Pulsars, transients and relativistic astrophysics

Abstract:

In this thesis I will present observational studies of transient systems that produce mildly to extremely relativistic outflows through a coupling to an accretion flow. I will focus on the analysis of data taken from three source classes: black hole X-ray binaries (BHXRBs; particularly the system MAXI J1820+070), gamma-ray bursts (GRBs; particularly the system GRB 171010A), and tidal disruption events (TDEs; particularly the systems ASASSN-14li and Swift J1644+57).

I will present an extensive radio monitoring campaign on MAXI J1820+070 utilising five different interferometers, along with extensive X-ray observations, during the system's 2018 outburst. Together these data allowed me to probe the coupling between accretion and jet production throughout an entire outburst cycle, as well as during multiple hard accretion state only re-brightenings, tracked over a two year time frame. As MAXI J1820+070 transitioned from the hard to soft accretion state, contemporaneous time-series indicators of the launch of bipolar relativistic ejections were observed at both radio and X-ray frequencies (manifesting as a radio flare and an evolving quasi-period oscillation, respectively). I then confirmed the presence of these ejecta utilising multiple interferometers, and was able to track the evolution of both the approaching and receding ejecta for over ~150 d. Through utilising interferometers sensitive to very different angular scales, I was able to infer the internal energy of the ejection, and found it to be much larger than the value implied from the state transition radio flare. This is strong evidence for ongoing particle acceleration as ejections interact with the surrounding interstellar medium. In addition to the study of MAXI J1820+070, I will also present a broader population study of state transition radio flares from black hole X-ray binaries, and demonstrate that commonly employed models (which attribute flares to an optical depth evolution from an expanding region) are not appropriate for the majority of flares in the sample studied. I describe the ability of extended periods of particle acceleration to explain the flare profiles.

GRB 171010A was a luminous and nearby long GRB detected at early times by the Arcminute Microkelvin Imager Large Array interferometer, as well as with the Swift X-ray telescope. Long GRBs produce highly relativistic outflows that are best studied through their interaction with the circumburst material. This interaction produced a broadband synchrotron afterglow. I present a study of the radio through X-ray afterglow of GRB 171010A in the context of the fireball model (which details the interaction of the jet and interstellar medium). By fitting the time evolving spectra, the values and evolution of the characteristic synchrotron frequencies can be inferred. GRB 171010A is one of the most energetic GRBs detected below z~0.5, allowing for our theoretical understanding of afterglows to be investigated. While I find general agreement with the canonical models (particularly the spectral indices either side of the minimum energy frequency) a number of deviations are seen. I discuss possible solutions to these deviations, which likely include the addition of a second spectral component resulting from a reverse shock.

Finally, I will present late time radio and X-ray observations of the thermal TDE ASASSN-14li, and late time radio only observations of the relativistic TDE Swift J1644+57. Tidal disruption events occur when a star passes too close to a supermassive black hole and is torn apart by tidal forces. Approximately half of the stellar material is accreted and the rest is unbound. ASASSN-14li is a radio bright thermal TDE, and the origin of this radio emission is disputed. I will show that the late time radio properties are now consistent with background AGN activity, but that while the TDE was the dominant radio component, the radio emission was correlated with the X-ray emission. This provides evidence that thermal TDEs produce jets. It is not disputed that the relativistic TDE Swift J1644+57 produced a jet, and said jet's radio emission has now been monitored for ~10 yrs. I will present the most recent monitoring of Swift J1644+57 in the context of previously proposed jet models for the source. I will additionally discuss the up-to-date population of radio loud TDEs.

My conclusions contain a comparison of the outflows produced by these sources, and how they are analysed in different frameworks.

Abstract:

Abstract:

Abstract:

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.