Short gamma-ray bursts with extended emission

The recent association of several short gamma-ray bursts ( GRBs) with early-type galaxies with low star formation rates demonstrates that short bursts arise from a different progenitor mechanism than long bursts. However, since the duration distributions of the two classes overlap, membership is not always easily established. The picture is complicated by occasional softer, extended emission lasting tens of seconds after the initial spikelike emission comprising an otherwise short burst. Using the large BATSE sample with time-tagged event ( TTE) data, we show that the fundamental defining characteristic of the short-burst class is that the initial spike exhibits negligible spectral evolution at energies above similar to 25 keV. The behavior is nearly ubiquitous for the 260 bursts with T(90) < 2 s for which the BATSE TTE data type completely included the initial spike. We find this same signature-negligible spectral lag-for six Swift BAT short bursts and one HETE-2 short burst. We also analyze a small sample of "short'' BATSE bursts-those with the most fluent, intense extended emission. The same lack of evolution on the pulse timescale obtains for the extended emission in the brighter bursts for which significant measurements can be made. We also show that the dynamic range in the ratio of peak intensities, spike: extended, is similar to 10(4). However, for our BATSE sample the total counts fluence of the extended component equals or exceeds that in the spike by a factor of several. A high Lorentz factor, similar to 500-1000, might explain the negligible lags.