A STRONGLY MAGNETIC NEUTRON-STAR IN A NEARLY FACE-ON BINARY-SYSTEM

THE first example of a new type of transient X-ray source recently app eared in the direction of the Galactic Centre(1), During the peak of i ts outburst, the new source, GRO J1744 - 28, was very bright in X-rays (2) and produced both 2.1-Hz periodic pulsations(3) and intense bursts lasting tens of seconds(1,4). Before the discovery of this source, it was thought that X-ray stars could not display these different types of activity simultaneously. Here we discuss the nature of the source, which seems to be a strongly magnetic neutron star accreting matter fr om a low-mass companion star in a low-inclination orbit. The dipole co mponent of its magnetic field is less than or similar to 10(11) G. Whe n the source was at its brightest, its X-ray luminosity between bursts was close to the Eddington critical luminosity(5), at which the outwa rd force of the escaping radiation balances the inward force of gravit y. The X-ray bursts probably occur when matter that has accumulated in the inner part of the accretion disk briefly overcomes the forces tha t oppose its inflow, and the gas falls onto the neutron star.