Multiwavelength observations of the black hole transient Swift J1745-26 during the outburst decay

We characterized the broad-band X-ray spectra of Swift J1745-26 during the decay of the 2013 outburst using INTEGRAL ISGRI, JEM-X and Swift XRT. The X-ray evolution is compared to the evolution in optical and radio. We fit the X- ray spectra with phenomenological and Comptonization models. We discuss possible scenarios for the physical origin of a ~50 day flare observed both in optical and X- rays ~170 days after the peak of the outburst. We conclude that it is a result of enhanced mass accretion in response to an earlier heating event. We characterized the evolution in the hard X-ray band and showed that for the joint ISGRI-XRT fits, the e-folding energy decreased from 350 keV to 130 keV, while the energy where the exponential cut-off starts increased from 75 keV to 112 keV as the decay progressed.We investigated the claim that high energy cut-offs disappear with the compact jet turning on during outburst decays, and showed that spectra taken with HEXTE on RXTE provide insufficient quality to characterize cut-offs during the decay for typical hard X-ray fluxes. Long INTEGRAL monitoring observations are required to understand the relation between the compact jet formation and hard X-ray behavior. We found that for the entire decay (including the flare), the X-ray spectra are consistent with thermal Comptonization, but a jet synchrotron origin cannot be ruled out.Comment: Accepted for publication by MNRA

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

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.<br/