Observing run: Radio observations of Gamma-Ray Burst host galaxies with the Australian ATCA radio telescope
Final Report Abstract
Gamma-Ray Bursts (in the following GRBs) are the most luminous electromagnetic phenomena in the universe. They separate into two classes, long (> 2 s) and short (<= 2 s). Long bursts are due to the gravitational collapse of very massive stars, short bursts most likely signal the merger of two compact stellar objects. The latter were the target of this observing run, more precisely their host galaxies. GRB host galaxies: Short GRBs occur in all morphological types of galaxies, from elliptical to irregular. About 2 dozen have been identified so far. The picture emerges that the hosts of short bursts are not primarily passively evolving ellipticals. In about 50% of the cases they are actively star-forming galaxies. Some of them have star-formation rates (SFRs) even exceeding 10 solar masses per year. The latter is surprising and triggers the question whether the short-burst progenitors hosted in these star-forming galaxies belong to a young stellar population, i.e. whether they are just millions of years old. This is contrary to the short-burst progenitors hosted in ellipticals, which are obviously members of an old stellar population (i.e., they are billions of years old). In order to go deeper into this question, we have to measure the unobscured SFR in these host galaxies. Scientific background of this observing run: Contrary to optical light, radio waves do not suffer from dust extinction and provide an unobscured view on the star-formation activity within galaxies. Semi-empirical relations link the radio continuum flux of a galaxy to its SFR averaged over about 100 million years. The goal of this observing run was to measure the radio continuum flux of a small number of host galaxies of short bursts, hosts which are not elliptical galaxies. The radio observations were performed end of July 2013 in the ATCA operation facilities near Narrabri, Australia, about 550 km west of Sydney. We observed six host galaxies of short bursts with redshifts z<0.5 in the 5.5 and 9.0 GHz bands using all six antennae. Nearly 70 hours of telescope time were used, nearly 90 Gb of raw data were obtained. Data reduction and scientific results: Data reduction has to be performed using the MIRIAD data reduction package for ATCA radiointerferometry data. For beginners (optical observers as we are), this data reduction is not an easy task. While it can be automated to a certain degree, the results are still very sensitive on the details of the procedure, in particular since it is our goal to reach a flux density limit in the order of 10 Jy, which would allow us to constrain (or to detect) SFR in our targets between about 1 and 15 solar masses per year.
