Long-term optical/X-ray correlations in LMXBs
Abstract
We have been monitoring the long-term variability of ~30 low-mass X-ray binaries at optical wavelengths with the two 2-m Faulkes Telescopes. With six to eight years of light curves in several filters for most sources, the data provide a rich database of accretion history in samples of transient, quiescent and persistently accreting LMXBs. Here, we correlate the long-term optical evolution (on week to years timescales) of LMXBs with the hard X-ray flux evolution from Swift BAT, RXTE and INTEGRAL. For black hole LMXBs in the hard state, the optical and X-ray fluxes are highly correlated. In some sources, the clear hard X-ray drop/rise over the hard-to-soft/soft-to-hard state transition is replicated (to a lesser extent) in the optical light curves, and are associated with optical colour changes. Lags between optical and X-ray fluxes are apparent, in both the timing of the peak fluxes (e.g. the optical peak of some outbursts occur several days later than the hard X-ray flux peak) and the drop/rises over state transitions. Optical/X-ray cross-correlations have revealed X-ray lags of several days in some persistent sources, which are likely to represent the viscous timescale of the accretion disc. We demonstrate how the properties of the correlations (correlation index values, lag timescales, dependency on spectral state) can be used to infer the optical emission mechanisms. High amplitude optical drop/rises over state transitions are associated with synchrotron emission from jets, and we discuss whether the same is seen in transient, state changing neutron star LMXBs. We also search for optical correlations to the long-term (hundreds of days) X-ray variability timescales seen in some LMXBs.
- Publication:
-
40th COSPAR Scientific Assembly
- Pub Date:
- 2014
- Bibcode:
- 2014cosp...40E2811R