The Nature and Evolutionary History of GRO J1744–28

GRO J1744-28 is the first known X-ray source to display bursts, periodic pulsations, and quasi-periodic oscillations. This source may thus provide crucial clues that will lead to an understanding of the differences in the nature of the X-ray variability from various accreting neutron stars. The orbital period is 11.8 days, and the measured mass function of 1.31 × 10-4 M☉ is one of the smallest among all known binaries. If we assume that the donor star is a low-mass giant transferring matter through the inner Lagrange point, then we can show that its mass is lower than ~0.7 M☉ and probably closer to 0.25 M☉. Higher mass, but unevolved, donor stars are shown to be implausible. We also demonstrate that the current He core mass of the donor star lies in the range of 0.20-0.25 M☉. Thus, this system is most likely in the final stages of losing its hydrogen-rich envelope, with only a small amount of mass remaining in the envelope. If this picture is correct, then GRO J1744-28 may well represent the closest observational link that we have between the low-mass X-ray binaries and recycled binary pulsars in wide orbits. We have carried out a series of binary evolution calculations and explored, both systematically and via a novel Monte Carlo approach, the range of initial system parameters and input physics that can lead to the binary parameters of the present-day GRO J1744-28 system. The input parameters include both the initial total mass and the core mass of the donor star, the neutron-star mass, the strength of the magnetic braking, the mass-capture fraction, and the specifics of the core mass/radius relation for giants. Through these evolution calculations, we compute probability distributions for the current binary system parameters (i.e., the total mass, core mass, radius, luminosity, and K-band magnitude of the donor star, the neutron star mass, the orbital inclination angle, and the semimajor axis of the binary). Our calculations yield the following values for the GRO J1744-28 system parameters (with 95% confidence limits in parentheses): donor star mass: 0.24 M☉ (0.2-0.7 M☉); He core mass of the donor star: 0.22 M☉ (0.20-0.25 M☉); neutron-star mass: 1.7 M☉ (1.39-1.96 M☉); orbital inclination angle: 18° (7°-22°); semimajor axis: 64 lt-s (60-67 lt-s); radius of the donor star: 6.2 R☉ (6-9 R☉); luminosity of donor star: 23 L☉ (15-49 L☉); and long-term mass transfer rate at the current epoch: 5 × 10-10 M☉ yr-1 (2 × 10-10 to 5 × 10-9 M☉ yr-1). We deduce that the magnetic field of the underlying neutron star lies in the range of ~1.8 × 1011 G to ~7 × 1011 G, with a most probable value of 2.7 × 1011 G. This is evidently sufficiently strong to funnel the accretion flow onto the magnetic polar caps and suppress the thermonuclear flashes that would otherwise give rise to the type I X-ray bursts observed in most X-ray bursters. We present a simple paradigm for magnetic accreting neutron stars wherein X-ray pulsars, GRO J1744-28, the Rapid Burster, and the type I X-ray bursters may form a continuum of possible behaviors among accreting neutron stars, with the strength of the neutron-star magnetic field serving as the crucial parameter that determines the mode of X-ray variability from a given object.