Bimodality of wind-fed accretion in high-mass X-ray binaries

Abstract

We study an influence of X-ray photo-ionization from an accreting neutron star in a high mass X-ray binary. Our aim is to unveil a new principle governing X-ray luminosities of X-ray binaries, with a simple analysis of fluid equations simulating line-driven wind flow under influence of X-ray irradiation. In this study, we solve equation of motion of the accretion flow taking into account the line-driven acceleration and X-ray photo-ionization. Under the influence of X-ray irradiation, we find the flow equations take two types of solutions. The first solution is characterized by a slow wind velocity which causes a large accretion rate. The second solution is a fast wind flow which results in a small accretion rate. We find that only the solution with a fast wind and faint X-ray luminosity is a steady solution. On the other hand, slow wind solution with a large X-ray luminosity is not a realizable solution. In bright X-ray binary systems, X-ray luminosity would increase until strong X-ray reduces the line-driven acceleration and causes a stagnation of the wind. This result implies an important consequence that the X-ray luminosity of the wind-fed, X-ray emitting binary will be settled by the wind stagnation limit. At the same time, the fast wind solution with small X-ray luminosity also can be a steady state. Bright X-ray sources, such as Vela X-1, would have limiting luminosities of wind stagnation, while faint systems such a quiescent Super giant Fast X-ray Transients could be on the faint solutions.

Measuring the stellar wind parameters in IGR J17544-2619 and Vela X-1 constrains the accretion physics in supergiant fast X-ray transient and classical supergiant X-ray binaries

“…We suggest that it might be the reason why IGR J17544-2619 shows higher ∞ than Vela X-1. X-ray action can also have an important impact on stellar wind velocity, as shown by Karino (2014). However, this effect is probably local, since we do not observe sizeable differences in the terminal velocity between eclipsing and non-eclipsing orbital phases in Vela X-1.…”

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

“…It is only close from the accretor, once all the elements have been deprived of their electrons, that the line-driven acceleration is halted, as previously emphasized in the literature (see e.g. Hatchett & McCray 1977;Ho & Arons 1987;Blondin et al 1990;Karino 2014). In an attempt to illustrate the dramatic impact of the efficiency of the line-driven acceleration on the subsequent properties of the accretion flow, and to encompass potential inaccuracies in the calculation of this acceleration in the context of Vela X-1, we consider the case of an artificially enhanced wind acceleration (by 50%) which leads to larger flow velocities by approximately 20%.…”

Revisiting the archetypical wind accretor Vela X-1 in depth

“…In spite of episodic flares in which Vela X-1 reaches a few 10 37 erg s −1 , the system has never been observed in such a high state, however. Karino (2014) performed a similar computation, but accounted for optical depth and a gradual effect of the X-rays on line acceleration. Karino (2014) retrieved a bimodal behavior, but in contrast to Ho & Arons (1987b), Vela X-1 was lying on the branch corresponding to a high luminosity, associated with a slow wind.…”

“…Karino (2014) performed a similar computation, but accounted for optical depth and a gradual effect of the X-rays on line acceleration. Karino (2014) retrieved a bimodal behavior, but in contrast to Ho & Arons (1987b), Vela X-1 was lying on the branch corresponding to a high luminosity, associated with a slow wind. Manousakis & Walter (2015a) revived the role of the X-ray ioninzing feedback that they found to be a possible origin of the observed off-states.…”