Liang 2009 XRT光变的单一幂律下降

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Title:		A Comprehensive Analysis of Swift/XRT Data: IV. Single Power-Law Decaying XRT lightcurves and Implications for the Unified Origin of the X-rays
Authors:		Liang, En-Wei; Lv, Hou-Jun; Zhang, Bin-Bin; Zhang, Bing
Publication:		eprint arXiv:0902.3504
Publication Date:		02/2009
Origin:		ARXIV
Keywords:		Astrophysics - High Energy Astrophysical Phenomena
Comment:		29 pages, including 2 tables and 8 figures. Submitted to ApJ. This is the forth paper of a series. Paper I, II, and III see astro-ph/0612246 (ApJ, 2007, 666,1002), arXiv:0705.1373 (ApJ, 2007, 670, 565), and arXiv:0708.2942 (ApJ, 2008, 675, 528)
Bibliographic Code:		2009arXiv0902.3504L

Abstract

By analyzing the Swift/XRT lightcurves detected before 2008 October, we find 17 cases that decay as a single power law (SPL) from tens of seconds to ~10^5 seconds post the GRB trigger. They are apparently different from the canonical ones that are characterized by a shallow-to-normal decay transition. We compare the distributions of the observables of the prompt gamma-rays and X-rays for the two groups of GRBs, but no statistical difference is found. Interestingly, the SPL XRT lightcurves in the burst frame merge into a conflux. The normal decay segment for the canonical lightcurves has the similar feature. These results likely suggest that both the prompt gamma-rays and the X-rays for the two groups of GRBs may share the similar physical origins, and the apparent difference between the groups of XRT lightcurves may not be intrinsic. If the shallow decay is due to energy injection into the fireball, this suggests that the total energy budget after injection for both types of GRBs is similar. More intriguingly, the picture is consistent with the scenario that the apparent shallow-to-normal behavior is an artifact because of the improper choice of the zero time point (T_0), as suggested by Yamazaki. We shift T_0 of the canonical lightcurves to an epoch prior to the GRB triggers to make the lightcurves close to a SPL with the temporal indices similar to those in the normal decay phase. It is found that the T_0-shifted lightcurves trace the lightcurves of the SPL type well. This result likely suggests that the X-rays might be a long-lasting emission component that is independent of the prompt gamma-rays. The GRBs with the SPL lightcurves may be those whose T_0's of X-ray decay are comparable to the trigger times. We discuss the prior X-ray emission from GRBs for both an external shock origin and an origin from a long-lasting central engine.

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