Journal Article

Observations of the quiescent X-ray transients GRS 1124–684 (=GU Mus) and Cen X-4 (=V822 Cen) taken with ULTRACAM on the VLT*

T. Shahbaz, V. S. Dhillon, T. R. Marsh, J. Casares, C. Zurita and P. A. Charles

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 403, issue 4, pages 2167-2175
Published in print April 2010 | ISSN: 0035-8711
Published online April 2010 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2010.16262.x
Observations of the quiescent X-ray transients GRS 1124–684 (=GU Mus) and Cen X-4 (=V822 Cen) taken with ULTRACAM on the VLT*

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We present high time-resolution multicolour optical observations of the quiescent X-ray transients GRS 1124–684 (=GU Mus) and Cen X-4 (=V822 Cen) obtained with ULTRACAM. Superimposed on the secondary stars' ellipsoidal modulation in both objects are large flares on time-scales of 30–60 min as well as several distinct rapid flares on time-scales of a few minutes, most of which show further variability and unresolved structure. Not significant quasi-periodic oscillations are observed and the power density spectra of GRS 1124–684 and Cen X-4 can be described by a power law. From the colour–colour diagrams of the flare events, for GRS 1124–684 we find that the flares can be described by hydrogen gas with a density of NH∼ 1024 nucleons cm−2, a temperature of ∼8000 K and arising from a radius of ∼0.3 R. Finally we compile the values for the transition radius (the radius of the hot advection-dominated accretion flow) estimated from quasi-periodic oscillations and/or breaks in the power density spectrum for a variety of X-ray transients in different X-ray states. As expected, we find a strong correlation between the bolometric luminosity and the transition radius.

Keywords: accretion, accretion discs; binaries: close; stars: individual: GRS 1124–684 (=GU Mus); stars: individual: Cen X-4 (=V822 Cen)

Journal Article.  4132 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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