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.
Highlights
According to the disc instability model (DIM; see Lasota 2001; Hameury 2020, for reviews of the model), accretion discs around compact objects are subject to a thermal–viscous instability if the rate at which matter is brought to their outer edge is less than a critical value strongly increasing with radius
We successfully extended and generalized the irradiated-DIM to the case of large accretion discs and super-Eddington accretion rates
Assuming that, during decay, the inner disc extending between the inner edge and the position of the cooling front is close to being steady with an accretion rate equal to the critical rate, we have been able to derive a relation between the peak accretion rate during an outburst and the maximum distance reached by the heating front, that closely matches the results of numerical simulations as long as the heating front does not reach the outer edge of the disc
Summary
According to the disc instability model (DIM; see Lasota 2001; Hameury 2020, for reviews of the model), accretion discs around compact objects are subject to a thermal–viscous instability if the rate at which matter is brought to their outer edge is less than a critical value strongly increasing with radius This basic tenet of the DIM has been convincingly confirmed by observations of dwarf novae (Dubus et al 2018) and transient X-ray sources (Coriat et al 2012). Since some of the usual, sub-Eddington X-ray transients show long-lasting outbursts (tens of years), this could be the case for some apparently steady ULXs
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