DISCOVERY OF HARD X-RAY PULSATIONS FROM THE TRANSIENT SOURCE GRO J1744-28

AN UNUSUAL astronomical source of hard X-ray bursts, GRO J1744 - 28, w as discovered recently(1,2). The properties of this source differ mark edly from those of other known high-energy burst sources-X-ray burster s, soft gamma-ray repeaters and gamma-ray bursters- suggesting that it may represent a new type of source. The bursts probably arise from un stable accretion of matter onto a compact object(2), such as a neutron star, but the nature of the object and the origin of the burst instab ility have not been revealed by observations of the bursts themselves. Here we report the detection of coherent X-ray pulsations, with a per iod of 467 milliseconds, from GRO J1744 - 28; these are the first pers istent pulsations seem in a bursting X-ray source. These pulses and th eir timing indicate that the object is a magnetized neutron star, accr eting gas from a low-mass companion star. The pulsation rate has been increasing during the period of our observations, indicating that an a ccretion disk has been formed and that the transfer of matter from the disk is spinning up the neutron star. The source of the instability t hat leads to the bursts remains unknown.