The X-ray pulsar 2A 1822−371 as a super-Eddington source

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.