High time resolution optical, UV, and IR observations of X-ray binaries
It has long been known that X-ray and optical/UV variability in active X-ray binaries is sometimes correlated. The expectation has been that this arises from rapid reprocessing of X-rays into longer wavelength photons, and hence that the lags between the bandpasses can be used to reconstruct an echo-map of the binary, revealing the geometry and spatial scale of reprocessing sites. I will review how this can be done, what can be learned, and what progress has been made recently with the new generation of fast cameras. In addition, an interesting twist to the story has emerged in the form of correlated variability in black holes that does not seem associated with reprocessing. The temporal properties, and IR spectrum of the variability, instead point to synchrotron emission from a jet. © 2008 American Institute of Physics.