Journal Article

Correlated optical/X-ray long-term variability in LMXB 4U 1636-536

I. C. Shih, P. A. Charles and R. Cornelisse

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 412, issue 1, pages 120-126
Published in print March 2011 | ISSN: 0035-8711
Published online March 2011 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2010.17895.x
Correlated optical/X-ray long-term variability in LMXB 4U 1636-536

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We have conducted a 3-month programme of simultaneous optical, soft and hard X-ray monitoring of the LMXB 4U 1636-536/V801 Ara using the SMARTS 1.3-m telescope and archival RXTE/ASM and Swift/XRT data. 4U 1636-536 has been exhibiting a large amplitude, quasi-periodic variability since 2002 when its X-ray flux dramatically declined by roughly an order of magnitude. We confirmed that the anticorrelation between soft (2–12 keV) and hard (>20 keV) X-rays, first investigated by Shih et al., is not an isolated event but a fundamental characteristic of this source's variability properties. However, the variability itself is neither strictly stable nor changing on an even longer characteristic time-scale. We also demonstrate that the optical counterpart varies on the same time-scale, and is correlated with the soft, and not the hard, X-rays. This clearly shows that X-ray reprocessing in LMXB discs is mainly driven by soft X-rays. The X-ray spectra in different epochs of the variability revealed a change of spectral characteristics which resemble the state change of black hole X-ray binaries. All the evidence suggest that 4U 1636-536 is frequently (∼monthly) undergoing X-ray state transitions, a characteristic feature of X-ray novae with their wide range of luminosities associated with outburst events. In its current behavioural mode, this makes 4U 1636-536 an ideal target for investigating the details of state changes in luminous X-ray binaries.

Keywords: accretion, accretion discs; binaries: close; stars: neutron; X-rays: binaries

Journal Article.  3391 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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