptps.143.125}, abstractNote={(ReceivedMarch 26, 2001) Some aspects of diffusive shock acceleration are reviewed with an emphasis on nonlinear effects from efficient acceleration. A direct application to G347.3 – 0.5, a supernova remnant showing a featureless X-ray spectrum believedprod ucedby synchrotron emission from TeV electrons, is also presented.}, number={143}, journal={Progress of Theoretical Physics. Supplement}, author={Ellison, D. C.}, year={2001}, pages={125–148} }
 @article{blondin_ellison_2001, title={Rayleigh-Taylor instabilities in young supernova remnants undergoing efficient particle acceleration}, volume={560}, ISSN={["0004-637X"]}, DOI={10.1086/322499}, abstractNote={We employ hydrodynamic simulations to study the effects of high shock compression ratios, as expected for fast shocks with efficient particle acceleration, on the convective instability of driven waves in supernova remnants. We find that the instability itself does not depend significantly on the compression ratio, σ, with the growth rates and the width of the mixing region at saturation being comparable for the range of ratios we studied; 4 ≤ σ ≤ 21. However, because the width of the interaction region between the forward and reverse shocks can shrink significantly with increasing σ, we find that convective instabilities can reach all the way to the forward shock front if compression ratios are high enough. Thus, if supernova blast waves accelerate particles efficiently, we expect the forward shock to be perturbed with small-amplitude, small-wavelength bumps and to find clumps and filaments of dense ejecta material in the vicinity of the shock. In addition and in contrast to situations in which σ ≤ 4, any enhancement of the radial magnetic field from Rayleigh-Taylor instabilities will also extend all the way to the shock front, and this may help explain the slight dominance of radial fields long seen in polarization measurements of young remnants like Tycho.}, number={1}, journal={ASTROPHYSICAL JOURNAL}, author={Blondin, JM and Ellison, DC}, year={2001}, month={Oct}, pages={244–253} }
 @misc{drury_ellison_aharonian_berezhko_bykov_decourchelle_diehl_meynet_parizot_raymond_et al._2001, title={Test of Galactic Cosmic-Ray source models - Working group report}, volume={99}, ISSN={["1572-9672"]}, DOI={10.1023/A:1013825905795}, number={1-4}, journal={SPACE SCIENCE REVIEWS}, author={Drury, LO and Ellison, DE and Aharonian, FA and Berezhko, E and Bykov, A and Decourchelle, A and Diehl, R and Meynet, G and Parizot, E and Raymond, J and et al.}, year={2001}, month={Oct}, pages={329–352} }
 @misc{decourchelle_ellison_2001, title={X-ray emission from SNRS undergoing efficient shock acceleration}, volume={99}, ISSN={["0038-6308"]}, DOI={10.1023/A:1013801200343}, number={1-4}, journal={SPACE SCIENCE REVIEWS}, author={Decourchelle, A and Ellison, DC}, year={2001}, month={Oct}, pages={219–228} }
 @article{drury_ellisson_meyer_2000, title={Interpreting the cosmic ray composition}, volume={663}, ISSN={["0375-9474"]}, DOI={10.1016/S0375-9474(99)00728-9}, abstractNote={Detailed composition measurements can be a very powerful means of tracing origins, a fact used regularly by forensic scientists and art historians. One of the main motivating factors for making detailed observations of cosmic rays was always the hope that a unique compositional signature could be found which pointed unambiguously to a particular source. This has proven much harder than expected, but we have now reached a point where it appears possible to begin to decipher the information contained in the compositional data; the key, we have discovered, is to read the data not in isolation, but in the context provided by our general astronomical knowledge and by recent developments in shock acceleration theory (Meyer, Drury and Ellison, 1997, 1998; Ellison, Drury and Meyer, 1997). In our view (not, it is only fair to warn the reader, yet universally accepted) the data show clearly that the Galactic cosmic ray particles originate predominantly from the gas and dust of the general interstellar medium.}, journal={NUCLEAR PHYSICS A}, author={Drury, LO and Ellisson, DC and Meyer, JP}, year={2000}, month={Jan}, pages={843C–846C} }
