Negative X-ray reverberation time delays from MCG-6-30-15 and Mrk 766
  We present an X-ray time lag analysis, as a function of Fourier frequency, for MCG-6-30-15 and Mrk 766 using long term XMM-Newton light curves in the 0.5-1.5 keV and the 2-4 keV energy bands, together with some physical modelling of the corresponding time lag spectra. Both the time lag spectra of MCG-6-30-15 and Mrk 766 show negative values (i.e. soft band variations lag behind the corresponding hard band variations) at high frequencies, around 0.001 Hz, similar to those previously observed from 1H 0707-495. The remarkable morphological resemblance between the time lag spectra of MCG-6-30-15 and Mrk 766 indicate that the physical processes responsible for the observed soft time delays is very similar in the two sources, favouring a reflection scenario from material situated very nearby to the central black hole. 
D. Emmanoulopoulos, I. M. McHardy and I. E. Papadakis
astro-ph.CO astro-ph.HE