Keck II spectroscopy of mHz quasi-periodic oscillations in Hercules X-1 |
| |
| Authors: | K O'Brien Keith Horne B Boroson † M Still ‡ R Gomer JB Oke P Boyd SD Vrtilek |
| |
| Institution: | 1School of Physics and Astronomy, University of St Andrews, St Andrews KY16 9SS;2Astronomical Institute 'Anton Pannekoek', University of Amsterdam, 1098-SJ Amsterdam, the Netherlands;3NASA/Goddard Space Flight Center, Code 662, Greenbelt, MD 20771, USA;4Howard Hughes Medical Institute and Department of Biochemistry and Cell Biology, MS-140, Rice University, Houston, TX 77005-1892, USA;5California Institute of Technology, Mail Stop 105-24, Pasadena, CA 91125, USA;6Dominion Astrophysical Observatory, Herzberg Institute of Astrophysics, National Research Council of Canada, 5071 West Saanich Road, Victoria, BC V8X 4M6, Canada;7Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA |
| |
| Abstract: | We present Keck II spectroscopy of optical mHz quasi-periodic oscillations (QPOs) in the light curve of the X-ray pulsar binary Hercules X-1. In the power spectrum it appears as 'peaked noise', with a coherency ～2, a central frequency of 35 mHz and a peak-to-peak amplitude of 5 per cent. However, the dynamic power spectrum shows it to be an intermittent QPO, with a lifetime of ～100 s, as expected if the lifetime of the orbiting material is equal to the thermal time-scale of the inner disc. We have decomposed the spectral time series into constant and variable components and used blackbody fits to the resulting spectra to characterize the spectrum of the QPO variability and constrain possible production sites. We find that the spectrum of the QPO is best fitted by a small hot region, possibly the inner regions of the accretion disc, where the ballistic accretion stream impacts on to the disc. The lack of any excess power around the QPO frequency in the X-ray power spectrum, created using simultaneous light curves from RXTE , implies that the QPO is not simply reprocessed X-ray variability. |
| |
| Keywords: | accretion accretion discs binaries: close stars: individual: Hercules X-1 stars: oscillations X-rays: stars |
|
|
