A Multi-scale Radio Study of Gamma-ray Burst Afterglows

Abstract

The radio afterglows of gamma-ray bursts (GRBs) reveal important details about the properties of the blast wave and the circumburst environment, while also lending themselves as cosmic laboratories for studying physics in the most extreme environments of the Universe. In this thesis, I demonstrate how modern radio interferometry on different angular scales - from wide-field surveys conducted with the Australian SKA Pathfinder (ASKAP) to high-resolution observations performed through Very Long Baseline Interferometry (VLBI) - can help answer some of the important open questions about GRBs and their afterglows.

I present a targeted search for radio emission from all long GRB afterglows located in the Rapid ASKAP Continuum Survey footprint. I show the resulting detection of late-time radio emission from the low-luminosity GRB 171205A, the first confirmed detection of a radio afterglow in a wide-field radio survey, with subsequent observations and analysis revealing a circumburst environment shaped by pre-explosion stellar winds. I describe a novel matched-filter approach to finding orphan afterglows in the multi-epoch ASKAP Variables and Slow Transients Pilot Survey. The matched-filter search yielded five candidates, including one synchrotron transient candidate, VAST J175036.1−181454, which I interpreted as either a likely off-axis GRB or tidal disruption event. I outline the radio observing campaign for the ultra-long GRB 220627A, which included VLBI observations aimed at testing a possible gravitational lensing scenario. While the VLBI observations were inconclusive, partly due to insufficient sensitivity, the wider observing campaign helped to characterise the properties of the ultra-long GRB radio afterglow, of which only a handful have been previously studied. I conclude by summarising the body of work in this thesis and describing avenues of future research that have been enabled by this work.