Abstract
We present deep optical and infrared observations of the short duration GRB 050906. Although no X-ray or optical/IR afterglow was discovered to deep limits, the error circle of the GRB (as derived from the Swift BAT) is unusual in containing the relatively local starburst galaxy IC328. This makes GRB 050906 a candidate burst from a soft-gamma repeater, similar to the giant flare from SGR 1806-20. The probability of chance alignment of a given BAT position with such a galaxy is small ( � < 1%), although the size of the error circle (2.6 arcminute radius) is such that a higher z origin can’t be ruled out. Indeed, the error circle also includes a moderately rich galaxy cluster at z = 0.43, which is a plausible location for the burst given the apparent preference that short GRBs have for regions of high mass density. No residual optical or infrared emission has been observed, either in the form of an afterglow or later time emission from any associated supernova-like event. We discuss the constraints these limits place on the progenitor of GRB 050906 based on the expected optical signatures from both SGRs and merging compact object systems.
Highlights
Until recently the revolution of our knowledge of gammaray burst (GRB) sources was limited almost exclusively to those with durations of t90 >2s – so called long bursts
The bright, nearby galaxy IC 328 lies within its positional error circle and makes a good case for a short GRB associated with an soft gamma-ray repeater (SGR) giant flare
In particular these may be responsible for the fraction of short-duration GRBs (S-GRBs) in the local universe reported by Tanvir et al (2005), and for the two subsequently detected IPN bursts which may have originated from M81/82 and M31
Summary
Until recently the revolution of our knowledge of gammaray burst (GRB) sources was limited almost exclusively to those with durations of t90 >2s – so called long bursts (see e.g Meszaros 2006 for a review). The discovery of a massive flare from soft gamma-ray repeater (SGR) 1806-20 (Hurley et al 2005; Palmer et al 2005) provided evidence that some fraction of the large sample of S-GRBs found by the Burst And Transient Source Experiment (BATSE) could be explained by SGR giant-flares in galaxies out to ∼ 30 − 40 Mpc, and potentially further with Swift. To date it has not been possible to identify with high confidence the individual host galaxies of short bursts which may be due to SGR giant flares, due to the large error regions associated with BATSE bursts and the relative dearth of smaller error boxes from eg. The bright, nearby galaxy IC 328 lies within its positional error circle and makes a good case for a short GRB associated with an SGR giant flare.
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