 @article{baring_jones_ellison_2000, title={Inverse bremsstrahlung in shocked astrophysical plasmas}, volume={528}, ISSN={["0004-637X"]}, DOI={10.1086/308194}, abstractNote={There has recently been interest in the role of inverse bremsstrahlung, the emission of photons by fast suprathermal ions in collisions with ambient electrons possessing relatively low velocities, in tenuous plasmas in various astrophysical contexts. This follows a long hiatus in the application of suprathermal ion bremsstrahlung to astrophysical models since the early 1970s. The potential importance of inverse bremsstrahlung relative to normal bremsstrahlung, i.e., where ions are at rest, hinges upon the underlying velocity distributions of the interacting species. In this paper, we identify the conditions under which the inverse bremsstrahlung emissivity is significant relative to that for normal bremsstrahlung in shocked astrophysical plasmas. We determine that, since both observational and theoretical evidence favors electron temperatures almost comparable to, and certainly not very deficient relative to, proton temperatures in shocked plasmas, these environments generally render inverse bremsstrahlung at best a minor contributor to the overall emission. Hence, inverse bremsstrahlung can be safely neglected in most models invoking shock acceleration in discrete sources such as supernova remnants. However, on scales ≳100 pc distant from these sources, Coulomb collisional losses can deplete the cosmic-ray electrons, rendering inverse bremsstrahlung, and perhaps bremsstrahlung from knock-on electrons, possibly detectable.}, number={2}, journal={ASTROPHYSICAL JOURNAL}, author={Baring, MG and Jones, FC and Ellison, DC}, year={2000}, month={Jan}, pages={776–788} }
 @article{ellison_berezhko_baring_2000, title={Nonlinear shock acceleration and photon emission in supernova remnants}, volume={540}, ISSN={["0004-637X"]}, DOI={10.1086/309324}, abstractNote={We have extended a simple model of nonlinear diffusive shock acceleration to include the injection and acceleration of electrons and the production of photons from bremsstrahlung, synchrotron, inverse-Compton, and pion-decay processes. We argue that the results of this model, which is simpler to use than more elaborate ones, offer a significant improvement over test-particle, power-law spectra which are often used in astrophysical applications of diffusive shock acceleration. With an evolutionary supernova remnant (SNR) model to obtain shock parameters as functions of ambient interstellar medium parameters and time, we predict broadband continuum photon emission from supernova remnants in general, and SN 1006 in particular, showing that our results compare well with the more complete time-dependent and spherically symmetric nonlinear model of Berezhko, Ksenofontov, & Petukhov. We discuss the implications nonlinear shock acceleration has for X-ray line emission and use our model to describe how ambient conditions determine the TeV/radio flux ratio, an important parameter for γ-ray observations of radio SNRs.}, number={1}, journal={ASTROPHYSICAL JOURNAL}, author={Ellison, DC and Berezhko, EG and Baring, MG}, year={2000}, month={Sep}, pages={292–307} }
 @article{bykov_chevalier_ellison_uvarov_2000, title={Nonthermal emission from a supernova remnant in a molecular cloud}, volume={538}, ISSN={["0004-637X"]}, DOI={10.1086/309103}, abstractNote={In evolved supernova remnants (SNRs) interacting with molecular clouds, a highly inhomogeneous structure consisting of a forward shock of moderate Mach number, a cooling layer, a dense radiative shell and an interior region filled with hot tenuous plasma is expected. We present a model of nonthermal electron injection, acceleration and propagation in that environment and find that these SNRs are efficient electron accelerators and sources of hard X- and γ -ray emission. A forward shock of velocity v s > 100 km s - 1 with an ionized precursor propagating into the molecular cloud accompanied by magnetohydrodynamic turbulence provides a spatially inhomogeneous distribution of nonthermal electrons. The energy spectrum of the nonthermal electrons is shaped by the joint action of first and second order Fermi acceleration in a turbulent plasma with substantial Coulomb losses. Bremsstrahlung, synchrotron, and inverse Compton radiation of the nonthermal electrons produce multiwavelength photon spectra in quantitative agreement with the radio and the hard emission observed by ASCA and EGRET from IC 443. We distinguish interclump shock wave emission from molecular clump shock wave emission; particles reach higher energies in the interclump shock and that is the likely source of γ -ray emission and radio synchrotron emission. Spatially resolved X- and γ -ray spectra from the supernova remnants IC 443, W44, and 3C391 as might be observed with BeppoSAX, Chandra XRO, XMM, INTEGRAL and GLAST would distinguish the contribution of the energetic lepton component to the γ -rays observed by EGRET , constraining the cosmic ray nuclear component spectra in these SNRs. These data would provide a valuable tool for studying the complex structure of molecular clouds where SNR radiative shocks interact with dense molecular clumps.}, number={1}, journal={ASTROPHYSICAL JOURNAL}, author={Bykov, AM and Chevalier, RA and Ellison, DC and Uvarov, YA}, year={2000}, month={Jul}, pages={203–216} }
