Observations of the Peculiar Hard X-Ray Transient X0331+53 (V0332+53)

Abstract Tenma observations were made of the transient 4.4-s X-ray pulsar X0331+53 (V0332+53) at several phases of its 34-d binary orbit during the period November 1983-January 1984. The measured X-ray flux and pulse periods agree with those obtained with EXOSAT. Rapid random fluctuations in the X-ray emission from this source were studied. It appears that this type of variability is not rare among accreting pulsars, but is particularly noticeable in X0331+53 due to the small pulse fraction. The X-ray spectra show a turn-over above ~ 15 keV, which is better described by a recently introduced cyclotron absorption formula than the commonly used exponential cutoff model. This also applies to the spectra of Her X-1 and Cen X-3. In terms of this model, a magnetic field strength of ~ 3 × 1012 G is suggested for X0331+53. The absence of iron K-emission lines in the spectra indicates a rather small circumstellar matter density. This, together with the observed high luminosity (~ 3 × 1036 erg s–1) and large intensity modulation along the orbit, suggests that the pulsar is powered by accretion from the equatorial envelope of the supposedly Be-type primary (BQ Cam).