Orbital elements of 4U 0115+63 and the nature of the hard X-ray transients

Extended SAS 3 timing observations of the hard transient X-ray source 4U 0115+63 are reported, and a definitive measurement of the binary orbit of this transient source is presented. It is shown that this source is in a long orbit (period of approximately 24.3 days) that is moderately eccentric (e about 0.34) and that the mean value of the rate of decrease of the pulse period is consistent with the expected spinup of a rotating neutron star that is accreting from a disk. A distance of about 2.5 kpc is inferred, and the B-star optical counterpart is estimated to have an absolute magnitude of approximately -1.5 and a mass of at least 5 solar masses. It is suggested that the companion is a Be star which does not fill its Roche lobe and that the eccentricity and transient nature of the source result from the large orbital separation. It is proposed that hard X-ray transients as a class are collapsed stars (perhaps all neutron stars) in binary systems that are substantially wider than the more persistent X-ray binaries and that the large orbital separation, the small radius of the companion, or both, result in episodic rather than continuous mass transfer onto the X-ray star.