 @article{decourchelle_ellison_ballet_2000, title={Thermal X-ray emission and cosmic-ray production in young supernova remnants}, volume={543}, ISSN={["0004-637X"]}, DOI={10.1086/318167}, abstractNote={We have developed a simple model to investigate the modifications of the hydrodynamics and nonequilibrium ionization X-ray emission in young supernova remnants due to nonlinear particle acceleration. In nonlinear, diffusive shock acceleration, the heating of the gas to X-ray-emitting temperatures is strongly coupled to the acceleration of cosmic-ray ions. If the acceleration is efficient and a significant fraction of the shock ram energy ends up in cosmic rays, compression ratios will be higher and the shocked temperature lower than test particle, Rankine-Hugoniot relations predict. We illustrate how particle acceleration impacts the interpretation of X-ray data using the X-ray spectra of Kepler's remnant, observed by ASCA and the Rossi X-Ray Timing Explorer. The thermal X-ray emission provides important constraints on the efficiency of particle acceleration, in complement to nonthermal emission. X-ray data from Chandra and XMM-Newton, plus radio observations, will be essential to quantify nonlinear effects.}, number={1}, journal={ASTROPHYSICAL JOURNAL}, author={Decourchelle, A and Ellison, DC and Ballet, J}, year={2000}, month={Nov}, pages={L57–L60} }
 @article{giacalone_ellison_2000, title={Three-dimensional numerical simulations of particle injection and acceleration at quasi-perpendicular shocks}, volume={105}, ISSN={["2169-9402"]}, DOI={10.1029/1999ja000018}, abstractNote={We present results from three‐dimensional numerical simulations of quasi‐perpendicular collisionless shocks in order to determine whether cross‐field diffusion, which is otherwise artificially suppressed in simulations which contain at least one ignorable spatial coordinate, is large enough to efficiently accelerate thermal particles and/or pickup ions. We find that although the simulated downstream distribution functions are quite steep, a fraction of the particles are accelerated to energies well above the thermal energy. This occurs only if the system contains fluctuations with wavelengths that are considerably larger than the gyroradii of the particles of interest (the high‐energy ones), We find that this is due to the fact that the transport of the particles normal to the shock, against the downstream convection, is mostly influenced by the meandering of field lines on large scales. We also show that if the system does not contain these long‐wavelength waves, the scattering is not sufficient to accelerate thermal particles or pickup ions. We use two different types of simulations to demonstrate this. In the first set of simulations we use the well‐known hybrid simulation which treats the interaction between the particles and fields self‐consistently. For computational tractability these simulations use very small spatial domains, and the effect of the large‐scale field‐line random walk is suppressed. However, this approximation accurately addresses the physics of the scattering at resonant wavelengths. Test‐particle simulations, which are more computationally tractable than the hybrid simulations, are also performed for larger systems and to illustrate the effect of the long‐wavelength waves.}, number={A6}, journal={JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS}, author={Giacalone, J and Ellison, DC}, year={2000}, month={Jun}, pages={12541–12556} }
