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Title: Low energy spectral index and E{sub p} evolution of quasi-thermal photosphere emission of gamma-ray bursts

Recent observations by the Fermi satellite suggest that a photosphere emission component is contributing to the observed spectrum of many gamma-ray bursts (GRBs). One important question is whether the photosphere component can interpret the typical 'Band' function of GRBs with a typical low energy photon spectral index α ∼ –1. We perform a detailed study of the photosphere emission spectrum by progressively introducing several physical ingredients previously not fully incorporated, including the probability distribution of the location of a dynamically evolving photosphere, superposition of emission from an equal arrival time 'volume' in a continuous wind, the evolution of optical depth of a wind with finite but evolving outer boundary, as well as the effect of different top-hat wind luminosity (L{sub w} ) profiles. By assuming a comoving blackbody spectrum emerging from the photosphere, we find that for an outflow with a constant or increasing L{sub w} , the low-energy spectrum below the peak energy (E{sub p} ), can be modified to F {sub ν} ∼ ν{sup 1.5} (α ∼ +0.5). A softer (–1 < α < +0.5) or flat (α = –1) spectrum can be obtained during the L{sub w} decreasing phase or high-latitude-emission-dominated phase. We also study the evolutionmore » of E{sub p} as a function of wind and photosphere luminosity in this photosphere model. An E{sub p} – L tracking pattern can be reproduced if a certain positive dependence between the dimensionless entropy η and L{sub w} is introduced. However, the hard-to-soft evolution pattern cannot be reproduced unless a contrived condition is invoked. In order to interpret the Band spectrum, a more complicated photosphere model or a different energy dissipation and radiation mechanism is needed.« less
Authors:
;  [1]
  1. Department of Physics and Astronomy, University of Nevada Las Vegas, Las Vegas, NV 89154 (United States)
Publication Date:
OSTI Identifier:
22357099
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 785; 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; COSMIC GAMMA BURSTS; DISTRIBUTION; EMISSION; EMISSION SPECTRA; ENERGY LOSSES; ENERGY SPECTRA; ENTROPY; EVOLUTION; LUMINOSITY; PHOTONS; PHOTOSPHERE; PROBABILITY; RELATIVISTIC RANGE; SATELLITES