A Hot Cocoon in the Ultralong GRB 130925A: Hints of a PopIII-like Progenitor in a Low-Density Wind Environment
Publication date: 11 July 2014
Authors: Piro, L., et al.
Journal: The Astrophysical Journal Letters
Copyright: IOP PublishingGRB 130925A is a peculiar event characterized by an extremely long gamma-ray duration (~7 ks), as well as dramatic flaring in the X-rays for ~20 ks. After this period, its X-ray afterglow shows an atypical soft spectrum with photon index Γ ~ 4, as observed by Swift and Chandra, until ~107 s, when XMM-Newton observations uncover a harder spectral shape with Γ ~ 2.5, commonly observed in gamma-ray burst (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.