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

The unusual properties of the bursting X-ray pulsar GRO J1744-28 areexplained in terms of a low-mass X-ray binary system consisting of an evolvedstellar companion transferring mass through Roche-lobe overflow onto a neutronstar, implying that the inclination of the system is < 18 degrees.Interpretation of the QPO at frequency nu(QPO) = 40 Hz using the beat-frequencymodel of Alpar \& Shaham and the measured period derivative with the Ghosh \&Lamb accretion-torque model implies that the persistent X-ray luminosity of thesource is approximately equal to the Eddington luminosity and that the neutronstar has a surface equatorial magnetic field = 2e10 [40 Hz/nu(QPO)] G forstandard neutron star parameters. This implies a distance to GRO J1744-28 of ~5 [nu(QPO)/40 Hz]^{1/6} b^{1/2} kpc, where b < 1 is a correction factor thatdepends on the orientation of the neutron star.Comment: 10 pages, 2 figures, uuencoded gzipped tarred file. Uses AAS macros v. 4. Revised shortened paper, accepted for publication in Astrophysical Journal Letter