X-ray and radio monitoring of GX 339-4 and CYG X-1

Previous work by Motch et al. [1985, Space Sci. Rev. 40, 219] suggested that in the low/hard state of GX, the soft X-ray power-law extrapolated backward in energy agrees with the IR flux level. Corbel and Fender [2002, ApJ 573, L35-L39] later showed that the typical hard state radio power-law extrapolated forward in energy meets the backward extrapolated X-ray power-law at an IR spectral break, which was explicitly observed twice in GX. This has been cited as further evidence that jet synchrotron radiation might make a significant contribution to the observed X-rays in the hard state. We explore this hypothesis with a series of simultaneous radio/X-ray hard state observations of GX. We fit these spectra with a simple, but remarkably successful, doubly broken power-law model that indeed requires a spectral break in the IR. For most of these observations, the break position as a function of X-ray flux agrees with the jet model predictions. We then examine the radio flux/X-ray flux correlation in CYG through the use of 15 GHz radio data, obtained with the Ryle radio telescope, and Rossi X-ray Timing Explorer data, from the All Sky Monitor and pointed observations. We find evidence of 'parallel tracks' in the radio/X-ray correlation which are associated with 'failed transitions' to, or the beginning of a transition to, the soft state. We also find that for CYG the radio flux is more fundamentally correlated with the hard, rather than the soft, X-ray flux.