The Photosphere Emission Spectrum of Hybrid Relativistic Outflow for Gamma-ray Bursts

TL;DR

This paper investigates the photospheric emission spectrum of hybrid relativistic outflows in gamma-ray bursts, showing that such models can reproduce observed spectral features and distributions.

Contribution

It introduces a hybrid outflow model with moderate magnetization to explain GRB spectra, extending previous pure fireball or Poynting-flux models.

Findings

High-energy spectrum is a power law, matching observed Band functions.

Low-energy index distribution aligns with GRB peak-flux spectra.

High-energy index distribution explained by magnetic acceleration and jet structure.

Abstract

The photospheric emission in the prompt phase is the natural prediction of the original fireball model for gamma-ray burst (GRB) due to the large optical depth ($\tau >1$) at the base of the outflow, which is supported by the quasi-thermal components detected in several Fermi GRBs. However, which radiation mechanism (photosphere or synchrotron) dominates in most GRB spectra is still under hot debate. The shape of the observed photosphere spectrum from a pure hot fireball or a pure Poynting-flux-dominated outflow has been investigated before. In this work, we further study the photosphere spectrum from a hybrid outflow containing both a thermal component and a magnetic component with moderate magnetization ($\sigma_{0}=L_{P}/L_{\text{Th}}\sim 1-10$), by invoking the probability photosphere model. The high-energy spectrum from such a hybrid outflow is a power law rather than an exponential cutoff, which is compatible with the observed Band function in large amounts of GRBs. Also, the distribution of the low-energy indices (corresponding to the peak-flux spectra) is found to be quite consistent with the statistical result for the peak-flux spectra of GRBs best-fitted by the Band function, with similar angular profiles of structured jet in our previous works. Finally, the observed distribution of the high-energy indices can be well understood after considering the different magnetic acceleration (due to magnetic reconnection and kink instability) and the angular profiles of dimensionless entropy with the narrower core.