We examine the effects of time dilation on the temporal profiles of gamma-ray burst (GRB) pulses. By using prescriptions for the shape and evolution of prompt gamma-ray spectra, we can generate a simulated population of single pulsed GRBs at a variet
y of redshifts and observe how their light curves would appear to a gamma-ray detector here on Earth. We find that the observer frame duration of individual pulses does not increase as a function of redshift as one would expect from the cosmological expansion of a Friedman-Lemaitre-Robertson-Walker Universe. In fact, the duration of individual pulses is seen to decrease as their signal-to-noise decreases with increasing redshift, as only the brightest portion of a high redshift GRBs light curve is accessible to the detector. The results of our simulation are consistent with the fact that a systematic broadening of GRB durations as a function of redshift has not materialized in either the Swift or Fermi detected GRBs with known redshift. We show that this fundamental duration bias implies that the measured durations and associated Eiso estimates for GRBs detected near an instruments detection threshold should be considered lower limits to their true values. We conclude by predicting that the average peak-to-peak time for a large number of multi-pulsed GRBs as a function of redshift may eventually provide the evidence for time dilation that has so far eluded detection.
I investigate the origin of the observed correlation between a GRBs nuFnu spectral peak Epk and its isotropic equivalent energy Eiso through the use of a population synthesis code to model the prompt gamma-ray emission from GRBs. By using prescriptio
ns for the distribution of prompt spectral parameters as well as the populations luminosity function and co-moving rate density, I generate a simulated population of GRBs and examine how bursts of varying spectral properties and redshift would appear to a gamma-ray detector here on Earth. I find that a strong observed correlation can be produced between the source frame Epk and Eiso for the detected population despite the existence of only a weak and broad correlation in the original simulated population. The energy dependance of a gamma-ray detectors flux-limited detection threshold acts to produce a correlation between the source frame Epk and Eiso for low luminosity GRBs, producing the left boundary of the observed correlation. Conversely, very luminous GRBs are found at higher redshifts than their low luminosity counterparts due to the standard Malquest bias, causing bursts in the low Epk, high Eiso regime to go undetected because their Epk values would be redshifted to energies at which most gamma-ray detectors become less sensitive. I argue that it is this previously unexamined effect which produces the right boundary of the observed correlation. Therefore, the origin of the observed correlation is a complex combination of the instruments detection threshold, the intrinsic cutoff in the GRB luminosity function, and the broad range of redshifts over which GRBs are detected. These simulations serve to demonstrate how selection effects caused by a combination of instrumental sensitivity and the cosmological nature of an astrophysical population can act to produce an artificially strong correlation between observed properties.
We investigate the nature of the mass-metallicity (M-Z) relation for long gamma-ray burst (LGRB) host galaxies. Recent studies suggest that the M-Z relation for local LGRB host galaxies may be systematically offset towards lower metallicities relativ
e to the M-Z relation defined by the general star forming galaxy (SDSS) population. The nature of this offset is consistent with suggestions that low metallicity environments may be required to produce high mass progenitors, although the detection of several GRBs in high-mass, high-metallicity galaxies challenges the notion of a strict metallicity cut-off for host galaxies that are capable of producing GRBs. We show that the nature of this reported offset may be explained by a recently proposed anti-correlation between the star formation rate (SFR) and the metallicity of star forming galaxies. If low metallicity galaxies produce more stars than their equally massive, high-metallicity counterparts, then transient events that closely trace the SFR in a galaxy would be more likely to be found in these low metallicity, low mass galaxies. Therefore, the offset between the GRB and SDSS defined M-Z relations may be the result of the different methods used to select their respective galaxy populations, with GRBs being biased towards low metallicity, high SFR, galaxies. We predict that such an offset should not be expected of transient events that do not closely follow the star formation history of their host galaxies, such as short duration GRBs and SN Ia, but should be evident in core collapse SNe found through upcoming untargeted surveys.
We present results of an extensive observing campaign of the short duration, hard spectrum gamma-ray burst (GRB) 070724A, aimed at detecting the radioactively-powered emission that might follow from a binary merger or collapse involving compact objec
ts. Our multi-band observations span the range in time over which this so-called Li-Paczynski mini-supernova could be active, beginning within 3 hours of the GRB trigger, and represent some of the deepest and most comprehensive searches for such emission. We find no evidence for such activity and place limits on the abundances and the lifetimes of the possible radioactive nuclides that could form in the rapid decompression of nuclear-density matter. Furthermore, our limits are significantly fainter than the peak magnitude of any previously detected broad-lined Type Ic supernova (SN) associated with other GRBs, effectively ruling out a long GRB-like SN for with this event. Given the unambiguous redshift of the host galaxy (z=0.456), GRB 070724A represents one of a small, but growing, number of short-hard GRBs for which firm physical/restframe quantities currently exist. The host of GRB 070724A is a moderately star-forming galaxy with an older stellar population component and a relatively high metallicity of 12+log(O/H)_KD02=9.1. We find no significant evidence for large amounts of extinction along the line of sight that could mask the presence of a SN explosion and estimate a small probability for chance alignment with the putative host. We discuss how our derived constraints fit into the evolving picture of short-hard GRBs, their potential progenitors, and the host environments in which they are thought to be produced.
