A STATISTICAL STUDY OF GRB X-RAY FLARES: EVIDENCE OF UBIQUITOUS BULK ACCELERATION IN THE EMISSION REGION

• Published in: The Astrophysical journal. Supplement series Vol. 225; no. 1; p. 17
• Main Authors: Jia, Lan-Wei, Uhm, Z. Lucas, Zhang, Bing
• Format: Journal Article
• Language: English
• Published: United States The American Astronomical Society 01.07.2016
• Subjects: Acceleration; ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; Confidence intervals; COSMIC GAMMA BURSTS; Decay; DIAGRAMS; EMISSION; Flares; FLUX DENSITY; Gamma ray bursts; gamma-ray burst: general; JETS; Light curve; methods: statistical; radiation mechanisms: non-thermal; RELATIVISTIC RANGE; Slopes; STANDARD MODEL; STATISTICS; STELLAR FLARES; VISIBLE RADIATION; X RADIATION; X rays
• ISSN: 0067-0049; 1538-4365
• DOI: 10.3847/0067-0049/225/1/17

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 and spectral index β during the steep decay phase of these flares. The values depend on the choice of the zero time point t0. 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.