Formation of wind-captured disks in supergiant X-ray binaries

Abstract

Context. In Supergiant X-ray binaries (SgXB), a compact object captures a fraction of the wind of an O/B supergiant on a close orbit. Proxies exist to evaluate the efficiency of mass and angular momentum accretion but they depend so dramatically on the wind speed that given the current uncertainties, they only set loose constrains. Furthermore, they often bypass the impact of orbital and shock effects on the flow structure. Aims. We study the wind dynamics and the angular momentum gained as the flow is accreted. We identify the conditions for the formation of a disc-like structure around the accretor and the observational consequences for SgXB. Methods. We use recent results on the wind launching mechanism to compute 3D streamlines, accounting for the gravitational and X-ray ionizing influence of the compact companion on the wind. Once the flow enters the Roche lobe of the accretor, we solve the hydrodynamics equations with cooling. Results. A shocked region forms around the accretor as the flow is beamed. For wind speeds of the order of the orbital speed, the shock is highly asymmetric compared to the axisymmetric bow shock obtained for a purely planar homogeneous flow. With net radiative cooling, the flow always circularizes for wind speeds low enough. Conclusions. Although the donor star does not fill its Roche lobe, the wind can be significantly beamed and bent by the orbital effects. The net angular momentum of the accreted flow is then sufficient to form a persistent disc-like structure. This mechanism could explain the proposed limited outer extension of the accretion disc in Cygnus X-1 and suggests the presence of a disc at the outer rim of the neutron star magnetosphere in Vela X-1, with dramatic consequences on the spinning up of the accretor.

A model for high-mass microquasar jets under the influence of a strong stellar wind

“…When the companion is a massive star, these systems are called high-mass microquasars (HMMQs) and accretion onto the CO takes place as the latter captures a fraction of the stellar wind. This mechanism is compatible with the presence of an accretion disk around the CO (El Mellah et al 2019a), which is a necessary condition for jet formation. In HMMQs, interaction between the jets and the stellar wind may play an important role in both the propagation and the radiation produced by the jet, as the combined effect of the wind and orbital motion deviates the jets from a straight trajectory.…”

Wind Roche lobe overflow in high-mass X-ray binaries

“…Vela X-1 has dimensionless parameters which lie in the same ranges as P13, but has a comparatively low stellar massloss rate and might suffer several effects which prevents a significant fraction of the mass captured from eventually being accreted. For instance, hydrodynamics simulations within the Roche lobe of the compact object suggest that, when the flow cannot radiate away the energy it gains as it is adiabatically compressed and/or shocked, the effective mass accretion rate can be lowered by a factor of approximately ten compared to the mass capture rate µṀ computed here (El Mellah et al 2019). Since the wind speeds are comparable in ULXs and SgXBs, the flow density in Vela X-1 must be much lower than in P13 or M101, leading to a less efficient radiative cooling.…”

“…Tutukov & Fedorova (2016) noticed that in IC 10 X-1 and NGC 300 X-1, two systems where a stellar mass BH accretes from a Wolf-Rayet companion, no wind speed could explain both the accretion disk and the limited X-ray luminosity of 10 38 erg s −1 . The wind-RLOF mechanism, fully compatible with the formation of a wind-captured disk while not necessarily leading to large mass-accretion rates, solves this issue (El Mellah et al 2019).…”

“…Given the maximal µ we found at ∼20%, this analysis suggests that mass transfer via wind-RLOF in a ULX is only possible for stellar mass-loss rates above ∼10 −6 M yr −1 . We note here that matching a mass-transfer rateṀ high enough to reach ULX levels is a necessary but not sufficient condition: the final X-ray luminosity is determined by the physics at stake within the Roche lobe of the accretor, where the flow geometry can be disk-like (El Mellah et al 2019) or spherical (Shakura et al 2012) and where, for an accreting NS, the magnetosphere eventually dominates the dynamics (Bozzo et al 2008). For accreting BHs, powerful disk outflows come into play in the vicinity of the compact object, lowering the final mass-accretion rate (Pinto et al 2016).…”

High mass X-ray binaries: Beacons in a stormy universe

“…Calculating the wind mass transfer rate requires both atmospheric and hydrodynamical models of wind flows and radiative transfer codes (El Mellah, Kurfurst, Sander), but sophisticated three-dimensional models now exist that deal with the flows on scales from the orbital, through the accretion zone, and into the vicinity of the compact component. El Mellah et al (2019) show how such models predict that the wind-accreted gas in sgXRBs has sufficient angular momentum to create an accretion disk around the neutron star or black hole.…”