Ghisellini 2009 辐射情形的外激波解释LAT高能观测

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相比惯用的绝热情形，辐射情形可以使光变更陡。

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Title:		GeV emission from gamma-ray bursts: a radiative fireball?
Authors:		Ghisellini, G.; Ghirlanda, G.; Nava, L.; Celotti, A.
Affiliation:		AA(INAF - Osservatorio Astronomico di Brera, via E. Bianchi 46, I-23807 Merate, Italy), AB(SISSA, via Beirut 2-4, I-34014 Trieste, Italy), AC(INAF - Osservatorio Astronomico di Brera, via E. Bianchi 46, I-23807 Merate, Italy; SISSA, via Beirut 2-4, I-34014 Trieste, Italy), AD(SISSA, via Beirut 2-4, I-34014 Trieste, Italy)
Publication:		Monthly Notices of the Royal Astronomical Society, Volume 403, Issue 2, pp. 926-937. (MNRAS Homepage)
Publication Date:		04/2010
Origin:		WILEY
MNRAS Keywords:		radiation mechanisms: non-thermal, gamma-rays: bursts, gamma-rays: theory, X-rays: general
DOI:		10.1111/j.1365-2966.2009.16171.x
Bibliographic Code:		2010MNRAS.403..926G

Abstract

We study the emission observed at energies >100 MeV of 11 gamma-ray bursts (GRBs) detected by the Fermi-Large Area Telescope (LAT) until 2009 October. The GeV emission has three main properties: (i) its duration is often longer than the duration of the softer emission detected by the Gamma Burst Monitor onboard Fermi (this confirms earlier results from the Energetic Gamma-Ray Experiment Telescope); (ii) its spectrum is consistent with F_ν ~ ν^-1 and does not show strong spectral evolution; and (iii) for the brightest bursts the flux detected by the LAT decays as a power law with a typical slope t^-1.5. We argue that the observed >0.1 GeV flux can be interpreted as afterglow emission shortly following the start of the prompt phase emission as seen at smaller frequencies. The decay slope is what is expected if the fireball emission is produced in the radiative regime, i.e. all dissipated energy is radiated away. We also argue that the detectability in the GeV energy range depends on the bulk Lorentz factor Γ of the bursts, being strongly favoured in the case of large Γ. This implies that the fraction of bursts detected at high energies corresponds to the fraction of bursts having the largest Γ. The radiative interpretation can help to explain why the observed X-ray and optical afterglow energetics are much smaller than the energetics emitted during the prompt phase, despite the fact that the collision with the external medium should be more efficient than internal shocks in producing the radiation that we see.

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