Spectroscopic Monitoring of the BE/X-Ray Pulsar X Persei-4U 0352+309 with the Orbiting and Retrievable Far and Extreme Ultraviolet Spectrometer SPAS II and the IUE

Abstract

We present phase-resolved spectra of the Be/X-ray pulsar system X Persei/4U 0352+309 obtained with the Tübingen far-UV echelle spectrometer of the Orbiting and Retrievable Far and Extreme Ultraviolet Spectrometer SPAS II (ORFEUS-SPAS II). Data were obtained in 1996 November and folded on the ~837.5 s pulsar period derived from contemporaneous X-ray observations. Contrary to optical results by several groups, we find no evidence for periodic modulation in our far-UV line profile data. The limits we can place on the presence of any periodic signal are ~1% of the continuum level in the Si IV λ1394 line and ~1.6% in N V λ1238. We have also combined these ORFEUS spectra with newly reprocessed IUE data to discover radial velocity variations in X Per that are consistent with a period of the order of 20-30 yr and a binary separation of ~20 AU. Even with a wide separation for this system, we argue that certain types of reported rapid variability as well as long-term X-ray enhancements could be caused by the residue of expanding Be disks falling onto the neutron star. The UV resonance lines appear to be typical for a classical Be star, showing variability on timescales of 1 day and also a few hours.