Speaker
Description
X-ray binaries radiate brightly from radio to X-ray due to the accretion and ejection of matter in the system. Complex, correlated flux variations probe links between emitting components, on short as well as long timescales. Here I review the progression of multi-wavelength studies of X-ray binaries, focusing on long-term (day+ timescales) correlated behaviour between radio, optical/infrared and X-ray emission. Power law correlations exist between these different wavelength regimes, especially in the hard X-ray state, with interesting exceptions. Broadly speaking, the slopes and normalizations of these correlations can be explained with a combination of disc, corona and jet contributions, and in most cases are consistent with analytical model predictions. As well as being used to probe the accretion - ejection paradigm in these systems, both the radio/X-ray and optical/X-ray correlations can be used to constrain the nature of the compact object for new X-ray transients. Recently, dependencies of the correlation slopes on the choice of waveband have added an extra dimension to these studies. Correlations tend to break down in the soft state, due to quenching of the jet and evolution of the corona. Black hole systems are systematically brighter at radio, and optical/IR frequencies, at a given X-ray luminosity compared to neutron star X-ray binaries. We demonstrate that using these correlations in conjunction with spectral energy distributions and colour-magnitude diagrams can successfully isolate the different emitting components in order to measure accurate outer disc temperatures and jet synchrotron spectral indices. Some correlations have been confirmed to also exist on short timescales from fast timing data. Lags between components, on both long and short timescales, can produce scatter in global correlations. Finally, while it is well known that the radio/X-ray correlation extends, with a mass term, to supermassive black holes in AGN (the Fundamental Plane of black hole activity), we present a new optical/X-ray correlation that also extends to AGN. This power law correlation, with a slope consistent with theoretical expectations from X-ray binaries, is between the optical emission due to reprocessing on the disc surface, and the Bolometric X-ray luminosity, with a term for the disc size included. These results imply that the process of X-ray reprocessing is universal for all accretion disks, from stellar-mass black holes to supermassive black holes.