2000/11/30 by Yoram Lithwick, Re'em Sari · 1 citation
Physics and Astronomy · #astro-ph
paper · pdf · doi:10.1086/321455
published as ApJ, 555, 540 (2001) · ApJ accepted, 5 pages
arxiv created 2001/03/21 · arxiv updated 2009/12/01
As is well-known, the requirement that gamma ray bursts (GRB's) be optically thin to high energy photons yields a lower limit on the Lorentz factor (γ) of the expansion. In this paper, we provide a simple derivation of the lower limit on γdue to the annihilation of photon pairs, and correct the errors in some of the previous calculations of this limit. We also derive a second limit on γdue to scattering of photons by pair-created electrons and positrons. For some bursts, this limit is the more stringent. In addition, we show that a third limit on γ, which is obtained by considering the scattering of photons by electrons which accompany baryons, is nearly always less important than the second limit. Finally, we evaluate these limits for a number of bursts.