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GRB 130925A is a peculiar event characterized by an extremely long gamma-rayduration ($\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 softspectrum with photon index $\Gamma$$\sim$4, as observed by Swift and Chandra,until $\approx 10^7$ s, when XMM-Newton observations uncover a harder spectralshape with $\Gamma$$\sim$2.5, commonly observed in GRB afterglows. We find thattwo distinct emission components are needed to explain the X-ray observations:a thermal component, which dominates the X-ray emission for several weeks, anda non-thermal component, consistent with a typical afterglow. A forward shockmodel well describes the broadband (from radio to X-rays) afterglow spectrum atvarious epochs. It requires an ambient medium with a very low density windprofile, consistent with that expected from a low-metallicity blue supergiant(BSG). The thermal component has a remarkably constant size and a total energyconsistent with those expected by a hot cocoon surrounding the relativisticjet. We argue that the features observed in this GRB (its ultralong duration,the thermal cocoon, and the low density wind environment) are associated with alow metallicity BSG progenitor and, thus, should characterize the class ofultralong GRBs.

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