Photospheric emission from stratified jets

TL;DR

This paper investigates how stratified relativistic jets with velocity shear can produce high-energy photon spectra in gamma-ray bursts through a Fermi-like acceleration mechanism at the photosphere, explaining observed spectral features.

Contribution

It introduces a model of photospheric emission from stratified jets with velocity shear, demonstrating photon acceleration and spectral features consistent with GRB observations.

Findings

High-energy power-law tails can form in spectra due to photon acceleration at shear boundaries.

The model explains the extra hard component in some GRB spectra.

The empirical Ep-Lp relation can be reproduced by varying outflow properties.

Abstract

We explore photospheric emissions from stratified two-component jets, wherein a highly relativistic spine outflow is surrounded by a wider and less relativistic sheath outflow. Thermal photons are injected in regions of high optical depth and propagated until they escape at the photosphere. Due to the presence of shear in velocity (Lorentz factor) at the boundary of the spine and sheath region, a fraction of the injected photons are accelerated via a Fermi-like acceleration mechanism such that a high energy power-law tail is formed in the resultant spectrum. We show, in particular, that if a velocity shear with a considerable variance in the bulk Lorentz factor is present, the high energy part of observed Gamma-ray Bursts (GRBs) photon spectrum can be explained by this photon acceleration mechanism. We also show that the accelerated photons may also account for the origin of the extra hard power-law component above the bump of the thermal-like peak seen in some peculiar bursts (e.g., GRB 090510, 090902B, 090926A). It is demonstrated that time-integrated spectra can also reproduce the low energy spectrum of GRBs consistently due to a multi-temperature effect when time evolution of the outflow is considered. Finally, we show that the empirical Ep-Lp relation can be explained by differences in the outflow properties of individual sources.