Unstable GRB photospheres and electron-positron annihilation lines

TL;DR

This paper introduces a new emission mechanism for prompt gamma-ray bursts that explains their non-thermal spectra and predicts observable electron-positron annihilation lines, emphasizing the role of pair dynamics and instabilities.

Contribution

It proposes a novel model involving pair-driven instabilities and shock heating to explain GRB spectra and predicts observable annihilation lines, advancing understanding of GRB emission processes.

Findings

Non-thermal spectra with >50% radiative efficiency explained

Predicted electron-positron annihilation lines above continua

Pair instabilities occur near the progenitor star radius

Abstract

We propose an emission mechanism of prompt gamma-ray bursts (GRBs) that can reproduce the observed non-thermal spectra with high radiative efficiencies, >50%. Internal dissipation below a photosphere can create a radiation-dominated thermal fireball. If electron-positron pairs outnumber protons, radiative acceleration of pairs drives the two-stream instabilities between pairs and protons, leading to the ``proton sedimentation'' in the accelerating pair frame. Pairs are continuously shock heated by proton clumps, scattering the thermal photons into the broken power-law shape, with the non-thermal energy that is comparable to the proton kinetic energy, consistent with observations. Pair photospheres become unstable around the radius of the progenitor star where strong thermalization occurs, if parameters satisfy the observed spectral (Yonetoku) relation. Pair annihilation lines are predicted above continua, which could be verified by GLAST.