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Astron. Astrophys. 331, 1037-1045 (1998)

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

J.F. Dolan 1, R.F. Hill 1, 2, P.T. Boyd 1, 3, J.M. Silvis 2, J.W. Percival 4 and G.W. van Citters 5

1 Laboratory for Astronomy & Solar Physics, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA (e-mail: tejfdsplvin.gsfc.nasa.gov)
2 Hughes STX Corporation, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA (e-mail: hill@noether.gsfc.nasa.gov; silvisxfiles.gsfc.nasa.gov)
3 Universities Space Research Association, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA (e-mail: padidragons.gsfc.nasa.gov)
4 Department of Astronomy & Space Astronomy Laboratory, University of Wisconsin, Madison, WI 53706, USA (e-mail: jwpmaddog.sal.wisc.edu)
5 Division of Astronomical Sciences, National Science Foundation, 4201 Wilson Blvd. Arlington, VA 22230, USA (e-mail: gvancittnotel.nsf.gov)

Received 3 December 1996 / Accepted 30 October 1997


A search for reprocessed X-rays causing UV pulsations in the companion stars to X-ray pulsars was carried out in a 200 Å wide bandpass centered at 1450 Å  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.

Key words: binaries: close – stars: individual: HD 77581 – stars: neutron – pulsars: individual: 4U 0900-40 – ultraviolet: stars – X-rays: stars

* Based in part on observations with the Hubble Space Telescope obtained at the Space Telescope Science Institute, which is operated by AURA, Inc. under NASA contract NAS 5-26555

Send offprint requests to: J.F. Dolan

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Online publication: March 3, 1998