The observational phenomenology of a pulsating ultraluminous X-ray source

Abstract

The extreme apparent luminosities of ultraluminous X-ray sources suggests that they are powered either by sub-Eddington accretion onto intermediate mass black holes, or a population of super-critically accreting stellar mass compact objects. The latter hypothesis is thought to apply for the majority of known ULXs based on a wealth of growing evidence, including the recent discoveries of pulsating neutron star ULXs (or PULXs). Among the key questions driving on-going research are whether a majority of ULXs contain neutron star accretors, and how such objects can overcome the Eddington limit. In this thesis, we examine the nature of ULXs in the context of super-Eddington accretion. We report the discovery of X-ray pulsations in an archetypal ULX, NGC 1313 X-2, which adds to
the currently sparse number of PULXs. Notably, this object has a weaker pulsed fraction and a lower pulsation duty cycle compared to other sources, requiring frequent and long
exposure observations to be detected with current facilities, but would be a key science driver for future missions like Athena (see chapter 3). Further, we undertake multi-band (near-infrared to X-ray) observations of NGC 1313 X-2 to discriminate between the proposed physical mechanisms responsible for its optical emission. This is likely to have
contributions from multiple components, including the donor star and the X-ray irradiated accretion disc, and we use simultaneous HST and XMM-Newton data to disentangle these components. The results of this work are discussed in chapter 4. Finally, we perform a search for diffuse X-ray emission associated with shock-ionised ULX bubbles, driven by outflows launched from the ULX accretion disc. The aim was to investigate whether such a process can explain the narrow emission lines found in high-resolution XMM-Newton spectra of the particular ULX Holmberg IX X-1 (see chapter 2). We summarise our findings in chapter 5.