Reprocessed UV pulses from the binary companions of X-ray pulsars

A search for reprocessed X-rays causing UV pulsations in the companion stars to X-ray pulsars was carried out in a 200 Angstrom wide bandpass centered at 1450 Angstrom using the High Speed Photometer on the Hubble Space Telescope. We observed the systems A0535+26 = HD245770, 4U0900-40 (Vela XR-1)= HD77581, A1118-61 = He3-640, and 4U1145-619 = HD102567. We confirm the existence of reprocessed X-rays reported in 4U0900-40 by Boroson et al. (1996) and give upper limits on the pulsed fraction in our UV bandpass present in the other systems. Archival IUE spectra are consistent with the finding of Boroson et al. that the UV radiation from HD77581 is emitted primarily in resonance emission lines from multiply ionized metals. The existence of two different pulse frequencies (UV and X-ray) in an X-ray binary system makes it dynamically equivalent to a double-lined spectroscopic binary. If the UV pulses are emitted from a site in the primary's outer atmosphere rather than its stellar wind, then the mass of the neutron star can be derived from the orbital phase dependence of the two frequencies.