GRB 241030A: a prompt thermal X-ray emission component and diverse origin of the very early UVOT WHITE and U band emission

TL;DR

This paper analyzes GRB 241030A, revealing a thermal photosphere component, variable early optical emission likely from external shocks, and providing insights into the diverse radiation mechanisms of gamma-ray bursts.

Contribution

It presents the first detailed joint analysis of prompt thermal emission and early optical variability in GRB 241030A, highlighting multiple radiation origins.

Findings

Detection of a thermal photosphere with a temperature of a few keV.

Rapid early optical rise consistent with external shock onset.

Simultaneous optical and X-ray data constrain emission mechanisms.

Abstract

We present a detailed analysis of the long-duration GRB 241030A detected by {\it Swift}. Thanks to the rapid response of XRT and UVOT, the strongest part of the prompt emission of GRB 241030A has been well measured simultaneously from optical to hard X-ray band. The time-resolved WHITE band emission shows strong variability, largely tracing the activity of the prompt gamma-ray emission, may be produced by internal shocks too. The joint analysis of the XRT and BAT data reveals the presence of a thermal component with a temperature of a few keV, which can be interpreted as the photosphere radiation, and the upper limit of the Lorentz factor of this region is found to range between approximately 20 and 80. The time-resolved analysis of the initial U-band exposure data yields a very rapid rise ($ \sim t^{5.3}$) with a bright peak reaching 13.6 AB magnitude around 410 seconds, which is most likely attributed to the onset of the external shock emission. The richness and fineness of early observational data have made this burst a unique sample for studying the various radiation mechanisms of gamma-ray bursts.