Theory of photon scattering in shearing plasma: Applications to GRBs

TL;DR

This paper introduces a novel photon acceleration mechanism in shearing relativistic plasmas, explaining high-energy spectra in GRBs and AGNs through multiple scatterings that produce power-law photon distributions.

Contribution

It presents a new photon acceleration process in shearing plasma flows, supported by numerical and theoretical analysis, offering an alternative explanation for observed high-energy spectra in astrophysical sources.

Findings

High-energy photon spectra follow a power-law distribution.

The mechanism reproduces observed photon indices in GRBs.

Numerical simulations confirm the theoretical predictions.

Abstract

We explore a new mechanism for photon energy gain in a relativistic plasma with velocity shear. This process takes place in optically thick plasma and resembles conventional Fermi acceleration, where photons undergo multiple scatterings between regions with varying Lorentz factors, leading to an overall energy increase. The resulting high-energy spectra from the escaped photons exhibit a power-law form. The mechanism is an alternative to the classical radiation spectrum from power-law accelerated particles, which can produce power-law spectra in sources like Gamma-ray bursts (GRBs) and Active Galactic Nuclei (AGNs). By employing both numerical simulations and theoretical analysis, we calculate the expected spectra for GRBs, and show that they match the observed photon indices ($\beta$) at high energies.