X‐Ray Hardness Variations as an Internal/External Shock Diagnostic

Abstract

The early, highly time-variable X-ray emission immediately following gamma-ray bursts (GRBs) exhibits strong spectral variations that are unlike the temporally smoother emission that dominates after t $ 10 3 s. The ratio of hardchannel (1.3Y10.0 keV ) to soft-channel (0.3Y1.3 keV ) counts in the Swift X-ray telescope provides a new measure delineating the end time of this emission. We define T H as the time at which this transition takes place and measure for 59 events a range of transition times that spans 10 2 to 10 4 s, on average 5 times longer than the prompt T 90 duration observed in the gamma-ray band. It is very likely that the mysterious light-curve plateau phase and the later powerlaw temporal evolution, both of which typically occur at times greater than T H and hence exhibit very little hardness ratio evolution, are both produced by external shocking of the surrounding medium and not by the internal shocks thought responsible for the earlier emission. We use the apparent lack of spectral evolution to discriminate among proposed models for the plateau phase emission. We favor energy injection scenarios with a roughly linearly increasing input energy versus time for six well-sampled events with nearly flat light curves at t % 10 3 Y10 4 s. Also, using the transition time T H as the delineation between the GRB and afterglow emission, we calculate that the kinetic energy in the afterglow shock is typically a factor of 10 lower than that released in the GRB. Three very bright events suggest that this presents a missing X-ray flux problem rather than an efficiency problem for the conversion of kinetic energy into the GRB. Lack of hardness variations in these three events may be due to a very highly relativistic outflow or due to a very dense circumburst medium. There are a handful of rare cases of very late time t > 10 4 s hardness evolution, which may point to residual central engine activity at very late time.

Effects of a low electron distribution cutoff on multiwavelength spectra and light curves of GRB afterglows

“…11. Our model-derived hardness ratio shows a characteristic signature that is compatible with observations at least during the early stages (Butler & Kocevski 2007;Liang et al 2007). On the other hand, it is still inconclusive regarding the late stages (P. O'Brien -priv.…”

On the Late-Time Spectral Softening Found in X-Ray Afterglows of Gamma-Ray Bursts

“…In general, the observed multiwavelength afterglows have been found consistent with the external forward shock models (e.g., Piran 2004; Mészáros 2006 for reviews). In contrast to the remarkable variations of the X-ray light curves, most spectra of X-ray afterglows show little variation (Butler & Kocevski 2007;Evans et al 2009;Shao et al 2010), which is consistent with the prediction of standard external forward shock models.…”

TheSwift Burst Analyser

“…positive-valued) hardness ratio bin are discarded and not converted into flux. At later times the spectral evolution is minimal (Butler & Kocevski 2007b), thus for XRT any light curve bins which occur after the final hardness ratio bin are converted to flux using the conversion factors from the final hardness bin.…”