The Nature and Evolutionary History of GRO J1744−28

Abstract

GRO J1744-28 is the first known X-ray source to display bursts, periodic pulsations, andquasi-periodic oscillations. This source may thus provide crucial cluesthat will lead to an understanding of the differences in the natureof the X-ray variability from various accreting neutron stars. Theorbital period is 11.8 days, and the measured mass function of 1.31x 10-4 M�� is one of the smallest among all known binaries. If weassume that the donor star is a low-mass giant transferring matterthrough the inner Lagrange point, then we can show that its mass islower than ~0.7 M�� and probably closer to 0.25 M��. Higher mass, butunevolved, donor stars are shown to be implausible. We also demonstratethat the current He core mass of the donor star lies in the range of0.20--0.25 M��. Thus, this system is most likely in the final stagesof losing its hydrogen-rich envelope, with only a small amount ofmass remaining in the envelope. If this picture is correct, then GROJ1744-28 may well represent the closest observational link that wehave between the low-mass X-ray binaries and recycled binary pulsarsin wide orbits. We have carried out a series of binary evolutioncalculations and explored, both systematically and via a novel MonteCarlo approach, the range of initial system parameters and inputphysics that can lead to the binary parameters of the present-day GROJ1744-28 system. The input parameters include both the initial totalmass and the core mass of the donor star, the neutron-star mass, thestrength of the magnetic braking, the mass-capture fraction, and thespecifics of the core mass/radius relation for giants. Through theseevolution calculations, we compute probability distributions forthe current binary system parameters (i.e., the total mass, coremass, radius, luminosity, and K-band magnitude of the donor star, theneutron star mass, the orbital inclination angle, and the semimajoraxis of the binary). Our calculations yield the following values forthe GRO J1744-28 system parameters (with 95% confidence limits inparentheses): donor star mass: 0.24 M�� (0.2--0.7 M��) He core mass of thedonor star: 0.22 M�� (0.20--0.25 M��) neutron-star mass: 1.7 M��(1.39--1.96 M��) orbital inclination angle: 18 deg (7 deg--22 deg)semimajor axis: 64 lt-s (60--67 lt-s); radius of the donor star: 6.2R�� (6--9 R��) luminosity of donor star: 23 L�� (15--49 L��) andlong-term mass transfer rate at the current epoch: 5 x 10-10 M��yr-1 (2 x 10-10 to 5 x 10-9 M�� yr-1). We deduce that the magneticfield of the underlying neutron star lies in the range of ~1.8 x1011 G to ~7 x 1011 G, with a most probable value of 2.7 x 1011 G.This is evidently sufficiently strong to funnel the accretion flowonto the magnetic polar caps and suppress the thermonuclear flashesthat would otherwise give rise to the type I X-ray bursts observedin most X-ray bursters. We present a simple paradigm for magneticaccreting neutron stars wherein X-ray pulsars, GRO J1744-28, theRapid Burster, and the type I X-ray bursters may form a continuum ofpossible behaviors among accreting neutron stars, with the strength ofthe neutron-star magnetic field serving as the crucial parameterthat determines the mode of X-ray variability from a given object.