X-ray Analysis of Gamma-Ray Burst Flares and Underlying Afterglows: Insights into Origin of Flares

Abstract

Gamma-ray burst (GRB) X-ray light curves exhibit a variety of complex temporal structures, such as flares and plateaus. The origin of flares seen in many GRB early afterglows is still uncertain. Here, we analyze a sample of 89 GRBs, 61 of them with flares, both with and without a "plateau" phase. We fit the Swift-XRT light curves with synchrotron emission from a forward shock propagating into either a constant-density ISM or a stellar wind, and flares on top of that. We find that the flare light curves are not symmetric, with a decay time that is $\sim$five times longer than the rise time. We do not find any differences in flare properties between GRBs with and without a "plateau" phase. Moreover, additional afterglow properties such as the electron power-law index and the end time of the plateau are consistent between bursts with and without flares. These results strongly indicate that flares originate from a mechanism distinct from that producing the plateau and afterglow. When looking at the prompt emission properties, we do find some tendencies: GRBs with flares tend to be brighter and longer lasting than GRBs without flares. We therefore conclude that, unlike plateaus, flares are unlikely to arise from an external origin and are more plausibly associated with prolonged central engine activity that lasts longer than the main episode that produces the prompt phase. As the plateau cannot have the same origin, this result excludes models of late-time energy injection as the source of the GRB plateau.