Discovery of mHz UV Quasiperiodic Oscillations in Hercules X-1
  Observations of the ultraviolet continuum of the X-ray binary system Her X-1/HZ Herculis with the Space Telescope Imaging Spectrograph on the Hubble Space Telescope show quasiperiodic oscillations (QPOs) at frequencies of 8+/-2 and 43+/-2 mHz, with rms amplitudes of 2 and 4% of the steady flux. Observations with the Keck telescope confirm the presence of the higher frequency QPO in the optical continuum, with a rms amplitude of 1.6+/-0.2%. The QPOs are most prominent in the HST data near phi=0.5 (where phi=0 is the middle of the X-ray eclipse), suggesting that they arise not in the accretion disk but on the X-ray heated face of the companion star. We discuss scenarios in which the companion star reprocesses oscillations in the disk which are caused by either Keplerian rotation or a beat frequency between the neutron star spin and Keplerian rotation at some radius in the accretion disk. 
B. Boroson (1), K. O'Brien (2), K. Horne (2), T. Kallman (1), M. Still   (1), P. T. Boyd (1), H. Quaintrell (3), S. D. Vrtil...