Spectral Transitions in X-Ray Binaries

We systematically analyzed about a terabyte of data from Galactic binarysystems to characterize how these spectra change as a function of massaccretion rate. With this volume of data we are able to identify thedifferences in behaviour of black holes and neutron stars, implying that blackholes really do have an event horizon. The spectral evolution seen as afunction of L/L_Edd can be linked to current state-of-the-art numericalsimulations of accretion flows, and we develop qualitative and quantitativemodels of the spectral changes in all types of binary systems. The keyingredient of this picture is that the major hard-soft spectral transition isdriven by a changing inner radius of the accretion disc. This alsoqualitatively explains the changes seen in the variability power spectra ofthese objects. This lays the foundation for a self-consistent, theoreticallymotivated framework in which to explain both the spectral and timing propertiesof accreting compact objects.Comment: Proceedings of the conference "Stellar-mass, intermediate-mass, and supermassive black holes", Kyoto, October 200