Evidence for rapid iron K alpha line flux variability in MCG-6-30-15

This paper employs direct spectral fitting of individual orbital data in or der to measure rapid X-ray iron K alpha line and continuum spectral slope v ariations in Seyfert 1 galaxies with unprecedented temporal resolution. App lication of this technique to a long RXTE observation of MCG -6-30-15 indic ates that the line flux does vary on short timescales, but that these varia tions are not correlated with (less than or similar to 1 day) changes in th e continuum flux or slope. These rapid variations indicate that the line do es indeed originate close to the black hole, confirming predictions based o n its very broad profile. However, the lack of a correlation with the conti nuum presents problems for models in which the line variations are driven b y those in the continuum, modified only by light-travel time effects. Inste ad, it may be that the line responds according to a physical process with a different timescale, such as ionization instabilities in the disk, or perh aps that the geometry and physical picture is more complex than implied by the simplest disk-corona models. These data also indicate that the slope of the underlying power-law continuum (Gamma) shows strong variability and is tightly correlated with the continuum flux in the sense that the spectrum steepens as the source brightens. All of these results have been checked wi th extensive simulations, which also indicated that a spurious correlation between Gamma and Compton reflection fraction (R) will result if these quan tities are measured from the same spectra. This casts serious doubts on pre vious claims of such a Gamma -R correlation.