SUBMITTED TO APJ Preprint typeset using LATEX style emulateapj v. 10/09/06 AN UP-SCATTERED COCOON EMISSION MODEL OF GAMMA-RAY BURST HIGH-ENERGY LAGS

The Fermi Gamma-ray Space Telescope recently detected the most energetic gamma-ray burst so far, GRB080916C, and reported its detailed temporal properties in an extremely broad spectral range: (1) the time-resolved spectra are well described by broken power-law forms over the energy range of 10 keV-10 GeV, (2) the high-energy emission (at epsilon > 100 MeV) is delayed by approximate to 5 s with respect to the epsilon less than or similar to 1 MeV emission, and (3) the emission onset times shift toward later times in higher energy bands. We show that this behavior of the high-energy emission can be explained by a model in which the prompt emission consists of two components: one is the emission component peaking at epsilon less than or similar to 1 MeV due to the synchrotron-self-Compton radiation of electrons accelerated in the internal shock of the jet and the other is the component peaking at epsilon similar to 100 MeV due to up-scattering of the photospheric X-ray emission of the expanding cocoon (i.e., the hot bubble produced by dissipation of the jet energy inside the progenitor star) off the same electrons in the jet. Based on this model, we derive some constraints on the radius of the progenitor star and the total energy and mass of the cocoon of this GRB, which may provide information on the structure of the progenitor star and the physical conditions of the jet propagating in the star. The up-scattered cocoon emission could be important for other Fermi GRBs as well. We discuss some predictions of this model, including a prompt bright optical emission and a soft X-ray excess.

[1]  M. Rees,et al.  Steep Slopes and Preferred Breaks in Gamma-Ray Burst Spectra: The Role of Photospheres and Comptonization , 1999 .

[2]  Re'em Sari,et al.  Lower Limits on Lorentz Factors in Gamma-Ray Bursts , 2000, astro-ph/0011508.

[3]  K. Ioka,et al.  CAN WE PROBE THE LORENTZ FACTOR OF GAMMA-RAY BURSTS FROM GeV–TeV SPECTRA INTEGRATED OVER INTERNAL SHOCKS? , 2009, 0904.4878.

[4]  Davide Lazzati,et al.  VERY HIGH EFFICIENCY PHOTOSPHERIC EMISSION IN LONG-DURATION γ-RAY BURSTS , 2009, 0904.2779.

[5]  Y. Lyubarsky.  ASYMPTOTIC STRUCTURE OF POYNTING-DOMINATED JETS , 2009, 0902.3357.

[6]  Kenji Toma,et al.  Probing the Efficiency of Electron-Proton Coupling in Relativistic Collisionless Shocks through the Radio Polarimetry of Gamma-Ray Burst Afterglows , 2008 .

[7]  Pawan Kumar,et al.  On the generation of high-energy photons detected by the Fermi Satellite from gamma-ray bursts , 2009, 0905.2417.

[8]  Yi-Zhong Fan,et al.  Interpretation and implications of the non‐detection of GeV spectrum excess by the Fermi Gamma‐ray Space Telescope in most gamma‐ray bursts , 2009, 0905.0908.

[9]  E. Waxman,et al.  THE EFFICIENCY OF ELECTRON ACCELERATION IN COLLISIONLESS SHOCKS AND GAMMA-RAY BURST ENERGETICS , 2005, astro-ph/0502070.

[10]  T. Piran,et al.  The possible high-energy emission from GRB 080319B and origins of the GeV emission of GRBs 080514B, 080916C and 081024B , 2008, 0811.2997.

[11]  K. Toma,et al.  A Possible Origin of Bimodal Distribution of Gamma-Ray Bursts , 2004, astro-ph/0407012.

[12]  F. A. Harrison,et al.  A Study of the Afterglows of Four GRBs: Constraining the Explosion and Fireball Model , 2003, astro-ph/0307056.

[13]  D. Burrows,et al.  Physical Processes Shaping Gamma-Ray Burst X-Ray Afterglow Light Curves: Theoretical Implications from the Swift X-Ray Telescope Observations , 2005, astro-ph/0508321.

[14]  Akira Mizuta,et al.  ANGULAR ENERGY DISTRIBUTION OF COLLAPSAR-JETS , 2008, 0812.4813.

[15]  Ramesh Narayan,et al.  GRB 080319B: evidence for relativistic turbulence, not internal shocks , 2008, 0812.0021.

[16]  A. Pe’er,et al.  EVIDENCE OF AN INITIALLY MAGNETICALLY DOMINATED OUTFLOW IN GRB 080916C , 2009, 0904.2943.

[17]  M. J. Rees,et al.  Dissipative photosphere models of gamma-ray bursts and X-ray flashes , 2005 .

[18]  C. Matzner.  Supernova hosts for gamma-ray burst jets: dynamical constraints , 2002, astro-ph/0203085.

[19]  Tsvi Piran,et al.  Observational limits on inverse Compton processes in gamma-ray bursts , 2008, 0807.3954.

[20]  J. Chiang,et al.  THE LARGE AREA TELESCOPE ON THE FERMI GAMMA-RAY SPACE TELESCOPE MISSION , 2009, 0902.1089.

[21]  N. Vlahakis,et al.  Relativistic Magnetohydrodynamics with Application to Gamma-Ray Burst Outflows. II. Semianalytic Super-Alfvénic Solutions , 2003, astro-ph/0303483.

[22]  S. B. Cenko,et al.  The redshift and afterglow of the extremely energetic gamma-ray burst GRB 080916C , 2009, 0902.0761.

[23]  D. Palmer,et al.  BATSE observations of gamma-ray burst spectra. I: Spectral diversity , 1993 .

