Models of ultraluminous X-ray transient sources

Abstract

Context. It is now widely accepted that most ultraluminous X-ray sources (ULXs) are binary systems whose large (above 1039 erg s−1) apparent luminosities are explained by super-Eddington accretion onto a stellar-mass compact object. Many of the ULXs, especially those containing magnetized neutron stars, are highly variable; some exhibit transient behaviour. Large luminosities might imply large accretion discs that could be therefore prone to the thermal–viscous instability known to drive outbursts of dwarf novae and low-mass X-ray binary transient sources. Aims. The aim of this paper is to extend and generalize the X-ray transient disc-instability model to the case of large (outer radius larger than 1012 cm) accretion discs and apply it to the description of systems with super-Eddington accretion rates at outburst and, in some cases, super-Eddington mass transfer rates. Methods. We have used our disc-instability-model code to calculate the time evolution of the accretion disc and the outburst properties. Results. We show that, provided that self-irradiation of the accretion disc is efficient even when the accretion rate exceeds the Eddington value, possibly due to scattering back of the X-ray flux emitted by the central parts of the disc on the outer portions of the disc, heating fronts can reach the disc’s outer edge generating high accretion rates. We also provide analytical approximations for the observable properties of the outbursts. We have successfully reproduced the observed properties of galactic transients with large discs, such as V404 Cyg, as well as some ULXs such as M51 XT-1. Our model can reproduce the peak luminosity and decay time of ESO 243-39 HLX-1 outbursts if the accretor is a neutron star. Conclusions. Observational tests of our predicted relations between the outburst duration and decay time with peak luminosity would be most welcome.

A Chandra Virgo cluster survey of spiral galaxies. I. Introduction to the survey and a new ULX sample

“…The predicted number of old-population ULXs does, however, drop rapidly with age, decreasing by a factor of 20 from 1 to 5 Gyr after a star formation burst, and by another order of magnitude from 5 to 10 Gyr (Wiktorowicz et al 2017). In addition to those steady mass-transfer systems, old-population ULXs should include also transient systems with a large (outer radius 10 12 cm) accretion disk, subject to the thermal-viscous disk instability, which may exceed the Eddington limit during outburst (Hameury & Lasota 2020).…”

“…Instead, in old-population ULXs, the donor star may be a Roche-lobe-filling low-mass or intermediatemass star; such systems would be analogous to transient low-mass X-ray binaries. If the Roche lobe of the compact object, and therefore its accretion disk, is very large, the thermal-viscous disk instability is predicted to cause long outburst phases and super-Eddington peak luminosities (Lasota et al 2015;Hameury & Lasota 2020). Alternatively, even intermediate-and low-mass stars can go through evolutionary phases when they can transfer > 10 −6 ⊙ yr −1 onto their compact companion for a sustained period of time of ∼10 5 -10 6 yr (Wiktorowicz et al 2017;Misra et al 2020), for example when they cross the Hertzsprung gap or as they ascend the red giant or asymptotic giant branch.…”

Wind-powered ultraluminous X-ray sources

“…Some ULXs are transient (see, e.g. Earnshaw et al 2020;Brightman et al 2020) and if this luminosity variability is due to the disc thermal-viscous instability the MT rate should be lower than the value mentioned above (Hameury & Lasota 2020).…”

Wind-powered Ultraluminous X-ray Sources

“…Some ULXs are transient (see, e.g., Earnshaw et al 2020;Brightman et al 2020). If this luminosity variability is due to the disk thermal-viscous instability, then the MT rate should be lower than the value mentioned above (Hameury & Lasota 2020).…”