Discovery of a 272 second periodic variation in the X-ray source GX 304-1

Observations of GX 304-1 (3U 1258-61) performed with the SAS-3 satellite revealed regular X-ray pulsations with a period of 272 s. Average pulse profiles, derived from 7 days of observation, are presented in four energy intervals covering the range 1-19 keV. The pulses are basically characterized by a single broad peak, and the amount of modulation decreases rapidly with increasing energy. The pulse arrival times are analyzed for effects of possible orbital motion of the X-ray star, and significant constraints are placed on the orbital parameters. For example, if it is established that the companion is an early-type star of at least 5 solar masses, then the orbital period is likely to be greater than about 15 days. Three 100-s flares were observed during which the source intensity rose by a factor of 4 in approximately 10 s.