Retrograde wind accretion -- an alternative mechanism for long spin-period of SFXTs

TL;DR

This paper proposes a retrograde wind accretion model to explain the unusually long spin periods observed in supergiant fast X-ray transients, a new class of high-mass X-ray binaries.

Contribution

It introduces a novel retrograde accretion scenario based on three characteristic radii to explain long neutron star spin periods in SFXTs.

Findings

Retrograde accretion can produce long spin periods in SFXTs.

The accretion regime depends on the system's spin and orbital periods.

The model aligns with observational evidence of accretion reversals.

Abstract

A new class of high-mass X-ray binaries (HMXBs) --- supergiant fast X-ray transients (SFXTs) --- are discovered by INTEGRAL, which are associated with OB supergiants and present long spin periods. Observational evidence indicates that some accreting neutron stars in HMXBs display accretion reversals. It has been suggested that the inverted torque can lead to a very slow rotator. According to three characteristic radii in wind-fed accretion, we developed a retrograde accretion scenario and divided the accretion phase into three regimes, to interpret the formation of the long spin period of SFXTs. The accretion regime in some SFXT systems can be determined by their spin and orbital periods.