Optical & X-ray Studies of Ultraluminous X-ray Sources

Abstract

Ultraluminous X-ray Sources (ULXs) are point like X-ray sources situated external to the nucleus of their host galaxy, with inferred X-ray luminosities in excess of 10^ erg s^ . Although first observed ∼ 30 years ago, these sources are yet to be fully understood. Some have suggested that these fascinating objects may contain intermediate mass black holes, whilst others have proposed they are stellar mass black holes residing in a new extreme accretion state. This thesis works towards the conclusion of this debate, by developing our understanding of these systems and their environments.

This work begins with a photometric survey of the optical counterparts of ULXs. The main aim of this survey is to find plausible candidates to gain radial velocity measurements and therefore mass functions of these systems. However, the collation of this sample also provides the opportunity to classify the stellar objects held within these systems. From this work, we find seven good candidates for optical spectroscopic follow-up. Our results also show that many of our sample are consistent with OB type stars, while some contain later type bright giants/supergiants.

Possibly our best chance to gain precise measurements of M_, and settling the debate over the nature of these systems, is by using radial velocity curves of their optical counterparts to calculate a mass function of ULXs. We are currently undertaking a programme to pursue mass function measurements for these systems. To date, we have received the pilot spectra of three optical counterparts.
We discuss the progress of this programme to date and perform analysis on both the absorption/emission features and the continuum of these spectra. Initial analysis reveals the presence of the He ii 4686 A line in two of our pilot spectra. This line may be associated with the accretion disc of these systems, and could therefore be used in our pursuit of the mass function. We also find the presence of both low and high ionization lines, with some
evidence for shock ionisation, and electron temperature of 7,000 – 10,000 K. This Balmer decrement also indicates that the extinction can be highly variable across ULX field. This
combination may suggest a ‘patchy’ environment with separate shock and photoionisation emission regions. While the continuum emission of one of our sample can be explained by either the spectra of an OB star or of a standard accretion disc, the steep slopes of two of our sample indicates non-stellar origins that could represent the optical spectrum of a super-Eddington accretion disc. Finally, this work highlights the need for further observations of these sources in order to unlock their nature.

We present results of X-ray spectral variability studies of the ULX population contained within NGC 4485 & 4490. We collate Chandra and XMM-Newton observations of the interacting galaxy pair, to analyse the emission from the six ULXs previously identified, and one additional source observed in the a recent exposure. This provides us with
an opportunity to study variability on both short and longer time scales. The spectral variability is generally characterised by a hardening of the source spectra as their luminosities increase. The sources show a variety of long-term light curves; however, short-term (intra-observational) temporal variability is conspicuous by its absence. This survey also reveals the detection of a possible change in accretion state that could be used to gain crude mass estimates of the compact objects. Finally, we explore further the variability of these systems with the aid of two new proprietary observations.

Finally, analysis of some of the best quality X-ray spectral data publicly available on these sources has provided the opportunity to explore the nature of these systems. We apply phenomenological models to characterise the spectra of these objects and more physically motivated models in order to explore the physical processes underlying these characteristics. Results show that the spectra of these sources are fundamentally different to that of Galactic X-ray binaries, whilst the application of physical models indicates a more extreme version of the highest known luminosity state, the very high state. We
therefore speculate that in observing ULXs we are observing stellar mass black holes residing in a new ‘ultraluminous’ state.