Kirk 2010 相对论激波的辐射机制

主要内容:
可能要分几种情况，包括加速和辐射。有时候是jitter有时候是同步辐射。

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Title:		Radiative Signatures of Relativistic Shocks
Authors:		Kirk, John G.; Reville, Brian
Affiliation:		AA(Max-Planck-Institut für Kernphysik, Postfach 10 39 80, 69029 Heidelberg, Germany john.kirk@mpi-hd.mpg.de), AB(Max-Planck-Institut für Kernphysik, Postfach 10 39 80, 69029 Heidelberg, Germany)
Publication:		The Astrophysical Journal Letters, Volume 710, Issue 1, pp. L16-L20 (2010). (ApJL Homepage)
Publication Date:		02/2010
Origin:		IOP
ApJ Keywords:		acceleration of particles, gamma-ray burst: general, radiation mechanisms: non-thermal
DOI:		10.1088/2041-8205/710/1/L16
Bibliographic Code:		2010ApJ...710L..16K

Abstract

Particle-in-cell simulations of relativistic, weakly magnetized collisionless shocks show that particles can gain energy by repeatedly crossing the shock front. This requires scattering off self-generated small length-scale magnetic fluctuations. The radiative signature of this first-order Fermi acceleration mechanism is important for models of both the prompt and afterglow emission in gamma-ray bursts and depends on the strength parameter a = λe|δB|/mc ² of the fluctuations (λ is the length scale and |δB| is the magnitude of the fluctuations). For electrons (and positrons), acceleration saturates when the radiative losses produced by the scattering cannot be compensated by the energy gained on crossing the shock. We show that this sets an upper limit on both the electron Lorentz factor \gamma \,<\,10^6 (n/1\,cm^{-3})^{-1/6}{\bar{\gamma }}^{1/6} and on the energy of the photons radiated during the scattering process \hbar \omega _max\,<\,40\,Max(a,1) (n/1\, cm^{-3})^{1/6}{\bar{\gamma }}^{-1/6}\,eV, where n is the number density of the plasma and {\bar{\gamma }} is the thermal Lorentz factor of the downstream plasma, provided a < a _crit ~ 10⁶. This rules out "jitter" radiation on self-excited fluctuations with a < 1 as a source of gamma rays, although high-energy photons might still be produced when the jitter photons are upscattered in an analog of the synchrotron self-Compton process. In fluctuations with a > 1, radiation is generated by the standard synchrotron mechanism, and the maximum photon energy rises linearly with a, until saturating at 70 MeV, when a = a _crit.

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