One Fits All: A Unified Synchrotron Model Explains GRBs with FRED-shape Pulses

Abstract

The analysis of gamma-ray burst (GRB) spectra often relies on empirical models lacking a distinct physical explanation. Previous attempts to couple physical models with observed data focus on individual burst studies, fitting models to segmented spectra with independent physical parameters. However, these approaches typically neglect to explain the time evolution of observed spectra. In this study, we propose a novel approach by incorporating the synchrotron radiation model to provide a self-consistent explanation for a selection of single-pulse GRBs. Our study comprehensively tests the synchrotron model under a unified physical condition, such as a single injection event of electrons. By tracing the evolution of cooling electrons in a decaying magnetic field, our model predicts time-dependent observed spectra that align well with the data. Using a single set of physical parameters, our model successfully fits all time-resolved spectra within each burst. Our model suggests that the rising phase of the GRB light curve results from the increasing number of radiating electrons, while the declining phase is attributed to the curvature effect, electron cooling, and the decaying magnetic field. Our model provides a straightforward interpretation of the peak energy’s evolution, linked to the decline of the magnetic field and electron cooling due to the expansion of the GRB emission region. Our findings strongly support the notion that spectral and temporal evolution in GRB pulses originates from the expansion of the GRB emission region, with an initial radius of approximately 1015 cm, and synchrotron radiation as the underlying emission mechanism.

Significant Cocoon Emission and Photosphere Duration Stretching in GRB 211211A: A Burst from a Neutron Star−Black Hole Merger

“…After more than 50 yr since the discovery, the radiation mechanism of gamma-ray burst (GRB) prompt emission (photosphere emission or synchrotron emission) is still under debate (Mészáros & Rees 2000;Pe'er 2008;Zhang & Yan 2011;Uhm & Zhang 2014;Zhang et al 2018a;Geng et al 2018;Ito et al 2019;Burgess et al 2020;Zhang 2020;Liu et al 2022;Yang et al 2023;Yan et al 2024). The existence of the photospheric emission is the basic prediction of the classical fireball model (Goodman 1986;Paczynski 1986) for GRB, because the optical depth τ at the jet base is much larger than unity (Piran 1999).…”