Astronomy and Astrophysics – Astrophysics
Scientific paper
2008-11-25
Astrophys.J.702:1211-1229,2009
Astronomy and Astrophysics
Astrophysics
44 pages, submitted to the Astrophysical Journal Updated discussion on the non-thermal component
Scientific paper
10.1088/0004-637X/702/2/1211
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 ($\sim$ 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.
Pe'er Asaf
Ryde Felix
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