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 disk 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 three 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-fit inner and outer radii of the accretion disk) responsible for iron line emission actually narrows with increasing flux to a region around 4–5 rg. In contrast to 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 (Γ) vs. flux correlation: Γ appears to approach a limiting value of Γ ∼ 2.1 at high flux. Two models are proposed to explain both the new feature in the Γ vs. flux correlation and the uncorrelated iron-line flux: a phenomenological two power-law model, and the recently proposed “thundercloud” model of Merloni & Fabian (2001). Both models are capable of reproducing the data well, but because they are poorly constrained by the observed Γ vs. flux relation, they cannot at present be tested meaningfully by the data. The various implications and the physical interpretation of these models are discussed.