A Statistical Study of GRB X-Ray Flares: Evidence of Ubiquous Bulk Acceleration in the Emission Region

Document Type

Article

Publication Date

1-1-2016

Publication Title

Astrophysical Journal, Supplement Series

Volume

225

Issue

1

Abstract

When emission in a conical relativistic jet ceases abruptly (or decays sharply), the observed decay light curve is controlled by the high-latitude "curvature effect." Recently, Uhm & Zhang found that the decay slopes of three gamma-ray burst (GRB) X-ray flares are steeper than what the standard model predicts. This requires bulk acceleration of the emission region, which is consistent with a Poynting-flux-dominated outflow. In this paper, we systematically analyze a sample of 85 bright X-ray flares detected in 63 Swift GRBs and investigate the relationship between the temporal decay index α and spectral index β during the steep decay phase of these flares. The α values depend on the choice of the zero time point t 0. We adopt two methods. "Method I" takes as the first rising data point of each flare and is the most conservative approach. We find that at the 99.9% confidence level 56/85 flares have decay slopes steeper than the simplest curvature effect prediction and therefore are in the acceleration regime. "Method II" extrapolates the rising light curve of each flare backward until the flux density is three orders of magnitude lower than the peak flux density, and it defines the corresponding time as the zero time point (). We find that 74/85 flares fall into the acceleration regime at the 99.9% confidence level. This suggests that bulk acceleration is common and may even be ubiquitous among X-ray flares, pointing toward a Poynting-flux-dominated jet composition for these events. © 2016. The American Astronomical Society. All rights reserved..

Language

English

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