Dashwood Brown, Cordelia (2025) State of the art constraints on black hole formation: The story of black holes told with high-precision kinematics. University of Southampton, Doctoral Thesis, 206pp.
Abstract
Black holes have been the subject of study by mathematicians and physicists alike for over 100 years, though the challenges associated with observing them mean they remain the most enigmatic objects in the Universe. This is an era of astronomy defined by unprecedented access to data, including groundbreaking astrometric surveys and advanced spectrographs. Such high-precision kinematic measurements offer insight into the most fundamental characteristics of black holes: their masses, through the orbital motion of companion stars; and their velocities, through analysis of their motion with respect to the Galactic potential. These observations act as keystones, around which to build and develop theory, enabling a deeper understanding of physics in the most extreme environments.
Taking advantage of groundbreaking astrometric surveys, this thesis presents the most extensive study of X-ray binary natal kicks to date, analysing 68 X-ray binaries including both neutron stars and black holes. This investigation finds that, contrary to theoretical predictions, the natal kicks applied to neutron stars and black holes are formally indistinguishable. This suggests that natal kicks are governed by similar physics irrespective of remnant type, and necessitates a revision of supernova hydrodynamic theory. With these results comes an observationally motivated distribution for implementation in future models: a Gamma distribution with mean 147 km/s. This prescription is agnostic to the drivers of these natal kicks, thereby removing a key degeneracy in population synthesis studies.
The biggest caveat to this study, and to much of the research on compact objects, is that the vast majority of confirmed black holes exist in X-ray binaries, meaning they are actively accreting matter from a stellar companion. In light of this, high-precision astrometry and dynamical observations were used to identify and characterise a small sample of black hole candidates. These putative black holes were identified as ordinary stellar binaries, highlighting the complexities of astrometric studies of two-component systems. Nevertheless, the number of confirmed non-interacting black holes is growing, providing further constraints on black hole evolution beyond the X-ray binary population.
An additional issue in the study of black holes is the difficulty of observing their luminous companions (and, consequently, determining the mass of the compact object) in the heavily extincted regions within the Galactic plane. The development of microcalorimeters marks a significant step forward in X-ray astronomy and presents an opportunity to study new high-precision observations in the X-ray waveband, offering the potential to probe regions that are significantly obscured in the optical. The narrow component of the iron fluorescent line, originating from the X-ray irradiated companion, provides a potential means to further constrain the binary system properties, including the mass of the compact object. Modelling the geometry of the binary system and considering the composition of the stellar surface indicates the equivalent width of this narrow iron line should be 2-40 eV (dependent on the binary mass ratio). The precision of cutting-edge X-ray spectrographs means velocity deviations can be measured within < 40 km/s (depending on X-ray flux), enabling dynamical mass measurements of systems which are inaccessible with optical and infrared observations.
State of the art observing instruments and techniques provide exceptionally precise measurements of the kinematics of black holes, which in turn offer insight into their velocities and provide dynamical measurements of their masses. The mass and velocity distributions of black holes are intrinsically linked to their formation processes and subsequent evolution, many aspects of which remain uncertain, and have appreciable implications for gravitational wave astronomy. This thesis aims to highlight the interconnection of observation and theory, with the ultimate goal of understanding the genesis of black holes.
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