Simultaneous X-ray/optical observations of GX 9+9 (4U 1728−16) |
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Authors: | A K H Kong P A Charles L Homer E Kuulkers D O'Donoghue |
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Institution: | Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA;Department of Physics and Astronomy, University of Southampton, Southampton SO17 1BJ;Astronomy Department, University of Washington, Box 351580, Seattle, WA 98195-1580, USA;ISOC, ESA/ESAC, Urb. Villafranca del Castillo, PO Box 50727, 28080 Madrid, Spain;South African Astronomical Observatory, PO Box 9, Observatory 7935, Cape Town, South Africa;Department of Astrophysics, University of Oxford, Keble Road, Oxford OX1 3RH |
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Abstract: | We report on the results of the first simultaneous X-ray ( RXTE ) and optical South African Astronomical Observatory (SAAO)] observations of the luminous low-mass X-ray binary (LMXB) GX 9+9 in 1999 August. The high-speed optical photometry revealed an orbital period of 4.1958 h and confirmed previous observations, but with greater precision. No X-ray modulation was found at the orbital period. On shorter time-scales, a possible 1.4-h variability was found in the optical light curves which might be related to the MHz quasi-periodic oscillations seen in other LMXBs. We do not find any significant X-ray/optical correlation in the light curves. In X-rays, the colour–colour and hardness-intensity diagrams indicate that the source shows characteristics of an atoll source in the upper banana state, with a correlation between intensity and spectral hardness. Time-resolved X-ray spectroscopy suggests that two-component spectral models give a reasonable fit to the X-ray emission. Such models consist of a blackbody component which can be interpreted as the emission from an optically thick accretion disc or an optically thick boundary layer, and a hard Comptonized component for an extended corona. |
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Keywords: | accretion accretion discs binaries: close stars: individual: GX 9+9 X-rays: stars |
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