Fermi Gamma-ray Space Telescope

Particle acceleration in mildly relativistic shocks

Patrick Crumley
(D. Caprioli, S. Markoff, A. Spitkovsky)


Many astrophysical objects such as radio supernovae, microquasars, and tidal disruption events launch supersonic outflows that travel at mildly relativistic speeds (v~c and Lorentz factor ~ 1). Shocks inside these mildly relativistic outflows have an advantage in producing high energy particles because the acceleration time is short, but the shock is subluminal for a larger range of upstream magnetic field inclinations compared to ultra-relativistic shocks. We study particle acceleration in weakly magnetized mildly relativistic shocks using fully kinetic particle-in-cell simulations. We find that quasi-parallel shocks are mediated by a filamentary non-resonant (Bell) instability driven by non-thermal ions, producing magnetic fluctuations on scales comparable to the ion gyro-radius. In quasi-parallel shocks, both electrons and ions are accelerated into non-thermal power-laws whose maximum energy grows linearly with time. Reflected non-thermal ions evacuate upstream cavities and amplify magnetic fields. At late times, 10% of the shock’s energy goes into non-thermal protons, 10% into magnetic fields, but only .05% of the energy goes into non-thermal electrons in a Mach number 15 quasi-parallel shock traveling at .75c. Increasing the velocity of such a shock from non-relativistic to relativistic increases the energy fraction in non-thermal electrons to several percent, while the energy fraction of ions is roughly constant. Therefore, the radiative efficiency of a quasi-parallel shock increases as its shock speed increases. In quasi-perpendicular shocks, no non-thermal power-law develops in ions or electrons. These findings have implications when modeling radiation from outflows that transition from relativistic to non-relativistic like the afterglows of GRBs.