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 ? ~ 4, as observed by Swift and Chandra, until 107?s, when XMM-Newton observations uncover a harder spectral shape with ? ~ 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.

[1]  A. Burrows,et al.  Shock breakout in SN 1987A , 1992 .

[2]  E. Quataert,et al.  Swift 1644+57: The Longest Gamma-ray Burst? , 2011, 1105.3209.

[3]  S. E. Woosley,et al.  How Massive Single Stars End Their Life , 2003 .

[4]  T. Sakamoto,et al.  LONG-DURATION X-RAY FLASH AND X-RAY-RICH GAMMA-RAY BURSTS FROM LOW-MASS POPULATION III STARS , 2012, 1207.2835.

[5]  S. Woosley,et al.  EVOLUTION AND EXPLOSION OF MASSIVE STARS * , 1978, Reviews of Modern Physics.

[6]  P. Jakobsson,et al.  A NEW POPULATION OF ULTRA-LONG DURATION GAMMA-RAY BURSTS , 2013, 1302.2352.

[7]  M. Feroci,et al.  The BeppoSAX catalog of GRB X-ray afterglow observations , 2005, astro-ph/0507708.

[8]  Ramesh Narayan,et al.  Mass fall-back and accretion in the central engine of gamma-ray bursts , 2008, 0807.0441.

[9]  D. Nakauchi,et al.  LUMINOUS SUPERNOVA-LIKE UV/OPTICAL/INFRARED TRANSIENTS ASSOCIATED WITH ULTRA-LONG GAMMA-RAY BURSTS FROM METAL-POOR BLUE SUPERGIANTS , 2012, 1212.6431.

[10]  D. Lazzati,et al.  Universal GRB Jets from Jet-Cocoon Interaction in Massive Stars , 2005, astro-ph/0502084.

[11]  C. Kouveliotou,et al.  Identification of two classes of gamma-ray bursts , 1993 .

[12]  D. Malesani,et al.  GRB 130925A: VLT/UVES observations. , 2013 .

[13]  J. P. Osborne,et al.  Methods and results of an automatic analysis of a complete sample of Swift-XRT observations of GRBs , 2008, 0812.3662.

[14]  Re'em Sari,et al.  The Shape of Spectral Breaks in Gamma-Ray Burst Afterglows , 2001 .

[15]  R. Starling,et al.  A search for thermal X-ray signatures in gamma-ray bursts - I. Swift bursts with optical supernovae , 2012, 1207.1444.

[16]  E. Mazets,et al.  Konus-wind observation of GRB 100625A. , 2010 .

[17]  B. Paczyński,et al.  The appearance of highly relativistic, spherically symmetric stellar winds , 1991 .

[18]  K. Cook,et al.  Line-driven winds , ionizing fluxes and UV-spectra of hot stars at extremely low metallicity . I . Very massive O-stars , 2003 .

[19]  Lifan Wang,et al.  The supernova-gamma-ray burst connection , 1998, astro-ph/9806212.

[20]  Michael L. Norman,et al.  The Formation of the First Star in the Universe , 2001, Science.

[21]  R. Starling,et al.  A search for thermal X‐ray signatures in gamma‐ray bursts – II. The Swift sample , 2012, 1207.1447.

[22]  P. Brown,et al.  The association of GRB 060218 with a supernova and the evolution of the shock wave , 2006, Nature.

[23]  M. Rees,et al.  POPULATION III GAMMA-RAY BURSTS , 2010, 1004.2056.

[24]  C. Kouveliotou,et al.  The connection between thermal and non-thermal emission in gamma-ray bursts: general considerations and GRB 090902B as a case study , 2010, 1007.2228.

[25]  Chris L. Fryer,et al.  X-RAY SPECTRAL COMPONENTS OBSERVED IN THE AFTERGLOW OF GRB 130925A , 2014, 1402.6755.

[26]  A. Levinson,et al.  COLLIMATION AND CONFINEMENT OF MAGNETIC JETS BY EXTERNAL MEDIA , 2012, 1209.5261.

[27]  R. Kudritzki.  Line-driven Winds, Ionizing Fluxes, and Ultraviolet Spectra of Hot Stars at Extremely Low Metallicity. I. Very Massive O Stars , 2002, astro-ph/0205210.

[28]  R. Schneider,et al.  Population III stars: hidden or disappeared? , 2007, 0707.1433.

[29]  S. Woosley,et al.  LONG GAMMA-RAY TRANSIENTS FROM COLLAPSARS , 2011, 1110.3842.

[30]  Volker Bromm,et al.  The Formation of the First Stars. I. The Primordial Star-forming Cloud , 2002 .

[31]  A. Fruchter,et al.  GRB 130925A: HST imaging. , 2013 .

[32]  London,et al.  Mass-loss predictions for O and B stars as a function of metallicity , 2001, astro-ph/0101509.

[33]  B. Gendre,et al.  THE ULTRA-LONG GAMMA-RAY BURST 111209A: THE COLLAPSE OF A BLUE SUPERGIANT? , 2012, 1212.2392.

[34]  Felix Ryde,et al.  QUASI-BLACKBODY COMPONENT AND RADIATIVE EFFICIENCY OF THE PROMPT EMISSION OF GAMMA-RAY BURSTS , 2008, 0811.4135.

[35]  Abraham Loeb,et al.  In the Beginning: The First Sources of Light and the Reionization of the Universe , 2000 .

[36]  Alan A. Wells,et al.  The Swift Gamma-Ray Burst Mission , 2004, astro-ph/0405233.

[37]  K. Ioka,et al.  CAN GAMMA-RAY BURST JETS BREAK OUT THE FIRST STARS? , 2010, 1009.6001.

[38]  E. Rol,et al.  TESTING THE STANDARD FIREBALL MODEL OF GAMMA-RAY BURSTS USING LATE X-RAY AFTERGLOWS MEASURED BY SWIFT , 2006 .

[39]  S. Komissarov.  Numerical simulations of relativistic magnetized jets , 1999 .

[40]  D. Watson,et al.  THERMAL EMISSION IN THE EARLY X-RAY AFTERGLOWS OF GAMMA-RAY BURSTS: FOLLOWING THE PROMPT PHASE TO LATE TIMES , 2013, 1305.3165.

[41]  D. Wei,et al.  THE PHOTOSPHERIC RADIATION MODEL FOR THE PROMPT EMISSION OF GAMMA-RAY BURSTS: INTERPRETING FOUR OBSERVED CORRELATIONS , 2012, 1204.4881.