Astronomy and Astrophysics – Astrophysics
Scientific paper
Feb 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010head...11.4504s&link_type=abstract
American Astronomical Society, HEAD meeting #11, #45.04; Bulletin of the American Astronomical Society, Vol. 41, p.737
Astronomy and Astrophysics
Astrophysics
1
Scientific paper
The termination shock of pulsar winds and internal shocks in gamma-ray bursts and AGN jets are likely to be seeded with a substantial magnetic field (ratio of magnetic to kinetic energy density > few percent), oriented mostly perpendicular to the shock normal. By means of 2.5D particle-in-cell simulations, we study how the efficiency of particle acceleration in relativistic shocks depends on the magnetization "sigma" of the pre-shock flow and the inclination angle between the field and the shock normal, for mass ratios between 1 and 100. If sigma>0.03, we find that only "subluminal" shocks, where relativistic particles can escape ahead of the shock along the field, lead to particle acceleration. The downstream ion spectrum in such shocks consists of a relativistic Maxwellian and a high-energy power-law tail, which accounts for 3% of ions and 10% of flow energy. For electron-proton shocks (i.e., mass ratio > few tens), the number of supra-thermal electrons is typically smaller than ions by a factor of 5-10, since electrons are effectively more magnetized and get quickly advected downstream. For "superluminal" shocks, self-generated turbulence is not strong enough to overcome the kinematic constraints, and the downstream particle spectrum does not show any significant supra-thermal tail. As seen from the upstream frame, efficient acceleration in relativistic (Lorentz factor gamma>5) magnetized (sigma>0.03) flows exists only for nearly-parallel magnetic fields (obliquity<30/gamma deg). Since most of magnetized astrophysical shocks are quasi-perpendicular, our findings may place challenging constraints on the models of non-thermal radiation that invoke particle acceleration in relativistic magnetized shocks.
Sironi Lorenzo
Spitkovsky Anatoly
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