Spectral variability of Cygnus X-1 during an intermediate state

We report the results of a simultaneous radio (Ryle telescope) and hard X-ray INTEGRAL observation of Cygnus X-1 during an intermediate state. During the 4 day long observation the broad band (3-200 keV) luminosity varied by up to a factor of 2.6 and the source showed an important spectral variability. A principal component analysis demonstrates that most of this variability occurs through 2 independent modes. The first mode consists in changes in the overall luminosity on time scale of hours with almost constant spectra (responsible for 68% of the variance). We interpret this variability mode as variations of the dissipation rate in the corona, possibly associated with magnetic flares. The second variability mode consists in a pivoting of the spectrum around ~10 keV (27% of the variance). It acts on a longer time-scale: initially soft, the spectrum hardens in the first part of the obervation and then softens again. This pivoting pattern is strongly correlated with the radio (15 GHz) emission: radio fluxes are stronger when the INTEGRAL spectrum is harder. We propose that the pivoting mode represents a "mini" state transition from a nearly High Soft State to a nearly Low Hard State, and back. This mini-transition would be caused by changes in the soft cooling photons flux in the hot Comptonising plasma associated with an increase of the temperature of the accretion disc. The jet power then appears to be anti-correlated with the disc luminosity and unrelated to the coronal power.