2015ApJ...811....4K

Extended emission in short gamma-ray bursts (SGRBs) is a mystery. By conducting time-resolved spectral analyses of the nine brightest events observed by the Swift-XRT, we classify the early X-ray emission of SGRBs into two types. One is the extended emission with exponentially rapid decay, which shows significant spectral softening for hundreds of seconds after the SGRB trigger and is also detected by the Swift-BAT. The other is a dim afterglow that only shows power-law decay over 10⁴ s. The correlations between the temporal decay and spectral indices of the extended emissions are inconsistent with the α-β correlation expected for the high-latitude curvature emission from a uniform jet. The observed too-rapid decay suggests that the emission is from a photosphere or a patchy surface, and manifests the stopping via a central engine such as magnetic reconnection at the black hole.