as tr o - ph / 9 60 21 02 20 F eb 1 99 6 On the Nature of the Bursting X - Ray Pulsar GRO J 1744 - 28 Steven

The unusual properties of the bursting X-ray pulsar GRO J1744-28 are explained in terms of a low-mass X-ray binary system consisting of an evolved stellar companion transferring mass through Roche-lobe overrow onto a neutron star, implying that the inclination of the system is < 18. Interpretation of the QPO at frequency QPO = 40 Hz using the beat-frequency model of Alpar & Shaham and the measured period derivative _ P with the Ghosh & Lamb accretion-torque model implies that the persistent X-ray luminosity of the source is approximately equal to the Eddington luminosity and that the neutron star has a surface equatorial magnetic eld B eq = 4:5 10 10 (QPO =40 Hz) 1 G for standard neutron star parameters. This implies a distance to GRO J1744-28 of = 3:7 (QPO =40 Hz) 1=6 b 1=2 kpc, where b < 1 is a correction factor that depends on the orientation of the neutron star.