ENABLING THE FULL CAPACITY OF MEERKAT FOR STUDYING X-RAY BINARIES
Creators: Hughe, Andrew K.
Publisher: name: University of Alberta Library
Year: 2024

<p>Accretion is ubiquitous in the Universe, observed in various systems spanning many orders of magnitude in their length and mass scales. The X-ray-bright inward-moving accretion flow is often accompanied by highly-relativistic radio-bright outflows that drive material away from the accretor, facilitating interactions with distant environments and the subsequent deposition of the outflowing energy and matter. This process, known as feedback, is critical in regulating the local evolution in these interaction regions. Thus, a comprehensive understanding of accretion flows and accretion-powered jets is crucial for building a coherent picture of our Universe. Despite the ubiquity of accretion and jets, we are still uncertain of the energetics of jets and the mechanisms that form and launch jets, warranting further high-quality monitoring. In this work, I present recent multi-wavelength monitoring campaigns of three outbursting X-ray binaries --- accreting stellar-mass black holes and neutron stars. X-ray binaries function as naturally occurring time domain laboratories with which we can study accretion-jet coupling.  These observations, taken as a part of the ThunderKAT and X-KAT collaborations, utilize quasi-simultaneous radio (taken with the radio interferometer MeerKAT) and X-ray observations (from various telescopes) to probe the time-evolution of outflows and inflows. In addition to the targeted observations, I discuss source-agnostic tools I developed to maximize the astrometric and polarimetric information we can derive from MeerKAT observations. My first target, the neutron star X-ray binary SAX J1810.8$-$2609, underwent a bright outburst in 2021. ThunderKAT monitoring revealed that  SAX J1810.8$-$2609 is a member of the sub-population of X-ray binaries that fail to undergo the standard outburst evolution through X-ray accretion states. Moreover, I detected persistent radio emission in the absence of X-ray emission, which I found was likely due to contamination from an unassociated foreground/background source. This thesis component revealed the need for high-precision astrometry, as the variable source position was the most substantial evidence for the unrelated background source, preventing over-interpretation of the ThunderKAT data. Thus, I developed (and later upgraded) a routine to quantify the astrometric precision in MeerKAT observations (\textsc{astkat}); I found that the current \textit{ad hoc} approach to astrometry significantly overestimates errors. The techniques in \textsc{astkat} are not MeerKAT specific and can be applied to other radio interferometers. High-precision astrometry is critical for the accurate modelling of jet propagation and interactions as this type of kinematic modelling that provides the best estimate for jet energetics and, thus, jet-based feedback as a whole. My second target, 1A 1744$-$361, another neutron star X-ray binary, showed evidence for ballistic outflows during the canonical state transition. While commonly observed in black hole X-ray binaries, my work shows that 1A 1744-361 provides the best evidence that these ejections also occur in standard neutron star X-ray binary systems, implying that both neutron stars and black holes have properties or processes conducive to jet ejections. While this thought was widely adopted, until now it has lacked empirical evidence. My third and final target, Swift J1727.8$-$1613, is a black hole X-ray binary that, in late 2023, underwent one of the brightest outbursts in the past decade. At this point in my research, I improved the ThunderKAT / X-KAT calibration and analysis pipeline (\textsc{polkat}) to handle polarisation observations. Swift J1727.8$-$1613 demonstrated exotic signatures in its photometric, polarimetric, and spatial properties, tied to repeated ejections of ballistic outflows. The utility of polarisation became immediately apparent as I detected, for the first time, evidence for a transient increase in the Faraday rotation measure during a ballistic ejection event, favouring an electron-proton (rather than an electron-positron) internal composition in X-ray binary ejecta. Since protons are much more massive than electrons, electron-proton jets have significantly more kinetic energy for feedback (at a fixed bulk velocity). Moreover, \textsc{polkat} can be readily adapted to any synthesis imaging polarization observations that aim to measure polarization properties using short tracks on interferometric arrays with linear polarization feeds. In addition to the MeerKAT array, the Advanced Atacama Large Millimeter Array, Australian Square Kilometre Array Pathfinder, and the Square Kilometer Array Mid are all examples of relatively new or soon-to-be-built interferometric arrays with linearly polarized feeds. </p>