The astrophysics of relativistic radio transients
Keywords: high energy astrophysics
radio astronomy

Astrophysical jets are ubiquitously associated with the most energetic phenomema in the Universe. In this thesis, I will present and discuss radio observations of two types of transient systems: neutron star X-ray binaries and gamma-ray bursts (GRBs).
In Chapter 2, I present the basics of radio interferometry: the method by which I collect data for this thesis. This is explained using a two dish interferometry. I then go on to explain the data reduction process whereby voltages from individual antennas can form an image of the sky. Finally, the observing facilities I have used for the work in this thesis are presented.
In Chapter 3, I present the last three years of GRB follow-up observations with the AMI-LA telescope. Several radio sources coincident with GRBs were detected with the AMI-LA telescope. I discuss two cases where a variable, radio-bright host galaxy was brightest than any afterglow emission preventing such a detection. I also discuss a single long GRB afterglow detection which I interpret as forward shock emission from a jet in an homogeneous environment. Finally, I discuss the results of the most up to date and complete radio monitoring campaigns of short GRBs that I have performed over the last three years. I have observed all short GRBs over the last three years on timescales of days with \textit{e}-MERLIN in the north, and MeerKAT in the south. Of these triggers, two radio counterparts have been detected. The first was associated with short-duration GRB 200826A (Rhodes et al 2021). Detections of a varying radio counterpart with \textit{e}-MERLIN confirmed that this source was the afterglow. I combined the 5GHz e-MERLIN light curve with data from \textit{Swift}-XRT and interpreted it in two separate scenarios. The first scenario used to describe the afterglow uses a transition from an optically thick to thin regime. The second scenario requires the presence of a jet break. I rule out the second scenario on the lack of a jet break in the X-ray data. Both the radio and X-ray data are consistent with a stellar wind environment and therefore inconsistent with a binary neutron star progenitor. The second counterpart was associated with short-duration GRB 210726A. The light curves show a sharp delayed rise, it is the longest detected cosmological short GRB to date, followed by an achromatic break. The broadband radio spectra show that the low-frequency emission is synchrotron self-absorbed. GRB 210726A so far, appears to be a cosmological analogue of gravitational wave event GW 170817.
In Chapter 4, I discuss a newly discovered sub-group of long-duration GRBs that have very high energy (VHE) counterparts. I have collected multi-band data on three of the five VHE GRBs and present the interpretations here (Rhodes et al, 2020; 2022a). All three events shows strong evidence of a forward shock component. Additionally, in the radio afterglow light curves of GRB 190829A I demonstrate the possible presence of a second shock: a reverse shock. Furthermore, the 15.5GHz radio light curve from GRB 190829A is one of the best, highest cadence radio light curves of any GRB afterglow. The data set for GRB 201216C had sparse coverage, and as a result I was able to demonstrate how flexible afterglow models are. I show that at 10s of days after the burst the jet launched gives way to a much wider, less energetic cocoon which is predicted in simulations. Finally, I present the beginning of a study into understanding whether the VHE GRBs are a separate population of GRBs or do all GRBs produce such high-energy photons. This is done by studying the luminosity functions of the VHE GRB population and comparing them to a flux limit sample as well as examining the variations in afterglow properties across the group.
In Chapter 5, I present the results of a long-term radio and X-ray monitoring campaign of a newly discovered neutron star X-ray binary Swift J1858.6-0814 (hereafter, J1858, Rhodes et al 2022b). J1858 went into outburst in late 2018, it remained radio-bright (i.e. in the hard state) for 18 months before undergoing a rapid transition to the soft state. I tracked the outburst of J1858 with radio interferometers AMI-LA and MeerKAT throughout the outburst. The radio emission was consistent with a compact, self-absorbed jet. When the X-ray and radio emission from J1858 is compared to other X-ray binaries, it is one of the most radio-luminous neutron star X-ray binaries.
The research presented in this thesis has demonstrated the broad range of astrophysical knowledge of both jets, their environments and stellar evolution, that can be extracted from radio transients both within the Milky Way and at extra-galactic distances. I will use this understanding to explore links between X-ray binary and gamma-ray burst jets in the future along with applying blast wave models to transient radio emission from tidal disruption events.