2017MNRAS.468..824B

The low-mass X-ray binary 2A 1822-371 is an eclipsing system with an accretion disc corona and with an orbital period of 5.57 h. The primary is a 0.59 s X-ray pulsar with a proposed strong magnetic field of 10¹⁰-10¹² G. In this paper, we study the spin evolution of the pulsar and constrain the geometry of the system. We find that, contrary to previous claims, a thick corona is not required and that the system characteristics could be best explained by a thin accretion outflow due to a super-Eddington mass transfer rate and a geometrically thick inner accretion flow. The orbital, spectral and timing observations can be reconciled in this scenario under the assumption that the mass transfer proceeds on a thermal time-scale that would make 2A 1822-371 a mildly super-Eddington source viewed at high inclination angles. The timing analysis on 13 yr of Rossi X-ray Timing Explorer data shows a remarkably stable spin-up that implies that 2A 1822-371 might quickly turn into a millisecond pulsar in the next few thousand years.