Adiabatic non-resonant acceleration in magnetic turbulence and hard spectra of gamma-ray bursts

TL;DR

This paper proposes a novel non-resonant acceleration mechanism in magnetic turbulence that explains the hard spectra observed in gamma-ray burst prompt emissions, linking turbulence processes to electron acceleration and synchrotron radiation.

Contribution

It introduces a new acceleration mechanism based on the second adiabatic invariant, combining Fermi processes to explain GRB spectra.

Findings

Accelerates electrons to a power-law distribution with index ~1.

Produces synchrotron emission extending to ~100 keV.

Explains the low-energy photon index ~-1 in GRB spectra.

Abstract

We introduce a non-resonant acceleration mechanism arising from the second adiabatic invariant in magnetic turbulence and apply it to study the prompt emission spectra of gamma-ray bursts (GRBs). The mechanism contains both the first- and second-order Fermi acceleration, originating from the interacting turbulent reconnection and dynamo processes. It leads to a hard electron energy distribution up to a cutoff energy at the balance between the acceleration and synchrotron cooling. The sufficient acceleration rate ensures a rapid hardening of any initial energy distribution to a power-law distribution with the index $p \sim 1$, which naturally produces a low-energy photon index $\alpha \sim -1$ via the synchrotron radiation. For typical GRB parameters, the synchrotron emission can extend to a characteristic photon energy on the order of $\sim 100$ keV.