X-ray Hardness Variations as an Internal/External Shock Diagnostic

The early, highly time-variable X-ray emission immediately following GRBs exhibits strong spectral variations that are unlike the temporally smoother emission which dominates after $t\sim 10^3$ s. The ratio of hard channel (1.3-10.0 keV) to soft channel (0.3-1.3 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 span $10^2$ s 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 against 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 $t\approx 10^3-10^4$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. Lack of hardness variations in these three events may be due to a very highly relatavistic 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.