@article{ellison_warren_bykov_2013, title={MONTE CARLO SIMULATIONS OF NONLINEAR PARTICLE ACCELERATION IN PARALLEL TRANS-RELATIVISTIC SHOCKS}, volume={776}, ISSN={["0004-637X"]}, DOI={10.1088/0004-637x/776/1/46}, abstractNote={We present results from a Monte Carlo simulation of a parallel collisionless shock undergoing particle acceleration. Our simulation, which contains parameterized scattering and a particular thermal leakage injection model, calculates the feedback between accelerated particles ahead of the shock, which influence the shock precursor and “smooth” the shock, and thermal particle injection. We show that there is a transition between nonrelativistic shocks, where the acceleration efficiency can be extremely high and the nonlinear compression ratio can be substantially greater than the Rankine–Hugoniot value, and fully relativistic shocks, where diffusive shock acceleration is less efficient and the compression ratio remains at the Rankine–Hugoniot value. This transition occurs in the trans-relativistic regime and, for the particular parameters we use, occurs around a shock Lorentz factor γ0 = 1.5. We also find that nonlinear shock smoothing dramatically reduces the acceleration efficiency presumed to occur with large-angle scattering in ultra-relativistic shocks. Our ability to seamlessly treat the transition from ultra-relativistic to trans-relativistic to nonrelativistic shocks may be important for evolving relativistic systems, such as gamma-ray bursts and Type Ibc supernovae. We expect a substantial evolution of shock accelerated spectra during this transition from soft early on to much harder when the blast-wave shock becomes nonrelativistic.}, number={1}, journal={ASTROPHYSICAL JOURNAL}, author={Ellison, Donald C. and Warren, Donald C. and Bykov, Andrei M.}, year={2013}, month={Oct} }
 @article{warren_blondin_2013, title={Three-dimensional numerical investigations of the morphology of Type Ia SNRs}, volume={429}, ISSN={1365-2966 0035-8711}, url={http://dx.doi.org/10.1093/mnras/sts566}, DOI={10.1093/mnras/sts566}, abstractNote={We explore the morphology of Type Ia supernova remnants (SNRs) using threedimensional hydrodynamics modeling and an exponential density profile. Our model distinguishes ejecta from the interstellar medium (ISM), and tracks the ionization age of shocked ejecta, both of which allow for additional analysis of the simulated remnants. We also include the adiabatic index as a free parameter, which aects the compressibility of the fluid and emulates the eciency of cosmic ray acceleration by shock fronts. In addition to generating 3-D images of the simulations, we compute line-of-sight projections through the remnants for comparison against observations of Tycho’s SNR and SN 1006. We find that several features observed in these two remnants, such as the separation between the fluid discontinuities and the presence of ejecta knots ahead of the forward shock, can be generated by smooth ejecta without any initial clumpiness. Our results are consistent with SN 1006 being dynamically younger than Tycho’s SNR, and more eciently accelerating cosmic rays at its forward shock. We conclude that clumpiness is not a necessary condition to reproduce many observed features of Type Ia supernova remnants, particularly the radial profiles and the fleecy emission from ejecta at the central region of both remnants.}, number={4}, journal={Monthly Notices of the Royal Astronomical Society}, publisher={Oxford University Press (OUP)}, author={Warren, Donald C. and Blondin, John M.}, year={2013}, month={Jan}, pages={3099–3113} }