On the Nature of the Bursting X-Ray Pulsar GRO J1744−28

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Abstract
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 overflow onto a neutron star, implying that the inclination of the system is 18°. Interpretation of the QPO at frequency νQPO = 40 Hz using the beat-frequency model of Alpar & Shaham and the measured period derivative 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 field Beq 2 × 1010 (νQPO/40 Hz)-1 G for standard neutron star parameters. This implies a distance to GRO J1744-28 of 5 (νQPO/40 Hz)1/6b1/2 kpc, where b < 1 is a correction factor that depends on the orientation of the neutron star.