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 r _ { g } .
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 ( \Gamma ) vs. flux correlation : \Gamma appears to approach a limiting value of \Gamma \sim 2.1 at high flux .
Two models are proposed to explain both the new feature in the \Gamma 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 \Gamma vs. flux relation , they can not at present be tested meaningfully by the data .
The various implications and the physical interpretation of these models are discussed .