Low-luminosity stellar wind accretion onto neutron stars in HMXBs

Abstract

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 $L_x\lesssim 4\times 10^{36}$ 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 $\gamma$ Cas Be-stars are consistent with failed settling accretion regime onto a propelling neutron star. The subsequent evolutionary stage of $\gamma$ Cas and its analogs should be the X Per-type binaries comprising low-luminosity slowly rotating X-ray pulsars.