Low-luminosity stellar wind accretion onto neutron stars in HMXBs
K. Postnov, L. Oskinova, J.M. Torrejon
Features and applications of quasi-spherical settling accretion onto rotating magnetized neutron stars in high-mass X-ray binaries are discussed. The settling accretion occurs in wind-fed HMXBs when the plasma cooling time is longer than the free-fall time from the gravitational capture radius, which can take place in low-luminosity HMXBs with Lx less than about 4 1036 erg/s. We briefly review the implications of the settling accretion, focusing on the SFXT phenomenon, which can be related to instability of the quasi-spherical convective shell above the neutron star magnetosphere due to magnetic reconnection from fast temporarily magnetized winds from OB-supergiant. If a young neutron star in a wind-fed HMXB is rapidly rotating, the propeller regime in a quasi-spherical hot shell occurs. We show that X-ray spectral and temporal properties of enigmatic γ Cas Be-stars are consistent with failed settling accretion regime onto a propelling neutron star. The subsequent evolutionary stage of γ Cas and its analogs should be the X Per-type binaries comprising low-luminosity slowly rotating X-ray pulsars.
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