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

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.