The X-ray 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 $t\sim 10^3$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-rays. 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. This worsens an efficiency problem for the conversion of shock kinetic energy into Gamma-ray's in the fireball model, unless the afterglow is driven not by the GRB but by a broadly-beamed, late-time or low Lorentz factor injection of energy on the order of the energy contained in the GRB. Furthermore, from a typically unchanging hardness ratio after $T_H$, 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 $t\approx 10^3-10^4$s. There are a handful of cases of very late time $t>10^4$s hardness evolution, which may point to residual central engine activity at very late time.