GRO J1744-28: Last Gasps of a Dying Low-mass X-ray Binary

Abstract

We argue that the bursting, transient X-ray source GRO J1744-28 is a binary consisting of a very low-mass ($M \approx 0.2 M_\sun$), highly evolved giant star that is transferring mass by Roche lobe overflow onto a high-mass ($M \approx 1.8 M_\sun$) neutron star. We explore a picture in which the bursts are due to thermonuclear flashes in matter that has accreted onto the neutron star. We attribute the unusually hard spectra of the bursts and the high burst rate compared to normal Type I X-ray burst sources -- indeed the existence of unstable nuclear burning itself -- to the unusually strong surface magnetic field, $ B_s \approx 1 \times 10^{13}$ G, of the neutron star.