Synthetic Spectra from PIC Simulations of Relativistic Collisionless Shocks

TL;DR

This paper uses particle-in-cell simulations to generate synthetic photon spectra from relativistic collisionless shocks, confirming synchrotron radiation as the primary emission mechanism and exploring conditions for jitter radiation, with implications for astrophysical phenomena.

Contribution

First-principles PIC simulations produce self-consistent spectra, demonstrating synchrotron origin and conditions for jitter radiation in relativistic shocks, informing astrophysical emission models.

Findings

Spectrum consistent with synchrotron radiation

Jitter regime achievable under reduced magnetic fields

Constraints on non-thermal emission in Gamma-Ray Bursts

Abstract

We extract synthetic photon spectra from first-principles particle-in-cell simulations of relativistic shocks propagating in unmagnetized pair plasmas. The two basic ingredients for the radiation, namely accelerated particles and magnetic fields, are produced self-consistently as part of the shock evolution. We use the method of Hededal & Nordlund (2005) and compute the photon spectrum via Fourier transform of the electric far-field from a large number of particles, sampled directly from the simulation. We find that the spectrum from relativistic collisionless shocks is entirely consistent with synchrotron radiation in the magnetic fields generated by Weibel instability. We can recover the so-called "jitter'' regime only if we artificially reduce the strength of the electromagnetic fields, such that the wiggler parameter K = qB lambda/mc^2 becomes much smaller than unity ("B" and "lambda" are the strength and scale of the magnetic turbulence, respectively). These findings may place constraints on the origin of non-thermal emission in astrophysics, especially for the interpretation of the hard (harder than synchrotron) low-frequency spectrum of Gamma-Ray Bursts.