Discovery of mHz UV Quasiperiodic Oscillations in Hercules X-1

Abstract

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.

6 pages, 6 figures (one two-part figure in color), accepted ApJ, Dec. 10, 2000, v545

References in corpus (7)

• High-Speed Optical Photometry of the Ultracompact X-ray Binary 4U 1626-67
• Quasi-Periodic Occultation by a Precessing Accretion Disk and Other Variabilities of SMC X-1
• BeppoSAX observation of 4U1626-67: Discovery of an absorption cyclotron resonance feature
• Recent X-ray measurements of the accretion-powered pulsar 4U 1907+09
• The Global-Normal Disk Oscillations and the Persistent Low Frequency QPO in X-ray Binaries
• Sidebands Due to Quasi-periodic Oscillations in 4U 1626-67
• Hercules X-1: Empirical Models of UV Emission Lines

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• Magnetically Driven Precession of Warped Disks and Milli-Hertz Variabilities in Accreting X-Ray Pulsars
• Large X-ray Flares from LMC X-4: Discovery of Milli-hertz Quasi-periodic Oscillations and QPO-modulated Pulsations
• Millihertz Optical/UV Oscillations in 4U 1626-67: Evidence for a Warped Accretion Disk
• Quasi periodic oscillations in XTE J0111.2--7317, highest frequency among the HMXB pulsars
• Hot Outflowing Gas from the X-ray Binary Hercules X-1
• Atmospheric reflection during an anomalous low-state of Hercules X-1
• RXTE Observations of Hercules X-1 During the July 1998 Short-high State
• Discovery of Coupling between Periodic and Aperiodic Variability and X-ray Quasi-periodic Oscillations from Her X-1
• High energy Millihertz quasi-periodic oscillations in 1A 0535+262 with Insight-HXMT challenge current models
• A Discovery of a Peculiar Pulsar in the Small Magellanic Cloud
• Keck II spectroscopy of mHz quasi-periodic oscillations in Hercules X-1
• SALT observation of X-ray pulse reprocessing in 4U 1626-67