Santana 2013 系统地限制伽马暴的磁场epsilonB

主要内容:
60个X射线，35个光学的暴，假定其它的参数，限定出epsilon_B.

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Title:		Magnetic Fields In Relativistic Collisionless Shocks
Authors:		Santana, Rodolfo; Barniol Duran, Rodolfo; Kumar, Pawan
Publication:		eprint arXiv:1309.3277
Publication Date:		09/2013
Origin:		ARXIV
Keywords:		Astrophysics - High Energy Astrophysical Phenomena
Comment:		21 Pages, 7 Figures, Submitted to ApJ
Bibliographic Code:		2013arXiv1309.3277S

Abstract

Using X-ray and optical afterglows, we present a systematic study on magnetic fields in Gamma-Ray Burst (GRB) external shocks. There are 60 (35) GRBs in our X-ray (optical) sample, mostly from Swift. We use two methods to study epsilon_B (fraction of energy in magnetic field in the shocked plasma). 1. For the X-ray sample, we use the constraint that the observed flux at the end of the X-ray steep decline is larger than or equal to the flux from the forward shock (FS). 2. For the optical sample, we use the condition that the observed flux arises from the FS (our optical sample light curves decline as ~t^-1 at early times as expected for the FS). Making a reasonable assumption on E (jet isotropic equivalent kinetic energy), we converted these conditions into an upper limit (measurement) on epsilon_B n^{2/(p+1)} for our X-ray (optical) sample, where n is the circumburst density and p is the electron distribution power-law index. Taking n=1 cm^-3, the distribution of epsilon_B measurements (upper limits) for our optical (X-ray) sample has a range of ~10^-8 -10^-3 (~10^-6 -10^-3) and median of ~few x 10^-5 (~few x 10^-5), showing that most bursts in our samples only require a weak magnetic field. The largest source of uncertainty in these results is n, since it has beed determined to vary over many orders of magnitude and its value is not known for each GRB. To characterize how much magnetic field amplification is needed, beyond shock compression of a seed magnetic field ~10 muG, we expressed our results in terms of an amplification factor, AF, which is very weakly dependent on n (AF propto n^0.21 for p=2.4). The range of AF measurements (upper limits) for our optical (X-ray) sample is ~ 1-1000 (~10-300) with a median of ~50 (~50). These results suggest that some amplification, in addition to shock compression, is needed to explain the majority of the afterglow observations in our samples.

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