The continuum variability of MCG-6-30-15:: a detailed analysis of the long 1999 ASCA observation

We report on an analysis in the 3-10 keV X-ray band of the long 1999 ASCA observation of MCG-6-30-15. The time-averaged broad iron K line is well described by disc emission near a Schwarzschild black hole, confirming the results of earlier analyses on the ASCA 1994 and 1997 data. The time-resolved iron-line profile is remarkably stable over a factor of 3 change in source flux, and the line and continuum fluxes are uncorrelated. Detailed fits to the variable iron-line profile suggest that the active region (parametrized by the best-fitting inner and outer radii of the accretion disc) responsible for iron-line emission actually narrows with increasing flux to a region around 4-5r (g) . In contrast with the iron line, the power-law continuum exhibits significant variability during the 1999 observation. Time-resolved spectral analysis reveals a new feature in the well-known photon index (Gamma) versus flux correlation: Gamma appears to approach a limiting value of Gammasimilar to2.1 at high flux. Two models are proposed to explain both the new feature in the Gamma versus flux correlation and the uncorrelated iron-line flux: a phenomenological two power-law model, and the recently proposed 'thundercloud' model of Merloni & Fabian. Both models are capable of reproducing the data well, but because they are poorly constrained by the observed Gamma versus flux relation, they cannot at present be tested meaningfully by the data. The various implications and the physical interpretation of these models are discussed.