Are Single-Zone Emission models Sufficient to Explain GRB 220426A and GRB 230812B?

TL;DR

This paper challenges single-zone emission models for GRBs by analyzing spectral width evolution in GRB 220426A and GRB 230812B, providing evidence for multiple emission zones during the prompt phase.

Contribution

It presents observational evidence that spectral width increases over time, questioning the adequacy of single-zone models and supporting multi-zone emission scenarios.

Findings

Spectral width $ ext{W}$ increases with time in the studied GRBs.

Single-zone models predict decreasing $ ext{W}$, contrary to observations.

Results suggest the prompt phase involves multiple evolving emission zones.

Abstract

Gamma-ray bursts (GRBs) are the universe's most energetic phenomena (isotropic luminosity $\sim 10^{51} - 10^{54}$ ergs/s) lasting for a very short duration ($\sim$ milliseconds - a few seconds). Even after an average of one GRB detected per day, their emission mechanism remains contentious. Inferences drawn from the empirical modelling of the GRB spectrum are often inconclusive. Some studies favor the emission from a thermal blast of hot plasma, while others suggest a synchrotron emission originating from a rapid acceleration of particles at the expense of the burst energy. Under these scenarios, the spectral width of the burst ($\mathcal{W}$), which is measured at half maxima, is expected to decrease with time. We show that for the GRB 220426A and GRB 230812B, $\mathcal{W}$ increases with time, raising serious concerns regarding the validity of these emission models. The results instead offer strong evidence that the GRB prompt phase involves the development of multiple emission zones, whose relative contributions change over time.