Hardness 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 ∼ 10s. 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 TH as the time at which this transition takes place and measure for 59 events a range of transition times that span 10s to 10s, on average 5 times longer than the prompt T90 duration observed in the Gamma-rays. Using the transition time TH 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. Two 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. Furthermore, from a typically unchanging hardness ratio after TH , we infer that the mysterious light curve plateau phase is produced by a mechanism separate from that which produced the earlier emission. We favor energy injection scenarios with a linearly increasing input energy versus time for six well sampled events with nearly flat light curves at t ≈ 10 − 10s. There are a handful of cases of very late time t > 10s hardness evolution, which may point to residual central engine activity at very late time. Subject headings: gamma rays: bursts — supernovae: general — X-rays: general