Combined X-ray and optical analysis to probe the origin of the plateau emission in $\gamma$-ray bursts afterglows

TL;DR

This study analyzes simultaneous X-ray and optical data from 30 gamma-ray bursts to investigate the origin of the plateau phase, finding that most are consistent with a single synchrotron emission region, while some require multiple processes.

Contribution

It provides the first systematic time-resolved spectral analysis combining X-ray and optical data during GRB plateaus, testing emission models and identifying cases needing multiple emission zones.

Findings

63% of GRBs are consistent with a single synchrotron spectrum.

Some GRBs show optical emission exceeding X-ray extrapolations, indicating multiple emission processes.

Results support models involving energy injection and structured jets for plateau emission.

Abstract

A large fraction of gamma-ray bursts (GRBs) shows a plateau phase during the X-ray afterglow emission, whose physical origin is still debated. In this work we define a sample of 30 GRBs with simultaneous X-ray and optical data during and after the plateau phase. Through a time-resolved spectral analysis of the X-ray plateaus, we test the consistency of the unabsorbed optical fluxes with those obtained via X-ray-to-optical spectral extrapolation by assuming a synchrotron spectrum. Combining X-ray with optical data, we find that 63% (19/30) GRBs are compatible with a single synchrotron spectrum thus suggesting that both the optical and X-ray radiations are produced from a single emitting region. For these GRBs we derive the temporal evolution of the break frequency and we compare it with the expectations predicted by several models. For 11/30 GRBs the optical emission is above the predicted range of values extrapolated from the X-rays in at least one temporal bin of the light curve. These GRBs may not be explained with a single zone emission, indicating the necessity of invoking two cooperating processes in order to explain the broad band spectral behaviour during X-ray plateaus. We discuss our findings in the framework of different scenarios invoked to explain the plateau feature, including the energy injection from a spinning-down magnetar and the high latitude emission from a structured jet.