Origin of bright flares in SFXTs

Abstract

In the settling accretion theory, which is applicable to quasi-spherical accreting slowly rotating magnetized neutron stars with X-ray luminosity $L_x\lesssim 4\times 10^{36}$~erg/s, bright X-ray flares ($\sim 10^{38}-10^{40}$~ergs) observed in supergiant fast X-ray transients (SFXT) may be produced by sporadic capture of magnetized stellar-wind plasma from the early-type supergiant. At sufficiently low steady accretion rates ($\lesssim 10^{15}$~g/s) through the shell around the neutron star magnetosphere at the settling accretion stage, magnetic reconnection can temporarily enhance the magnetospheric plasma entry rate, resulting in copious production of X-ray photons, strong Compton cooling, and ultimately in unstable accretion of the entire shell. A bright flare develops on the free-fall time scale in the shell, $R_B^{3/2}/\sqrt{GM}\sim 10^3-10^4$~s ($R_B$ is the classical Bondi capture radius), and the typical energy released in an SFXT bright flare corresponds to the mass of the shell.