Search

The detection and characterization of the afterglow emission and host galaxies of short-hard gamma-ray bursts (SHBs) is one of the most exciting recent astronomical discoveries. In particular, indications that SHB progenitors belong to old stellar populations, in contrast to the long-soft GRBs, provide a strong clue about the physical nature of these systems. Definitive conclusions are currently limited by the small number of SHBs with known hosts available for study. Here, we present our investigation of SHBs previously localized by the interplanetary network (IPN). We show that the brightest galaxy within the error box of SHB 000607, at z = 0.1405, is the probable host galaxy of this event, expanding the sample of SHBs with known hosts and distances. We find a spatial association of the bright SHB 790613 and the cataloged position of the rich galaxy cluster Abell 1892. However, we are unable to verify the reality of this cluster via spectroscopy or multicolor imaging, and we conclude that this association may well be spurious. In addition, we rule out the existence of galaxy overdensities (down to ≈21 mag, i.e., ≈0.1 L* at z = 0.2) near the locations of two other SHBs and set a lower limit on their probable redshift. We combine our SHB sample with a complete sample of events discovered by the Swift and HETE-2 missions and investigate the properties of the extended sample. We show that the progenitors of SHBs appear to be older than those of Type Ia SNe, on average, suggesting a typical lifetime of several Gyr. The low typical redshift of SHBs leads to a significant increase in the local SHB rate and bodes well for the detection of gravitational radiation from these events, should they result from compact binary mergers, with forthcoming facilities.

High-resolution spectroscopic observations in the wavelengths of the atomic species Na I, Ca II, and Ca I and the molecular species CH and CH + have been conducted toward stars behind a variety of high Galactic latitude molecular clouds (HLCs), and 17 new detections of molecular absorption are reported. For 13 regions, multiple detections of Na I absorption lines are used to map the spatial extent and possible dynamics of the HLCs, and to constrain the distance and velocities of the clouds more accurately. New and improved distance estimates are reported for several clouds, including the Ursa Major complex MBM 29-MBM 31 and MBM 151, which are found to be at 100 pc < d < 120 pc, and d < 50 pc, respectively