 @article{berezhko_ellison_1999, title={A simple model of nonlinear diffusive shock acceleration}, volume={526}, ISSN={["1538-4357"]}, DOI={10.1086/307993}, abstractNote={We present a simple model of nonlinear diffusive shock acceleration (also called first-order Fermi shock acceleration) that determines the shock modification, spectrum, and efficiency of the process in the plane-wave, steady state approximation as a function of an arbitrary injection parameter, η. The model, which uses a three-power-law form for the accelerated particle spectrum and contains only simple algebraic equations, includes the essential elements of the full nonlinear model and has been tested against Monte Carlo and numerical kinetic shock models. We include both adiabatic and Alfvén wave heating of the upstream precursor. The simplicity and ease of calculation make this model useful for studying the basic properties of nonlinear shock acceleration, as well as providing results accurate enough for many astrophysical applications. It is shown that the shock properties depend upon the shock speed u0 with respect to a critical value u ∝ ηp, which is a function of the injection rate η and maximum accelerated particle momentum pmax. For u0 < u, acceleration is efficient and the shock is strongly modified by the back pressure of the energetic particles. In this case, the overall compression ratio is given by rtot ≈ 1.3M if M > MA0, or by rtot ≈ 1.5M in the opposite case (MS0 is the sonic Mach number and MA0 is the Alfvén Mach number). If u0 > u, the shock, although still strong, becomes almost unmodified and accelerated particle production decreases inversely proportional to u0.}, number={1}, journal={ASTROPHYSICAL JOURNAL}, author={Berezhko, EG and Ellison, DC}, year={1999}, month={Nov}, pages={385–399} }
 @article{ellison_jones_baring_1999, title={Direct acceleration of pickup ions at the solar wind termination shock: The production of anomalous cosmic rays}, volume={512}, ISSN={["0004-637X"]}, DOI={10.1086/306739}, abstractNote={We have modeled the injection and acceleration of pickup ions at the solar wind termination shock and investigated the parameters needed to produce the observed anomalous cosmic-ray (ACR) fluxes. A nonlinear Monte Carlo technique was employed that in effect solves the Boltzmann equation and is not restricted to near-isotropic particle distribution functions. This technique models the injection of thermal and pickup ions, the acceleration of these ions, and the determination of the shock structure under the influence of the accelerated ions. The essential effects of injection are treated in a mostly self-consistent manner, including effects from shock obliquity, cross-field diffusion, and pitch-angle scattering. Using recent determinations of pickup ion densities, we are able to match the absolute flux of hydrogen in the ACRs by assuming that pickup ion scattering mean free paths, at the termination shock, are much less than an AU and that modestly strong cross-field diffusion occurs. Simultaneously, we match the flux ratios He+/H+ or O+/H+ to within a factor ~5. If the conditions of strong scattering apply, no pre-termination-shock injection phase is required and the injection and acceleration of pickup ions at the termination shock are totally analogous to the injection and acceleration of ions at highly oblique interplanetary shocks recently observed by the Ulysses spacecraft. The fact that ACR fluxes can be modeled with standard shock assumptions suggests that the much discussed "injection problem" for highly oblique shocks stems from incomplete (either mathematical or computer) modeling of these shocks rather than from any actual difficulty shocks may have in injecting and accelerating thermal or quasi-thermal particles.}, number={1}, journal={ASTROPHYSICAL JOURNAL}, author={Ellison, DC and Jones, FC and Baring, MG}, year={1999}, month={Feb}, pages={403–416} }
 @article{baring_ellison_reynolds_grenier_goret_1999, title={Radio to gamma-ray emission from shell-type supernova remnants: Predictions from nonlinear shock acceleration models}, volume={513}, ISSN={["1538-4357"]}, DOI={10.1086/306829}, abstractNote={Supernova remnants (SNRs) are widely believed to be the principal source of Galactic cosmic rays, produced by diffusive shock acceleration in the environs of the remnant's expanding blast wave. Such energetic particles can produce gamma rays and lower energy photons via interactions with the ambient plasma. The recently reported observation of TeV gamma rays from SN 1006 by the Collaboration of Australia and Nippon for a Gamma-Ray Observatory in the Outback (CANGAROO), combined with the fact that several unidentified EGRET sources have been associated with known radio/optical/X-ray-emitting remnants, provides powerful motivation for studying gamma-ray emission from SNRs. In this paper, we present results from a Monte Carlo simulation of nonlinear shock structure and acceleration coupled with photon emission in shelllike SNRs. These nonlinearities are a by-product of the dynamical influence of the accelerated cosmic rays on