Short Gamma‐Ray Bursts with Extended Emission

TLDR

Short GRBs are linked to early‑type, low‑star‑formation galaxies, indicating a distinct progenitor from long bursts, but overlapping durations and occasional extended emission make classification ambiguous. The study finds that short GRBs are defined by a lack of spectral evolution above ~25 keV in both the initial spike and any extended emission, a pattern seen in 260 BATSE bursts and several Swift/HETE‑2 events, with spike‑to‑extended intensity ratios spanning up to 10⁴ and suggesting Lorentz factors of 500–1000.

Abstract

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 spike-like emission. 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 completely included the initial spike. The same signature obtains for one HETE-2 and six Swift/BAT short bursts. Analysis of a small sample of "short" BATSE bursts with the most intense extended emission shows that the same lack of evolution on the pulse timescale obtains for the extended emission. The dynamic range in the ratio of peak intensities, spike : extended, is ~ 10^4. For some bursts, the extended emission is only a factor of 2-5 less intense. A high Lorentz factor, ~ 500-1000, might explain the negligible lags observed in short bursts.