The impact of thermal winds on the outburst lightcurves of black hole X-ray binaries

Abstract

Context. The observed signatures of winds from X-ray binaries are broadly consistent with thermal winds, driven by X-ray irradiation of the outer accretion disc. Thermal winds produce mass outflow rates that can exceed the accretion rate in the disc. Aims. We aim to study the impact of thermal wind mass loss on the stability and lightcurves of black hole X-ray binaries subject to the thermal-viscous instability, which drives their outbursts. Strong mass loss could shut off outbursts early, as proposed for the 2015 outburst of V404 Cyg. Methods. We use an analytical model for thermal (Compton) wind mass loss as a function of radius, X-ray spectrum and luminosity that has been calibrated against numerical simulations. We also estimate the fraction of the X-rays, emitted close to the compact object, that are scattered back to the outer disc in the wind. Scattering in the thermal wind couples irradiation to the disc size and inner mass accretion rate. The disc evolution equation are modified to include this wind mass loss and varying irradiation fraction. Results. Scattering in the strong wind expected of long P orb systems enhances the irradiation heating of the outer disc, keeping it stable against the thermal-viscous instability. This accounts very well for the existence of persistently bright systems with large discs such as Cyg X-2, 1E 1740.7−2942, or GRS 1758−258. Including mass loss from the thermal wind shortens the outburst, as expected, but insufficiently to explain the rapid decay timescale of black hole X-ray binary outbursts. However, including the wind-related varying irradiation fraction produces lightcurves with plateaus in long P orb systems like GRO J1655−40. Plateau lightcurves may be a dynamical signature of enhanced irradiation due to scattering in a thermal wind. Conclusions. Mass loss due to thermal winds is not a major driver for the outburst dynamics up to luminosities 0.1 − 0.2L Edd . Higher luminosities may produce stronger mass loss but their study is complicated since the wind becomes opaque. Magnetic winds, which extract angular momentum with little mass loss, seem more promising to explain the fast decay timescales generically seen in black hole X-ray binaries. Thermal winds can play an important role in the outburst dynamics through the varying irradiation heating. This may be evidenced by relating changes in wind properties, X-ray spectra or luminosity, with changes in the optical emission that traces the outer disc. Simulations should enable more accurate estimates of the dependence of the irradiation onto the disc as a function of irradiation spectrum, radius and disc wind properties.

Thermally driven disc winds as a mechanism for X-ray irradiation heating in black hole X-ray binaries: the case study of GX339–4

“…Evidence for this type of behaviour has been (i) to an extent, observed previously in BH-LMXBs XTEJ1817−330 and XTEJ1859+226, and (ii) recently predicted by Dubus et al (2019), who consider the impact a thermally-driven wind would have on BH-LMXB light-curves, in the context of the disc-instability model. By fitting an irradiated disc model to broadband SEDs observed throughout the 2006 outburst of XTE J1817−330, Gierliński et al (2009) found that while soft state observations were consistent with a near constant fraction of bolometric X-ray luminosity being reprocessed in the outer disc, the reprocessed fraction increased by a factor ∼ 6 as the source transitioned into the hard state.…”

“…While this thermal wind, predicted to be present in both the hard and soft accretion states, may not be a dominant mechanism for mass loss in this system, it does play an important role in the accretion process in terms of irradiating the disc. Using this wind as a medium in which to scatter X-rays back onto the disc, we are able to reproduce some of the features, independently predicted to be present (e.g., Kimura & Done 2019;Dubus et al 2019), in our computed C(t) outburst profiles (see Section 4.1). Such features include the peaks/drops in C occurring during accretion state transitions and complex (day-week timescale) variability.…”

“…It has slightly smaller best fit mass of 6.9M which is why this spectrum has higher L/L Edd ∼ 0.2 than the 0.1 for the soft state of GX339−4 shown in Figure 4. Dubus et al (2019) used the analytic estimates of scattering in a thermal-radiative wind to predict the time-series evolution of C, for a number of model BH-LMXB lightcurves with a range of orbital periods. They find complex variability in C throughout the outbursts, as the wind responds to changes in luminosity and spectral shape.…”

“…The fraction of intrinsic X-ray flux scattered in this Compton heated wind (C wind ) can be derived by integrating over the wind column density predicted by Done et al (2018), yielding (Dubus et al 2019),…”

A review of the disc instability model for dwarf novae, soft X-ray transients and related objects

“…The disc must therefore be warped, or the central irradiating source must be extended, or both. The source extension can be either due to the formation of a hot corona or to the existence of thermal winds as considered by Dubus et al (2019) that scatter the X-rays back to the disk. The problem then becomes extremely complex, and depends on many unknown parameters such as the shape of the disk or the source extent; Dubus et al (1999)…”