Mass Transfer in High-mass X-Ray Binaries: from Roche Lobe Overflow to Hoyle–Lyttleton Accretion

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We use a new overset dual spherical grid system to evolve the circumstellar gas flow in the High-Mass X-Ray Binary Vela X-1 to investigate the gravitational capture of the donor star’s wind and subsequent accretion onto the neutron star companion. For a fast, relatively undistorted stellar wind (small filling factor) we find results consistent with classical Hoyle–Lyttleton accretion with the formation of a steady, nearly axisymmetric bow shock. For models with a filling factor near unity, the mass transfer is dominated by a tidal stream feeding into a thin accretion disk. Finally, for slow winds without Roche Lobe Overflow we find the formation of a wind bow shock but with enough post shock angular momentum to form a thin, stable accretion disk.