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Title: 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 (≈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 ≈10{sup 7} 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 ofmore » ultralong GRBs.« less
Authors:
 [1] ; ;  [2] ;  [3] ;  [4] ;  [5] ;  [6] ; ;  [7] ;  [8]
  1. INAF-Istituto Astrofisica e Planetologia Spaziali, Via Fosso Cavaliere 100, I-00133 Rome (Italy)
  2. NASA/GSFC, Greenbelt, MD 20771 (United States)
  3. ARTEMIS, UMR 7250, Boulevard de l'Observatoire, F-06304 Nice, Cedex 4 (France)
  4. INAF-Osservatorio Astronomico di Brera, via E. Bianchi 46, I-23807 Merate (Italy)
  5. INAF-Istituto di Radioastronomia, Via Gobetti 101, I-40129 Bologna (Italy)
  6. CSIRO Astronomy and Space Science, Marsfield, NSW 2122 (Australia)
  7. INAF-Osservatorio Astronomico di Roma, via Frascati 33, I-00040 Monteporzio Catone (Italy)
  8. CSIRO Astronomy and Space Science, Locked Bag 194, Narrabri, NSW 2390 (Australia)
Publication Date:
OSTI Identifier:
22365503
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal Letters; Journal Volume: 790; Journal Issue: 2; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; AFTERGLOW; COSMIC GAMMA BURSTS; DENSITY; EMISSION; GAMMA RADIATION; JETS; METALLICITY; PHOTON EMISSION; PHOTONS; RELATIVISTIC RANGE; STARS; STELLAR FLARES; X RADIATION