Thermally driven winds in ultraluminous X-ray sources

Abstract

The presence of radiatively driven outflows is well established in ultraluminous X-ray sources (ULXs). These outflows are optically thick and can reprocess a significant fraction of the accretion luminosity. Assuming isotropic emission, escaping radiation from the outflow’s photosphere has the potential to irradiate the outer disc. Here, we explore how the atmosphere of the outer disc would respond to such irradiation, and specifically whether unstable heating may lead to significant mass loss via thermally-driven winds. We find that, for a range of physically relevant system parameters, this mass loss may actually switch off the inflow entirely and potentially drive limit-cycle behaviour (likely modulated on the timescale of the outer disc). In ULXs harbouring neutron stars, magnetic fields tend to have a slight destabilizing effect; for the strongest magnetic fields and highest accretion rates, this can push otherwise stable systems into the unstable regime. We explore the prevalence of the instability in a simulated sample of ULXs obtained from a binary population synthesis calculation. We find that almost all neutron star and black hole ULXs with Eddington-scaled accretion rates of $\dot{m}_0 < 100$ should be able to drive powerful outflows from their outer discs. Several known ULXs are expected to lie in this regime; the persistence of accretion in these sources implies the irradiation may be anisotropic which can be reconciled with the inferred reprocessed (optical) emission if some of this originates in the wind photosphere or irradiation of the secondary star.

Propeller states in locally supercritical ULXs

“…Should we be observing true propeller states in the ULXs studied in this paper, we must consider the process driving 𝑅 co to approach 𝑅 m . Should the accretion rate be constant over time (noting that there are means by which to modulate this at large radius: Middleton et al 2022), then the role of advection should be unchanged and 𝑅 co can only move inwards as the neutron star extracts angular momentum from the disc and spins up (noting that the accretion torque asymptotes to zero as 𝑅 m approaches 𝑅 co -Dai & Li 2006 -demanding that some other process triggers the onset of the propellor state itself). This would imply that 𝑅 co ≈ 𝑅 m and we should not observe propeller transitions with any preference for spectral appearance, instead spectral changes would need to be driven by precession (Middleton et al 2015a).…”

“…In the following, we will limit ourselves to the condition that the disc reaches the Eddington limit locally outside of R m , at the spherisation radius given by 𝑅 sph ≈ 𝑚 0 𝑅 in , where 𝑅 in is assumed to be the ISCO (whether reached by the disc or not, i.e. in the absence of truncating magnetic fields) and 𝑚 0 is the accretion rate at very large radius in Eddington units; this is typically the mass transfer rate from the donor star unless some mass is lost at large radius due to thermal winds (Middleton et al 2022). This condition (i.e.…”

The high energy X-ray probe (HEX-P): studying extreme accretion with ultraluminous X-ray sources

“…ULXs have been observed to change dramatically in brightness on timescales of days, weeks, and months (e.g., Walton et al, 2016a;Gúrpide et al, 2021a;. Possible causes of this variability include precession of the disk and wind (e.g., Pasham and Strohmayer, 2013;Middleton et al, 2015b;Luangtip et al, 2016;Amato et al, 2023) driven by a variety of processes [see the discussion in Middleton et al (2018Middleton et al ( ), (2019b], changes in accretion rate [for instance by the launching of a thermal wind at large radius (Middleton et al, 2022), or by modulating the mass transfer rate in a radiatively driven stellar wind], or the onset of a propeller state associated with a NS close to spin equilibrium. The latter has been extensively searched for (Earnshaw et al, 2018;Song et al, 2020), with the strongest evidence seen in the changing spin period of NGC 5907 ULX-1 (Fürst et al, 2023).…”

Orbital Decay in M82 X-2

“…Isotropic mass loss from the donor, instead, as one would expect from stellar winds, would have the opposite effect, expanding the orbit. Additional mass loss from the outer disk in the form of slow winds (e.g., Middleton et al 2022) would represent an intermediate case, carrying away specific angular momentum somewhere between that at the position of the neutron star and that at the first Lagrangian point, L1. Therefore, the estimate above is a lower limit to the mass transfer rate.…”