9/24: Lorenzo Sironi (CfA)
Non-Thermal Particle Acceleration in Relativistic Magnetized Astrophysical Flows
Dr. Lorenzo Sironi (CfA)
CAS 500, 12:00 pm, September 24, 2013
Most models of non-thermal radiation from Pulsar Wind Nebulae, gamma-ray bursts and AGN jets invoke particle acceleration in relativistic magnetized plasmas. By means of 2D and 3D particle-in-cell simulations, we study how the efficiency of particle acceleration in relativistic astrophysical flows depends on the plasma magnetization (ratio “sigma” of magnetic to kinetic energy density) and the geometry of the magnetic field. We investigate both uniform and alternating fields. For relativistic shocks propagating in a medium with uniform fields, we find that, if sigma>0.001, only configurations with nearly-parallel fields lead to particle acceleration. For quasi-perpendicular shocks, the level of self-generated turbulence is insufficient to give appreciable acceleration of particles. Weakly magnetized shocks with sigma<0.001 are efficient particle accelerators, with ~1% of particles and ~10% of energy stored in a power-law tail with slope around -2.5, whose upper energy cutoff grows as the square root of time. In the case of strongly magnetized flows (i.e., sigma>>1) with alternating fields, we find that field dissipation mediated by magnetic reconnection can efficiently transfer the energy from the fields to the particles, resulting in a flat power-law tail of non-thermal electrons with slope between -1.5 and -2. Our results can place important constraints on the origin of Ultra-High Energy Cosmic Rays, and provide physically-grounded inputs for astrophysical models of non-thermal radiation from Pulsar Wind Nebulae, gamma-ray bursts and AGN jets.