The orbital period of the accreting pulsar GX 1+4

We report strong evidence for a similar to 304 day periodicity in the spin history of the accretion-powered pulsar GX 1+4 that is most probably associ ated with the orbital period of the system. We have used data from the Burs t and Transient Source Experiment on the Compton Gamma-Ray Observatory to s how a clear periodic modulation of the pulsar frequency from 1991 to date, in excellent agreement with the ephemeris proposed by Cutler, Dennis, & Dol an in 1986. Our results indicate that the orbital period of GX 1+4 is 303.8 +1.1 days, making it the widest known low-mass X-ray binary system by more than 1 order of magnitude and putting this long-standing question to rest. A likely scenario for this system is an elliptical orbit in which the neutr on star decreases its spin-down rate (or even exhibits a momentary spin-up behavior) at periastron passages due to the higher torque exerted by the ac cretion disk onto the magnetosphere of the neutron star. These results are not inconsistent with either the X-ray pulsed flux light curve measured by BATSE during the same epoch or the X-ray flux history from the All-Sky Moni tor on board the Rossi X-Ray Timing Explorer.