Search

We present the discovery and high signal-to-noise spectroscopic observations of the optical afterglow of the long-duration gamma-ray burst GRB070125. Unlike all previously observed long-duration afterglows in the redshift range 0.5 < z < 2.0, we find no strong (rest-frame equivalent width W > 1.0 A) absorption features in the wavelength range 4000 - 10000 A. The sole significant feature is a weak doublet we identify as Mg II 2796 (W = 0.18 +/- 0.02 A), 2803 (W = 0.08 +/- 0.01) at z = 1.5477 +/- 0.0001. The low observed Mg II and inferred H I column densities are typically observed in galactic halos, far away from the bulk of massive star formation. Deep ground-based imaging reveals no host directly underneath the afterglow to a limit of R > 25.4 mag. Either of the two nearest blue galaxies could host GRB070125; the large offset (d >= 27 kpc) would naturally explain the low column density. To remain consistent with the large local (i.e. parsec scale) circum-burst density inferred from broadband afterglow observations, we speculate GRB070125 may have occurred far away from the disk of its host in a compact star-forming cluster. Such distant stellar clusters, typically formed by dynamical galaxy interactions, have been observed in the nearby universe, and should be more prevalent at z>1 where galaxy mergers occur more frequently., 8 pages, accepted in ApJ

We present a preliminary analysis of an extensive set of optical observations of the Type Ia SN 2005hk. We show that the evolution of SN 2005hk closely follows that of the peculiar SN 2002cx. SN 2005hk is more luminous than SN 2002cx, while still under-luminous compared to normal Type Ia supernovae. The spectrum at 9 days before maximum is dominated by conspicuous Fe III and Ni III lines, and the Si II 6355 line is also clearly visible. All these features have low velocity (~6000 km/s). The near maximum spectra show lines of Si II, S II, Fe II, Fe III, as well as other intermediate mass and iron group elements. Analysis with the code for synthetic spectra SYNOW indicates that all these spectral lines have similar velocities., 6 pages, 3 figures, submitted to the proceedings of the conference``The Multicoloured Landscape of Compact Objects and their Explosive Origins'', 2006 June 11--24, Cefalu, Sicily, to be published by AIP

If the progenitors of gamma‐ray bursts (GRBs) are massive stars, then we expect the GRB rate to follow the cosmic star formation rate at least to moderate redshifts. Here we attempt to quantify selection effects in the measurement of GRB redshifts as a function of the GRB energy release, afterglow luminosity and redshift. We apply these to a sample of 24 GRBs with optical afterglows and measured redshifts, and present the resultant measurement of the GRB rate over 0.2 7 during the 3 year Swift mission. This rate is largely limited by the relatively poor sensitivity in the near‐infrared, but may be increased if small telescopes are coupled with more sensitive NIR imaging in order to uncover a larger fraction of GRBs beyond the most distant known objects at z > 6.5.

We have developed an automated system for the rapid observation and identification of gamma‐ray burst (GRB) afterglows. We expect to observe GRB localisations from the High Energy Transient Explorer within 2 minutes of the GRB, and automatically identify an optical transient within 10 minutes. Such early observations are expected to yield new information on the GRB phenomenon, constrain possible progenitor models and allow rapid spectroscopic follow‐up.