Shen, Rongfeng 2011 星风环境下的减速时标前产生平降阶段

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Title:		Coasting External Shock in Wind Medium: An Origin for the X-Ray Plateau Decay Component in Swift Gamma-Ray Burst Afterglows
Authors:		Shen, Rongfeng; Matzner, Christopher D.
Affiliation:		AA(Department of Astronomy & Astrophysics, University of Toronto, Ontario M5S 3H4, Canada rfshen@astro.utoronto.ca), AB(Department of Astronomy & Astrophysics, University of Toronto, Ontario M5S 3H4, Canada matzner@astro.utoronto.ca)
Publication:		The Astrophysical Journal, Volume 744, Issue 1, article id. 36 (2012). (ApJ Homepage)
Publication Date:		01/2012
Origin:		IOP
Astronomy Keywords:		gamma-ray burst: general, radiation mechanisms: non-thermal, relativistic processes, shock waves, X-rays: bursts
DOI:		10.1088/0004-637X/744/1/36
Bibliographic Code:		2012ApJ...744...36S

Abstract

The plateaus observed in about one half of the early X-ray afterglows are the most puzzling feature in gamma-ray bursts (GRBs) detected by Swift. By analyzing the temporal and spectral indices of a large X-ray plateau sample, we find that 55% can be explained by external, forward shock synchrotron emission produced by a relativistic ejecta coasting in a ρvpropr ^-2, wind-like medium; no energy injection into the shock is needed. After the ejecta collects enough medium and transitions to the adiabatic, decelerating blast wave phase, it produces the post-plateau decay. For those bursts consistent with this model, we find an upper limit for the initial Lorentz factor of the ejecta, Γ₀ <= 46(epsilon _e /0.1)^-0.24(epsilon _B /0.01)^0.17; the isotropic equivalent total ejecta energy is E _iso ~ 10⁵³(epsilon _e /0.1)^-1.3(epsilon _B/0.01)^-0.09(t_b /10⁴ s) erg, where epsilon _e and epsilon _B are the fractions of the total energy at the shock downstream that are carried by electrons and the magnetic field, respectively, and t_b is the end of the plateau. Our finding supports Wolf-Rayet stars as the progenitor stars of some GRBs. It raises intriguing questions about the origin of an intermediate-Γ₀ ejecta, which we speculate is connected to the GRB jet emergence from its host star. For the remaining 45% of the sample, the post-plateau decline is too rapid to be explained in the coasting-in-wind model, and energy injection appears to be required.

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