Zhao 2010 Fermi-LAT暴的洛仑兹因子

主要内容:
如果低能和高能伽马射线来自同一个区的话，导出的洛仑兹因子会太高。
如果认为他们来自两个不同的区域的话，可以解释观测，不会导致低能段指数下降。

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Title:		The bulk Lorentz factors of Fermi-LAT GRBs
Authors:		Zhao, Xiaohong; Li, Zhuo; Bai, Jinming
Publication:		eprint arXiv:1005.5229
Publication Date:		05/2010
Origin:		ARXIV
Keywords:		Astrophysics - High Energy Astrophysical Phenomena
Comment:		6 pages, 5 figures, submitted to ApJ
Bibliographic Code:		2010arXiv1005.5229Z

Abstract

The Lorentz factor (LF) of gamma-ray burst (GRB) ejecta may be constrained by observations of high-energy (HE) spectral attenuation. The recent Fermi-LAT observations of prompt GeV emission from several bright GRBs have leaded to conclusions of unexpectedly large LFs, $\Gamma>10^3$. Here we revisit this problem with two main concerns. (1) With one-zone assumption where all photons are assumed to be generated in the same region (radius) and time, we {\em self-consistently} calculate the $\gamma\gamma$ optical depth by adopting a target photon spectrum with HE cutoff. We find that this might be important when the GRB LF is below a few hundreds. (2) Recent Fermi-LAT observations suggest that the bulk MeV-range and HE ($\ga100$ MeV) emission may arise from different regions. We then consider a two-zone case where HE emission is generated in large radii and mainly attenuated by prompt MeV-range emission generated at small radii. We find that the attenuated HE spectrum does not show an exponential spectral cutoff but a slight steepening. This suggests that there may be no abrupt cutoff due to $\gamma\gamma$ attenuation if relaxing the one-zone assumption. By studying the spectra of three bright Fermi-LAT GRBs 080916C, 090510 and 090902B, we show that a bulk LF of $300\la\Gamma\la600$ can be consistent with observations.

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