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Short-hard GRB: I. host galaxies & redshift distribution
It has been suspected for many years, and confirmed after the first year of BATSE data, that GRBs have a bimodal distribution in the duration and hardness of their prompt emission, dividing into long bursts with a softer spectrum (on average), and short-hard bursts (SHBs). The detection of afterglows of long GRBs in 1997 has led to considerable progress in our understanding of this class of GRBs, establishing their (cosmological) distances, energies, event rates, and several years later also their predominant progenitors. The afterglows of SHBs, however, were detected only 8 years later (in 2005), thanks to the launch of Swift, whose rapid and deep X-ray observations were finally able to detect their dimmer and more elusive afterglows. This has lead to the detection of several tentative host galaxies and redshift measurements for a handful of SHBs, which show that while SHBs occur at cosmological distances, their typical redshift is smaller than that of long GRBs (with a median of z~0.25 for SHBs compared to z~2.5 for long GRBs with Swift). Furthermore, their host galaxies are of both late type and early type, in stark contrast to the hosts of long GRBs which are all of late type, and with a much smaller specific star formation rate compared to long GRBs. There are also very good constraints against a potential accompanying supernova. This provides strong evidence that SHBs arise from different progenitors than long GRBs. In this meeting we will discuss the detection of host galaxies of SHBs (the host types, star formation rate, location of the SHBs relative to the hosts, and how secure the host associations are), as well as the implications of their (still small number) redshift distribution.
- Recommended Reading:
- (Nakar 2006): this is a very good new review on short-hard GRBs (SHBs). for this meeting only sections 1 & 2 are recommended, and especially subsections 2.3 and 2.4.
- (Prochaska et al. 2006): analyses the prospective host galaxies and large scale environments of the first 4 SHBs with detected afterglows, and contrasts them with those of long GRBs.
- (Fox et al. 2005): reports detailed observations of SHB 050709 and nicely compiles data for all 4 first SHBs with detected afterglows, comparing their properties to long GRBs.
- (Nakar et al. 2006): discusses the implications of the SHB redshift distribution.
- Additional Reading (not required):
- (Berger et al. 2006): discusses new evidence that a large fraction of the observed short-hard GRBs are from higher redshifts than previously thought on the basis of the first several redshift determinations.
- (Gehrels et al. 2005): reports the first detection of the (X-ray) afterglow of a short-hard GRB (050509B), and its tentative host galaxy - a giant elliptical at z=0.225.
- (Bloom et al. 2006): reports the first tentative host galaxy of a SHB (050509B), a giant elliptical galaxy at z=0.2248, that resides within a galaxy cluster, and draws tentative conclusions from its prompt and afterglow emission (e.g. that the efficiency of the gamma-ray emission is similar to that of long GRBs).
- (Hjorth et al. 2005a): constrains the optical emission from 1.85 days to 23 days after GRB 050509B (the first SHB with an afterglow), and constrains mini-supernova type emission that might be expected in binary mergers.
- (Hjorth et al. 2005b): reports the first detection of an optivcal afterglow from a SHB (050709).
- (Berger et al. 2005): reports the first radio afterglow from a SHB (050724) which is associated with an elliptical galaxy at z=0.257.
- (Covino et al. 2006): present a spectral analysis of the host of SHB 050709 which is of early type with relatively large star formation rate (SFR) for a SHB, demonstraiting the diversity of their hosts (both eraly and late types with a wide range of SFR).
- (Bloom et al. 2006): discusses the possible host galaxy of SHB 060502B, which if true implies a large offset of ~70 kpc. The probability that this is indeed the true host galaxy is estimated to be ~90%, demonstrating the difficulty in assinging a host galaxy in cases where the offset relative to the host is large (possibly causing some bias in favor of detecting small offsets).
MASS: Doug Applegate