A hot cocoon in the ultralong GRB 130925A: hints of a PopIII-like progenitor in a low density wind environment

TL;DR

This paper analyzes the peculiar ultralong GRB 130925A, revealing a thermal cocoon and low-density environment indicative of a low-metallicity blue supergiant progenitor, suggesting a new class of ultralong gamma-ray bursts.

Contribution

It provides the first detailed multi-component model of an ultralong GRB, linking thermal cocoon features and low-density environments to a PopIII-like progenitor.

Findings

Presence of a thermal cocoon dominating early X-ray emission

Low-density wind environment consistent with a low-metallicity blue supergiant

Spectral evolution from soft to harder X-ray spectra over time

Abstract

GRB 130925A is a peculiar event characterized by an extremely long gamma-ray duration ($\approx$7 ks), as well as dramatic flaring in the X-rays for $\approx$20 ks. After this period, its X-ray afterglow shows an atypical soft spectrum with photon index $\Gamma$$\sim$4, as observed by Swift and Chandra, until $\approx 10^7$ s, when XMM-Newton observations uncover a harder spectral shape with $\Gamma$$\sim$2.5, commonly observed in GRB afterglows. We find that two distinct emission components are needed to explain the X-ray observations: a thermal component, which dominates the X-ray emission for several weeks, and a non-thermal component, consistent with a typical afterglow. A forward shock model well describes the broadband (from radio to X-rays) afterglow spectrum at various epochs. It requires an ambient medium with a very low density wind profile, consistent with that expected from a low-metallicity blue supergiant (BSG). The thermal component has a remarkably constant size and a total energy consistent with those expected by a hot cocoon surrounding the relativistic jet. We argue that the features observed in this GRB (its ultralong duration, the thermal cocoon, and the low density wind environment) are associated with a low metallicity BSG progenitor and, thus, should characterize the class of ultralong GRBs.