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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 (&gt;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.

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