Supergiant fast X-ray transients and their place within the high mass X-ray binary population

Abstract

Supergiant Fast X-ray Transients (SFXT) are a class of HMXB that have only recently been unveiled by the wide field, high sensitivity monitoring of the Galactic Plane performed by the INTEGRAL observatory. Characterised by rapid X-ray flaring behaviour, on the order of hours, these extreme transients display X-ray dynamic ranges of 10⁴ - 10⁵ with peak outburst luminosities of ~10³⁶ erg s^-1. In this work, detailed studies of four of the twelve confirmed SFXTs are presented. Soft and hard X-ray observations are used to probe the nature of these individual sources and the accretion processes occurring within them. In each case the orbital period and hard X-ray outburst history is determined from INTEGRAL data. An RXTE study also discovered the likely neutron star spin period of IGR J17544-42619 and showed that a centrifugal barrier to the accretion flow may generate the deep X-ray quiescent states observed in this source. Orbital phase targeted INTEGRAL and XMM-Newton observations of IGR J16418-4532 allowed the detailed characterisation of this system. The detection of an X-ray intensity dip suggests that IGR J16418-4532 may undergo accretion regime transitions and implies the presence of a highly magnetised neutron star (B~10¹⁴ G). Through considerations of the orbital configurations of these systems it is concluded that the accretion processes occurring in SFXTs are heavily influenced by both the variable stellar wind environment and the action of higher order accretion processes, such as impeded accretion regimes and/or magnetic and centrifugal barriers. An assessment is made as to how the SFXTs may be reconciled with the classical, persistent supergiant X-ray binary population. Beyond the persistent versus transient segregation of the whole SgXRB population, a more physically meaningful divide may be drawn between systems that spend the majority of the time in an impeded or un-impeded accretion regime. Additionally, some members of the SFXT class may show a direct evolutionary link between HMXB systems with a main sequence and post main sequence companion for the first time.

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

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