[24]  Saclay,et al.  The expected thermal precursors of gamma-ray bursts in the internal shock model , 2002, astro-ph/0207456.

[25]  M. Rees,et al.  Radiation from an Expanding Cocoon as an Explanation of the Steep Decay Observed in GRB Early Afterglow Light Curves , 2006, astro-ph/0603343.

[26]  Bing Zhang,et al.  DISCERNING THE PHYSICAL ORIGINS OF COSMOLOGICAL GAMMA-RAY BURSTS BASED ON MULTIPLE OBSERVATIONAL CRITERIA: THE CASES OF z = 6.7 GRB 080913, z = 8.2 GRB 090423, AND SOME SHORT/HARD GRBs , 2009, 0902.2419.

[27]  I. France,et al.  Prompt high-energy emission from gamma-ray bursts in the internal shock model , 2008, 0811.2956.

[28]  Jonathan Granot,et al.  Images and spectra from the interior of a relativistic fireball , 1999 .

[29]  Tsvi Piran,et al.  Hydrodynamics of relativistic fireballs , 1993, astro-ph/9301004.

[30]  Caltech,et al.  Relativistic Jets in Collapsars , 2002, astro-ph/0207436.

[31]  P. M'esz'aros,et al.  PROMPT HIGH-ENERGY EMISSION FROM PROTON-DOMINATED GAMMA-RAY BURSTS , 2008, 0807.0951.

[32]  Bing Zhang.  Gamma-Ray Bursts in the Swift Era , 2007, astro-ph/0701520.

[33]  A. Panaitescu,et al.  Properties of Relativistic Jets in Gamma-Ray Burst Afterglows , 2001, astro-ph/0109124.

[34]  Takanori Sakamoto,et al.  Low-Luminosity GRB 060218: A Collapsar Jet from a Neutron Star, Leaving a Magnetar as a Remnant? , 2007 .

[35]  S. Komissarov,et al.  Magnetic acceleration of ultrarelativistic jets in gamma-ray burst sources , 2008, 0811.1467.

[36]  M. Rees,et al.  Events in the life of a cocoon surrounding a light, collapsar jet , 2002, astro-ph/0205108.

[37]  P. Mészáros,et al.  GRB 080916C: ON THE RADIATION ORIGIN OF THE PROMPT EMISSION FROM keV/MeV TO GeV , 2009, 0903.2086.

[38]  Ryo Yamazaki,et al.  Cosmological X-Ray Flashes in the Off-Axis Jet Model , 2003 .

[39]  TEMPORAL AND ANGULAR PROPERTIES OF GAMMA-RAY BURST JETS EMERGING FROM MASSIVE STARS , 2006, astro-ph/0609254.

[40]  A. Beloborodov,et al.  Optical and GeV-TeV Flashes from Gamma-Ray Bursts , 2004, astro-ph/0410050.

[41]  G. Ghirlanda,et al.  The Collimation-corrected Gamma-Ray Burst Energies Correlate with the Peak Energy of Their νFν Spectrum , 2004, astro-ph/0405602.

[42]  A. Loeb,et al.  Constraints on Off-Axis X-Ray Emission from Beamed Gamma-Ray Bursts , 1999, astro-ph/9903377.

[43]  Charles D. Dermer.  Curvature Effects in Gamma-Ray Burst Colliding Shells , 2004 .

[44]  M. Rees,et al.  Unsteady outflow models for cosmological gamma-ray bursts , 1994, astro-ph/9404038.

[45]  Re'em Sari,et al.  On the Synchrotron Self-Compton Emission from Relativistic Shocks and Its Implications for Gamma-Ray Burst Afterglows , 2000, astro-ph/0005253.

[46]  Bing Zhang,et al.  Prompt emission of high‐energy photons from gamma ray bursts , 2007, 0704.1329.

[47]  Zhuo Li.  PROMPT GeV EMISSION FROM RESIDUAL COLLISIONS IN GAMMA-RAY BURST OUTFLOWS: EVIDENCE FROM FERMI OBSERVATIONS OF GRB 080916c , 2008, 0810.2932.

[48]  M. Rees,et al.  Gasdynamics of relativistically expanding gamma-ray burst sources : kinematics, energetics, magnetic fields, and efficiency , 1993, astro-ph/9301007.

[49]  B. Metzger,et al.  Time-dependent models of accretion discs formed from compact object mergers , 2008, 0805.4415.

[50]  P. Meszaros.  Gamma-ray bursts , 1998 .

[51]  Takashi S. Nakamura,et al.  Peak Luminosity-Spectral Lag Relation Caused by the Viewing Angle of the Collimated Gamma-Ray Bursts , 2001, astro-ph/0105321.

[52]  Akira Mizuta,et al.  Collimated Jet or Expanding Outflow: Possible Origins of Gamma-Ray Bursts and X-Ray Flashes , 2006, astro-ph/0607544.

[53]  Neutrino, Neutron, and Cosmic-Ray Production in the External Shock Model of Gamma-Ray Bursts , 2000, astro-ph/0005440.

[54]  D. Guetta,et al.  High-Energy Emission from the Prompt Gamma-Ray Burst , 2002, astro-ph/0209578.

[55]  T. Piran,et al.  High-energy afterglow emission from gamma-ray bursts , 2007, 0704.2063.

[56]  B.Zhang,et al.  Gamma Ray Burst Section of the White Paper on the Status and Future of Ground-based TeV Gamma-ray Astronomy , 2008, 0810.0520.

[57]  E. Waxman,et al.  Collapsar Uncorking and Jet Eruption in Gamma-Ray Bursts , 2002, astro-ph/0206392.