Correlated Optical/X-ray Long-term Variability in LMXB 4U1636-536

TL;DR

This study presents simultaneous optical and X-ray observations of LMXB 4U1636-536, revealing correlated variability and state transitions similar to black hole binaries, highlighting its suitability for studying X-ray binary state changes.

Contribution

First simultaneous multi-wavelength monitoring of 4U1636-536 demonstrating correlated optical and X-ray variability and spectral state changes.

Findings

Confirmed anti-correlation between soft and hard X-rays.

Optical variability correlates with soft X-rays.

Evidence of frequent X-ray state transitions.

Abstract

We have conducted a 3-month program of simultaneous optical, soft and hard X-ray monitoring of the LMXB 4U1636-536/V801 Ara using the SMARTS 1.3m telescope and archival RXTE/ASM and Swift/XRT data. 4U1636-536 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 anti-correlation between soft (2-12 keV) and hard (> 20 keV) X-rays, first investigated by Shih et al. (2005), 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 timescale. We also demonstrate that the optical counterpart varies on the same timescale, 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 suggests that 4U1636-536 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 4U1636-536 an ideal target for investigating the details of state changes in luminous X-ray binaries.