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Bright Flares in Supergiant Fast X-ray Transients

At steady low-luminosity states, Supergiant Fast X-ray Transients (SFXTs) can be at the stage of quasi-spherical settling accretion onto slowly rotating magnetized NS from the OB-companion winds. At this stage, a hot quasi-static shell is formed above the magnetosphere, the plasma entry rate into magnetosphere is controlled by (inefficient) radiative plasma cooling, and the accretion rate onto the NS is suppressed by a factor of \sim 30 relative to the Bondi-Hoyle-Littleton value. Changes in the local wind velocity and density can only slightly increase the mass accretion rate (a factor of \sim 10) bringing the system into the Compton cooling dominated regime and led to the production of moderately bright flares (L_x\lesssim 10^{36} erg/s). To interpret the brightest flares (L_x>10^{36}~erg/s) displayed by the SFXTs, we propose that a larger increase in the mass accretion rate can be produced by sporadic capture of magnetized stellar wind plasma. At sufficiently low accretion rates, magnetic reconnection can 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, and the typical energy released in an SFXT bright flare corresponds to the mass of the shell. This view is consistent with the energy released in SFXT bright flares (\sim 10^{38}-10^{40}~ergs), their typical dynamic range (\sim 100), and with the observed dependence of these characteristics on the average unflaring X-ray luminosity of SFXTs. Thus the flaring behavior of SFXTs, as opposed to steady HMXBs, may be primarily related to their low X-ray luminosity allowing sporadic magnetic reconnection to occur during magnetized plasma entry into the magnetosphere.