2011MNRAS.412..120S

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.