Insights into the Nature of Supergiant Fast X-ray Transients: Refining and Comparing Orbital and Emission Properties

Abstract

Supergiant Fast X-ray Transients (SFXTs) are a sub-class of High Mass X-ray Binary (HMXB) that has been extensively studied for nearly thirteen years. They are characterised by their fast transient behaviour and can reach luminosities of ∼ 1036ergs−1, displaying large dynamic ranges of up to 106 that can be realised in a matter of hours. In this thesis, detailed studies of two confirmed SFXTs are presented as well as a comprehensive analysis of the whole sub-class allowing for comparison between the individual members as well as with other HMXBs. Temporal analysis of all available INTEGRAL data revealed a 5.7195 ± 0.0007 day periodicity in the SFXT AX J1845.0−0433, which was interpreted as the orbital period of the system. The data was also utilised to determine a comprehensive X-ray outburst history for this source. A simultaneous XMMNewton and INTEGRAL study of the SFXT IGR J17354−3255 revealed that the X-ray continuum can be fitted by a consistent model throughout the observation, suggesting that the observed flux variability is due to obscuration from an inhomogeneous wind rather than changes in accretion mode. The INTEGRAL data rules out extrapolation of the simple powerlaw model beyond the XMM-Newton energy range suggesting that the most suitable model to describe this data is a simple blackbody. A new way of visualising SFXTs is introduced enabling direct comparison between the systems such as their X-ray emission behaviour and orbital geometry. The outburst pattern analysis revealed fluence distributions that place the confirmed SFXTs into two sub-groups. On the Corbet diagram the sources from one group seem to occupy the same region as the HMXB sub-class of BeXRBs whilst the other group occupies the same region as the wind-fed sgXRBs. This leads to the hypothesis that SFXTs are better categorised as special cases of already known HMXB sub-classes, rather than a distinct sub-class of their own, which is likely why they exhibit such a wide range of X-ray phenomena.

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ORCID for Antony Bird: orcid.org/0000-0002-6888-8937

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