the shocked plasma and result in distributions of cosmic rays that deviate from pure power laws. Such deviations are crucial to acceleration efficiency considerations and impact photon intensities and spectral shapes at all energies, producing GeV/TeV intensity ratios that are quite different from test particle predictions. The Sedov scaling solution for SNR expansions is used to estimate important shock parameters for input into the Monte Carlo simulation. We calculate ion (proton and helium) and electron distributions that spawn neutral pion decay, bremsstrahlung, inverse Compton, and synchrotron emission, yielding complete photon spectra from radio frequencies to gamma-ray energies. The cessation of acceleration caused by the spatial and temporal limitations of the expanding SNR shell in moderately dense interstellar regions can yield spectral cutoffs in the TeV energy range that are consistent with Whipple's TeV upper limits on those EGRET unidentified sources that have SNR associations. Supernova remnants in lower density environments generate higher energy cosmic rays that produce predominantly inverse Compton emission observable at super-TeV energies, consistent with the SN 1006 detection. In general, sources in such low-density regions will be gamma-ray-dim at GeV energies.}, number={1}, journal={ASTROPHYSICAL JOURNAL}, author={Baring, MG and Ellison, DC and Reynolds, SP and Grenier, IA and Goret, P}, year={1999}, month={Mar}, pages={311–338} }
 @misc{meyer_drury_ellison_1998, title={A cosmic-ray composition controlled by volatility and A/Q ratio. SNR shock acceleration of gas and dust}, volume={86}, ISSN={["0038-6308"]}, DOI={10.1023/A:1005081721214}, number={1-4}, journal={SPACE SCIENCE REVIEWS}, author={Meyer, JP and Drury, LO and Ellison, DC}, year={1998}, pages={179–201} }
 @misc{klecker_mewaldt_bieber_cummings_drury_giacalone_jokipii_jones_krainev_lee_et al._1998, title={Anomalous cosmic rays - Report of working group 3}, volume={83}, ISSN={["1572-9672"]}, DOI={10.1023/A:1005031108919}, number={1-2}, journal={SPACE SCIENCE REVIEWS}, author={Klecker, B and Mewaldt, RA and Bieber, JW and Cummings, AC and Drury, L and Giacalone, J and Jokipii, JR and Jones, FC and Krainev, MB and Lee, MA and et al.}, year={1998}, month={Jan}, pages={259–308} }
 @misc{ellison_drury_meyer_1998, title={Cosmic rays from supernova remnants: A brief description of the shock acceleration of gas and dust}, volume={86}, ISSN={["0038-6308"]}, DOI={10.1023/A:1005019800296}, number={1-4}, journal={SPACE SCIENCE REVIEWS}, author={Ellison, DC and Drury, LO and Meyer, JP}, year={1998}, pages={203–224} }
 @article{meyer_drury_ellison_1997, title={Galactic cosmic rays from supernova remnants .1. A cosmic-ray composition controlled by volatility and mass-to-charge ratio}, volume={487}, ISSN={["1538-4357"]}, DOI={10.1086/304599}, abstractNote={We show that the Galactic cosmic-ray source (GCRS) composition is best described in terms of (1) a general enhancement of the refractory elements relative to the volatile ones, and (2) among the volatile elements, an enhancement of the heavier elements relative to the lighter ones. This mass dependence most likely reflects a mass-to-charge (A/Q) dependence of the acceleration efficiency; among the refractory elements, there is no such enhancement of heavier species, or only a much weaker one. We regard as coincidental the similarity between the GCRS composition and that of the solar corona, which is biased according to first ionization potential. In a companion paper, this GCRS composition is interpreted in terms of an acceleration by supernova shock waves of interstellar and/or circumstellar (e.g.,22Ne-rich Wolf-Rayet wind) gas-phase and, especially, dust material.}, number={1}, journal={ASTROPHYSICAL JOURNAL}, author={Meyer, JP and Drury, LOC and Ellison, DC}, year={1997}, month={Sep}, pages={182–196} }
