Hascoët, R. 2011 Fermi暴的GeV光子延后是因为早期被吸收

主要内容:
1. 改进光子-光子湮灭模型，使得对Gamma的限制小了几倍；
2. 认为早期对GeV光子来说光深较大，出不来，所以GeV的光子比较晚出来；（有个问题，GeV光子在MeV的没有了的时候还有呢）

从没有到有GeV光子出来，可以知道在什么时候光深为1，从而计算出洛伦兹因子，结果是340左右。

精彩摘抄:

文章信息:

· Find Similar Abstracts (with default settings below)

· Electronic Refereed Journal Article (HTML)

· Full Refereed Journal Article (PDF/Postscript)

· arXiv e-print (arXiv:1107.5737)

· References in the article

· Citations to the Article (6) (Citation History)

· Refereed Citations to the Article

· Also-Read Articles (Reads History)

·

· Translate This Page

Title:		Do Fermi Large Area Telescope observations imply very large Lorentz factors in gamma-ray burst outflows?
Authors:		Hascoët, R.; Daigne, F.; Mochkovitch, R.; Vennin, V.
Affiliation:		AA(UPMC-CNRS, UMR7095, Institut d'Astrophysique de Paris, F-75014 Paris, France), AB(UPMC-CNRS, UMR7095, Institut d'Astrophysique de Paris, F-75014 Paris, France), AC(UPMC-CNRS, UMR7095, Institut d'Astrophysique de Paris, F-75014 Paris, France), AD(UPMC-CNRS, UMR7095, Institut d'Astrophysique de Paris, F-75014 Paris, France)
Publication:		Monthly Notices of the Royal Astronomical Society, Online Early (MNRAS Homepage)
Publication Date:		01/2012
Origin:		WILEY
Astronomy Keywords:		radiation mechanisms: non-thermal, radiative transfer, gamma-ray burst: general, gamma-ray burst: individual: GRB 080916C
Abstract Copyright:		© 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS
DOI:		10.1111/j.1365-2966.2011.20332.x
Bibliographic Code:		2012MNRAS.tmp.2225H

Abstract

Recent detections of GeV photons in a few gamma-ray bursts (GRBs) by Fermi Large Area Telescope (LAT) imply huge bulk Lorentz factors to avoid a large γγ optical depth at high energy. Estimates can be as high as Γ≃ 10³ in the most extreme cases. This puts severe constraints on models of the central engine and the jet acceleration in GRBs. These estimates are, however, obtained from a simplified single-zone model. We present here a more realistic calculation of the γγ opacity which takes into account the time, space and direction dependent photon field existing in an outflow with several relativistically moving emitting zones. The formalism is very general and can be applied to many models of the prompt GRB emission. We present results obtained for a numerical implementation in the framework of the internal shock model. We show the following results in this paper. (i) The minimum Lorentz factor Γ_min in bright Fermi-LAT GRBs is reduced by a factor of ≃2-3 compared to previous estimates if the GeV and MeV emission are produced in the same region, and by an additional factor of ≃2-8 if the GeV emission is produced at larger radii. We provide an improved approximate formula for Γ_min which is in good agreement with our numerical results and which can be directly applied to Fermi-LAT GRB data. (ii) A delayed onset of the GeV emission can be due to the time evolution of the opacity in a GRB outflow. As an illustration of these first two results, we present a synthetic GRB that reproduces most features of GRB 080916C with a mean Lorentz factor of ≃340, an optically thin regime for γγ opacity at 3 GeV in time bin 'b', a variability time-scale of ≃0.5 s in the MeV light curve and a delayed onset of ≃5 s of the GeV emission. (iii) The γγ opacity can smooth the short time-scale variability in the GeV light curve. This last result implies that the observed variability at high energy is not necessarily a good test to distinguish between an internal and an external origin for the GeV emission in GRBs.

---

Bibtex entry for this abstract   Preferred format for this abstract (see Preferences)