The host galaxies and progenitors of core-collapse gamma-ray bursts

• Main Author: Chrimes, Ashley
• Format: Dissertation
• Language: English
• Published: University of Warwick 2020
• Subjects: QB Astronomy

Ever since their discovery over 50 years ago, gamma-ray bursts (GRBs) have been the subject of intense interest in transient astronomy. Despite being extremely luminous, their distance and transient nature have made studies challenging. By investigating their galactic environments, lightcurves and spectra, a consensus has arisen that long-duration (> 2s) GRBs are highly beamed relativistic explosions, arising from the collapse of massive, fast-spinning, stripped-envelope stars. However, the exact nature of their central engines and progenitor systems remain mysterious, not least due to the variety of environments they inhabit. The challenge is to explain all of the observed properties of long GRBs and their host galaxies simultaneously. In this thesis, I present multi-wavelength observational studies of unusual GRB host galaxies, the hosts of unusual GRBs, and a theoretical investigation into the progenitor systems of long GRBs using binary population synthesis techniques. I begin by characterising a population of GRB hosts which are detected in the WISE infrared all-sky survey. It is pertinent to ask whether such a sub-population of galaxies have physically distinct properties. Making use of specifically obtained data, in addition to a wealth of archival imaging, I show that _50% of the WISE-GRB associations are chance alignments, and that the genuine galaxy matches are in the dusty and nearby tails of the established GRB host distributions. I then describe an investigation into dark GRB host galaxies. Dark GRBs lack the optical emission expected from the spectral extrapolation of their X-ray afterglows. In this work, I use data from the Hubble Space Telescope and Chandra X-ray Observatory to measure how dark GRBs trace their host galaxy light. The findings indicate that dark bursts are not tracing light in a significantly different way to optically bright GRBs, and that at most _20% of dark GRBs occur at redshift > 5. It also suggests that the dust within lower redshift dark GRB hosts is uneven on sub-galactic scales, due to the absence of a morphological difference between the hosts of optically-dark and optically-bright GRBs. A follow-up study on GRB100205A, whose host is undetected in both the observed NIR and optical HST bands, is performed. This burst's afterglow indicates that it lies at redshift > 4:5, consistent with the host non-detections. This adds GRB100205A to the small sample of GRBs known at these redshifts. The fact that this burst had not already been characterised demonstrates the important of rapid, multi-wavelength follow-up for unambiguous high-redshift GRB identifications. Finally, I use BPASS (Binary Population and Spectral Synthesis) to model the occurrence rates of long GRBs from rapidly-spinning, stripped envelope progenitors. I introduce tidal interactions to the BPASS model outputs, and track stellar spins across a grid of models in binary and metallicity parameter space. A dual pathway model (of accretion and tidally driven spin) can reproduce the observed rates, the host galaxy metallicities, and theoretical GRB requirements for the angular momenta of accretion discs around newly formed black holes. Next-generation facilities, combined with new observational tests of these and other proposed models, will help to narrow the gap between GRB observation and theory over the next decade.