Accretion from a clumpy massive-star wind in supergiant X-ray binaries

Abstract

Supergiant X-ray Binaries (sgxb) host a compact object, often a neutron star (NS), orbiting an evolved O/B star. Mass transfer proceeds through the intense line-driven wind of the stellar donor, a fraction of which is captured by the gravitational field of the NS. The subsequent accretion process onto the NS is responsible for the abundant X-ray emission from sgxb. They also display peak-to-peak variability of the X-ray flux by a factor of a few 10 to 100, along with changes in the hardness ratios possibly due to varying absorption along the line-of-sight. We use recent radiation-hydrodynamic simulations of inhomogeneities (aka clumps) in the non-stationary wind of massive hot stars to evaluate their impact on the time-variable accretion process. For this, we run 3D hydrodynamic simulations of the wind in the vicinity of the accretor to investigate the formation of the bow shock and follow the inhomogeneous flow over several spatial orders of magnitude, down to the NS magnetosphere. In particular, we show that the impact of the wind clumps on the time-variability of the intrinsic mass accretion rate is severely tempered by the crossing of the shock, compared to the purely ballistic Bondi-Hoyle-Lyttleton estimation. We also account for the variable absorption due to clumps passing by the line-of-sight and estimate the final effective variability of the column density and mass accretion rate for different orbital separations. Finally, we compare our results to the most recent analysis of the X-ray flux and the hardness ratio in Vela X-1.

Theoretical wind clumping predictions of OB supergiants from line-driven instability simulations across the bi-stability jump

“…the prototype system Vela X-1 where the donor star is a early B supergiant with a slow and dense wind characteristic of a simulation below the bi-stability jump (Sander et al 2018). However, in the accretion models by El Mellah et al (2018) targeting Vela X-1, a multi-D LDI simulation of an O supergiant was used to simulate the clumpy wind accretion and the corresponding time-variation in X-ray luminosity. Preliminary calculations made by us suggest that also in such multi-D models the structure formation in B supergiants is still much weaker than in O supergiants.…”

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

“…For Vela X-1, the low-luminosity solution matches the observed luminosity of a few times 10 36 erg s −1 but once the wind is in the high ionization stage, the X-ray luminosity is 2000 times larger, matching a ULX level, although such a flare has never been observed in Vela X-1 (but see Krticka et al 2018, for the consequences for supergiant fast X-ray transients). To a lesser extent, the fraction of captured mass that ends up being accreted can also be altered by the clumpiness of the wind El Mellah et al 2018). These combined effects might explain why many SgXBs such as Vela X-1 have luminosities much lower than the Eddington luminosity of a NS or a stellar-mass BH, in spite of similar dimensionless parameters.…”

Bondi–Hoyle–Lyttleton accretion in supergiant X-ray binaries: stability and disc formation

“…Numerical simulation by Sundqvist et al (2018) shows that for typical O star parameters, the clumps in the wind can be over one order of magnitude denser, with size ∼ 0.05Rc at ∼ 2Rc (this is also the typical location of the NS in a SgXB undergoing wind accretion). For systems with fast wind (see Table 2) this clump size is ∼ Ra; this should make the accretion flow highly variable and reduce the accretion rate, as is shown in the simulation of El Mellah et al (2018b). Still, this does not mean that investigating accretion from a smooth (or slightly perturbed) upstream flow is irrelevant for SgXB systems, since the perturbation in the wind may not be as strong for different stellar parameters, and other physical mechanisms may suppress the initial perturbation that triggers the instability, making the variability small (if not zero) at the NS.…”