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Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
2009 (English)In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 702, no 2, 1211-1229 p.Article in journal (Refereed) Published
Abstract [en]

We perform time-resolved spectroscopy on the prompt emission in gamma-ray bursts (GRBs) and identify a thermal, photospheric component peaking at a temperature of a few hundreds keV. This peak does not necessarily coincide with the broad-band (keV-GeV) power peak. We show that this thermal component exhibits a characteristic temporal behavior. We study a sample of 56 long bursts, all strong enough to allow time-resolved spectroscopy. We analyze the evolution of both the temperature and flux of the thermal component in 49 individual time-resolved pulses, for which the temporal coverage is sufficient, and find that the temperature is nearly constant during the first few seconds, after which it decays as a power law with a sample-averaged index of -0.68. The thermal flux first rises with an averaged power-law index of 0.63 after which it decays with an averaged index of -2. The break times are the same to within errors. We find that the ratio of the observed to the emergent thermal flux typically exhibits a monotoneous power-law increase during the entire pulse as well as during complex bursts. Thermal photons carry a significant fraction (similar to 30% to more than 50%) of the prompt emission energy (in the observed 25-1900 keV energy band), thereby significantly contributing to the high radiative efficiency. Finally, we show here that the thermal emission can be used to study the properties of the photosphere, hence the physical parameters of the GRB fireball.

Place, publisher, year, edition, pages
2009. Vol. 702, no 2, 1211-1229 p.
Keyword [en]
gamma rays: bursts, gamma rays: observations, gamma rays: theory, radiation mechanisms: thermal
National Category
Astronomy, Astrophysics and Cosmology
URN: urn:nbn:se:su:diva-80704DOI: 10.1088/0004-637X/702/2/1211ISI: 000269245000036OAI: diva2:557742


Available from: 2012-09-28 Created: 2012-09-27 Last updated: 2012-09-28Bibliographically approved

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