Vanishing hardness-flux correlation in Cygnus X-1: Signs of the disc moving out

Abstract

Aims. We investigate observations of the X-ray binary Cygnus X-1 with unusually high hardness and low flux. In particular, we study the characteristic frequencies seen in the PDS and the hardness-flux correlation within and between these observations.Methods. We analyse observations of Cyg X-1 during periods when the source reaches its highest hardness levels (≳ 1 for the 9-20 keV over 2-4 keV RXTE/PCA count ratios, corresponding to Γ ≲ 1.6). Using the relativistic precession model to interpret the PDS, we estimate a value for the inner radius of the accretion disc. We also study the hardness-flux correlation.Results. In the selected observations, the characteristic frequencies seen in the power spectrum are shifted to the lowest end of their frequency range. Within a single observation, the hardness-flux correlation is very weak, contrary to the negative correlation normally observed in the hard state. We suggest that this could be interpreted as the inner disc boundary being at large radii (≳50Rg), thereby requiring more time to adjust to a changing accretion rate than allowed by a single RXTE observation, and compare our findings to estimates of the viscous time scale responsible for small scale variability in the system. © 2008 ESO.