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

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 degrees. Interpretation of the QPO at frequency nu(QPO) = 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 = 2e10 [40 Hz/nu(QPO)] G for standard 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 that depends on the orientation of the neutron star.

10 pages, 2 figures, uuencoded gzipped tarred file. Uses AAS macros v. 4. Revised shortened paper, accepted for publication in Astrophysical Journal Letters

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