On the lack of X-ray iron line reverberation in MCG-6-30-15: Implications for the black hole mass and accretion disk structure

We use the method of Press, Rybicki, & Hewitt to search for time lags and t ime leads between different energy bands of the RXTE data for MCG -6-30-15. We tailor our search in order to probe any reverberation signatures of the fluorescent iron Ka line that is thought to arise from the inner regions o f the black hole accretion disk. In essence, an optimal reconstruction algo rithm is applied to the continuum band (2-4 keV) light curve that smooths o ut noise and interpolates across the data gaps. The reconstructed continuum band light curve can then be folded through trial transfer functions in an attempt to find lags or leads between the continuum band and the iron line band (5-7 keV). We find reduced fractional variability in the line band. T he spectral analysis of Lee et al. reveals this to be due to a combination of an apparently constant iron line flux (at least on timescales of few x 1 0(4) s) and flux-correlated changes in the photon index. We also find no ev idence for iron line reverberation and exclude reverberation delays in the range 0.5-50 ks. This extends the conclusions of Lee et al. and suggests th at the iron line flux remains constant on timescales as short as 0.5 ks. Th e large black hole mass (>10(8) M.) naively suggested by the constancy of t he iron line flux is rejected on other grounds. We suggest that the black h ole in MCG -6-30-15 has a mass of M-BH similar to 10(6)-10(7) M. and that c hanges in the ionization state of the disk may produce the puzzling spectra l variability. Finally, it is found that the 8-15 keV band lags the 2-4 keV band by 50-100 s. This result is used to place constraints on the size and geometry of the Comptonizing medium responsible for the hard X-ray power l aw in this AGN.