Astronomy and Astrophysics – Astronomy
Scientific paper
Apr 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008apj...677..425l&link_type=abstract
The Astrophysical Journal, Volume 677, Issue 1, pp. 425-431.
Astronomy and Astrophysics
Astronomy
Gamma Rays: Bursts, Hydrodynamics, Radiative Transfer
Scientific paper
Many models of gamma-ray bursts (GRBs) as well as of soft gamma repeaters (SGRs) involve a fireball-an optically thick concentration of radiation energy with a high ratio of energy density to rest mass. We study the asymptotic behavior of an ultrarelativistic fireball consisting of electron-positron pairs and photons. We show that in the ultrarelativistic limit, after photons decouple from the pairs, the photon distribution function remains a blackbody spectrum in some appropriate Lorentz frame, allowing us to define an effective Lorentz factor and temperature for the photon gas. We also study the freezing out of electron-positron pairs and their asymptotic Lorentz factor γ∞. The dependence of these quantities on initial conditions can be described by simple scaling laws. We apply our results to SGR 1806-20 and find that the energy carried by electron-positron pairs is higher than calculated by former estimates, but is still an order of magnitude short of the minimum energy required to produce the observed afterglow. A viable solution of the energy budget is that the fireball is loaded by baryons or electromagnetic flux.
Li Chao
Sari Re'em
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