Origin of bright flares in SFXTs

TL;DR

This paper proposes that bright X-ray flares in supergiant fast X-ray transients (SFXTs) originate from sporadic magnetic reconnection events in the stellar wind, leading to unstable shell accretion onto neutron stars.

Contribution

It introduces a model linking magnetic reconnection in the stellar wind to the occurrence of bright flares in SFXTs within the settling accretion framework.

Findings

Magnetic reconnection can trigger unstable shell accretion in SFXTs.

Flares develop on the free-fall timescale of the shell, about 10^3-10^4 seconds.

The energy of flares corresponds to the mass of the accretion shell.

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.