 @article{ellison_drury_meyer_1997, title={Galactic cosmic rays from supernova remnants .2. Shock acceleration of gas and dust}, volume={487}, ISSN={["1538-4357"]}, DOI={10.1086/304580}, abstractNote={We present a quantitative model of Galactic cosmic-ray (GCR) origin and acceleration, wherein a mixture of interstellar and/or circumstellar gas and dust is accelerated by a supernova remnant blast wave. The gas and dust are accelerated simultaneously, but differences in how each component is treated by the shock leave a distinctive signature, which we believe exists in the cosmic-ray composition data. A reexamination of the detailed GCR elemental composition, presented in a companion paper, has led us to abandon the long-held assumption that GCR abundances are somehow determined by first ionization potential. Instead, volatility and mass (presumably mass-to-charge ratio) seem to better organize the data: among the volatile elements, the abundance enhancements relative to solar increase with mass (except for the slightly high H/He ratio); the more refractory elements seem systematically overabundant relative to the more volatile ones in a quasi-mass-independent fashion. If this is the case, material locked in grains in the interstellar medium must be accelerated to cosmic-ray energies more efficiently than interstellar gas-phase ions. Here we present results from a nonlinear shock model that includes (1) the direct acceleration of interstellar gas-phase ions, (2) a simplified model for the direct acceleration of weakly charged grains to ~100 keV amu-1 energies, simultaneously with the acceleration of the gas ions, (3) the energy losses of grains colliding with the ambient gas, (4) the sputtering of grains, and (5) the simultaneous acceleration of the sputtered ions to GeV and TeV energies. We show that the model produces GCR source abundance enhancements of the volatile, gas-phase elements that are an increasing function of mass, as well as a net, mass-independent enhancement of the refractory, grain elements over protons, consistent with cosmic-ray observations. We also investigate the implications of the slightly high H/He ratio. The GCR22Ne excess may also be accounted for in terms of the acceleration of 22Ne-enriched presupernova Wolf-Rayet star wind material surrounding the most massive supernovae. We also show that cosmic-ray source spectra, at least below ~1014 eV, are well matched by the model.}, number={1}, journal={ASTROPHYSICAL JOURNAL}, author={Ellison, DC and Drury, LOC and Meyer, JP}, year={1997}, month={Sep}, pages={197–217} }
 @article{giacalone_burgess_schwartz_ellison_bennett_1997, title={Injection and acceleration of thermal protons at quasi-parallel shocks: a hybrid simulation parameter survey}, volume={102}, DOI={10.1029/97JA01529}, abstractNote={Collisionless shocks that propagate along the mean magnetic field are known to accelerate some fraction of the incident charged particles directly from the thermal pool to energies that are considerably higher than the energy at which the plasma rams into the shock. Using hybrid simulations, we address two issues: (1) the dependence of the injection/acceleration of thermal protons to energies much higher than the plasma ram energy on various shock parameters such as Mach number, plasma beta, etc., and (2) the effect of the high‐energy particles, accelerated directly from the thermal population by the shock, on the macroscopic properties of the shock, most notably, on the density compression. We find that for supercritical Mach numbers the acceleration of the thermal plasma is efficient enough that the back pressure due to the energetic particles can significantly increase the density compression across the shock, above the value expected from the simple Rankine‐Hugoniot prediction. Additionally, at low Alfvén Mach number, where the acceleration of the thermal plasma is inefficient, the density compression is smaller than the simple Rankine‐Hugoniot prediction owing to the nonresonant fire hose instability. The acceleration efficiency increases with Mach number except at very high Alfvén Mach numbers, where it begins to decrease for Mach numbers greater than ∼ 10. This is due to the presence of a fixed, free‐escape boundary that limits the size of the foreshock region measured in units of the mean‐free paths of the accelerated particles. Additionally, we find that regardless of the upstream plasma parameters, the acceleration efficiency increases with both the density compression ratio across the shock and the distance to the free‐escape boundary measured in units of the mean‐free path of the energetic particles. Both of these are consistent with analytic theory and numerical models that use a phenomenological scattering law.}, number={A9}, journal={Journal of Geophysical Research. Space Physics}, author={Giacalone, J. and Burgess, D. and Schwartz, S. J. and Ellison, D. C. and Bennett, L.}, year={1997}, pages={19789–19804} }
 @misc{jones_ellison_1991, title={THE PLASMA PHYSICS OF SHOCK ACCELERATION}, volume={58}, ISSN={["0038-6308"]}, DOI={10.1007/BF01206003}, number={3-4}, journal={SPACE SCIENCE REVIEWS}, author={JONES, FC and ELLISON, DC}, year={1991}, month={Nov}, pages={259–346} }