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 rate 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 the occasional presence of 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 ~ 25 keV. This behavior is nearly ubiquitous for the 260 bursts with T90 < 2 s where the BATSE TTE data type completely included the initial spike: Their spectral lags measured between the 25–50 keV and 100–300 energy ranges are consistent with zero in 90–95% of the cases, with most outliers probably representing the tail of the long burst class. We find the 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 where significant measurements can be made. One possible inference is that both emission components may arise in the same region. We also show that the dynamic range in the ratio of peak intensities, spike : extended, is ~ 10, and that for some bursts, the extended emission is only a factor of 2–5 less intense. 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, Γ ~ 500–1000, might explain the negligible lags observed in short bursts.