Testing precession of super-eddington flows in ultraluminous X-ray sources.

Abstract

This thesis presents an investigation into the subset of accreting compact objects known as ultraluminous X-ray sources (ULXs). Studies of ULXs suggest that they may exist in a distinct supercritical accretion state, differing from that found in galactic stellar-mass black hole binaries, explaining their higher observed luminosities (due to anisotropic emission). Supercritical accretion is known to be relevant in a wide variety of situations (tidal disruption events, narrow line Seyfert 1 AGN) and is the key ingredient in explaining the rapid growth of high redshift supermassive black holes. This thesis aims to address outstanding questions related to ULXs and extreme rates of accretion: super-critical accretion predicts anisotropic emission, which may vary due to precession of the accretion disc -- how does this affect the observed population of ULXs? We do not know the underlying demographic of ULXs which hinders an understanding of how super-critical accretion operates -- how do changes in the proportion of neutron stars to black holes in the underlying ULX population affect our observations? If precession occurs, it should leave a discernible imprint in the correlation (or anti-correlation) between the X-rays and UV emission -- is this observed? Through a combination of theoretical, numerical, and observational approaches, this work endeavours to obtain answers to the above questions. In Chapter three, I explore the role of precession and beaming in shaping the observed ULX population, incorporating the use of stellar population synthesis code and a geometrical model for a ULX. This work enables the study of synthetic ULX populations under various scenarios, which may be compared to observational metrics such as X-ray luminosity functions or state-of-the-art surveys such as eROSITA's eRASS. Overall, the results from this work revealed that our view of the observed population of ULXs may be significantly altered by both beaming and precession, and that several key parameters, namely the black hole percentage in the underlying population as well as the maximum precessional angle, may change the ratio of hidden, transient and persistent sources detected in ongoing all-sky surveys. This work will therefore be of importance for interpreting the results of eRASS. Chapter four presents an observational study of a sample of approximately forty ULXs using the Swift observatory. I make predictions for the relative UV/optical to X-ray emission that might arise under a variety of different scenarios, such as irradiation of the secondary star, outer disc or UV emission emerging from the wind photosphere. I subsequently test these predictions by first creating long-term Swift light curves in various energy bands (XRT and UVOT), and then search for first-order linear correlations between the light curves. The results show a variety of correlation types, including positive, negative, and non-linear correlations. I go on to demonstrate that linear correlation tests may not be sensitive enough to detect more complex patterns in the data, but those we detect in a number of important instances may point towards precession of the disc. In future, more complex models may better elucidate the nature of these correlations and constrain the geometry of the accretion flow. In an attempt to eliminate the ambiguity of astrophysical source names, I have taken the care to provide SIMBAD identifiers for source names mentioned in this thesis, these are available via hyperlinks in the PDF version of this document. Some of the work presented in this thesis has appeared in Khan et al. 2022, as well as in (Khan et al., 2023 under peer review).

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Identifiers

ORCID for Norman Khan: orcid.org/0000-0002-3955-0697

ORCID for Adam Hill: orcid.org/0000-0003-3470-4834

ORCID for Poshak Gandhi: orcid.org/0000-0003-3105-2615

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