The discovery of 4.4 second X-ray pulsations from the rapidly variable X-ray transient V0332 + 53

Abstract

Using the EXOSAT Observatory, three outbursts from the transient X-ray source V0332 + 53 have been observed between November 1983 and January 1984. It is found that in addition to the rapid Cyg X-1-like variability previously reported by Tanaka et al. (1983), this source also displays stable pulsations with a period of 4.4 s. Doppler variations in the pulse period indicate that the pulsar is in a 34.25-day binary orbit with an eccentricity of 0.31. The times of periastron passage are close to those of the X-ray outbursts. The X-ray spectra and pulse profiles are quite similar to those seen from other X-ray pulsars. The rapid (about 1 s) variations are quantified in terms of the shot noise model previously applied to Cyg X-1, with the derived parameters found to be similar to those of Cyg X-1. It seems difficult to invoke the same mechanism to produce rapid fluctuations from both an accreting black hole and an accreting magnetized neutron star, and it is concluded that two physically distinct processes can give rise to similar temporal variability. The rapid variations from V0332 + 53 may result from instabilities in the magnetosphere of a spherically accreting neutron star.