AAS 195th Meeting, January 2000
Session 39. Neutron Stars with Partners
Display, Thursday, January 13, 2000, 9:20am-6:30pm, Grand Hall

## [39.07] UV Quasiperiodic Oscillations from Hercules X-1

B. Boroson (NASA GSFC), T. Kallman (NASA GSFC), S. D. Vrtilek (SAO)

Observations of the ultraviolet continuum of the X-ray binary system Her~X-1/HZ~Hercules with the Space Telescope Imaging Spectrograph on the Hubble Space Telescope reveal quasiperiodic oscillations (QPOs) at frequencies of 8 and 50 mHz, with rms amplitudes of ~0%\ of the steady flux. The observations took place when Her~X-1 was in an anomalous low'' state in July of 1999. The QPOs are most prominent near orbital phase \phi=0.5, suggesting they arise not in the accretion disk but on the X-ray heated face of the companion star, perhaps as a result of reprocessing of X-rays from the neutron star. If the frequency of the QPOs is related to the Keplerian rotation frequency in the disk, they originate 109~cm from the neutron star, near the Alfvèn shell radius.