Long-term monitoring of the archetype Seyfert galaxy MCG-6-30-15: X-ray,
  optical and near-IR variability of the corona, disc and torus
  We present long term monitoring of MCG-6-30-15 in X-rays, optical and near-IR
wavelengths, collected over five years of monitoring. We determine the power
spectrum density of all the observed bands and show that after taking into
account the host contamination similar power is observed in the optical and
near-IR bands. There is evidence for a correlation between the light curves of
the X-ray photon flux and the optical B-band, but it is not possible to
determine a lag with certainty, with the most likely value being around zero
days. Strong correlation is seen between the optical and near-IR bands. Cross
correlation analysis shows some complex probability distributions and lags that
range from 10 to 20 days, with the near-IR following the optical variations.
Filtering the light curves in frequency space shows that the strongest
correlations are those corresponding to the shortest time-scales. We discuss
the nature of the X-ray variability and conclude that this is intrinsic and
cannot be accounted for by absorption episodes due to material intervening in
the line of sight. It is also found that the lags agree with the relation tau ~
lambda^(4/3), as expected for an optically thick geometrically thin accretion
disc, although for a larger disc than that predicted by the estimated black
hole mass and accretion rate in MCG-6-30-15. The cross correlation analysis
suggests that the torus is located at ~20 light-days from the central source
and at most at ~50 light-days from the central region. This implies an AGN
bolometric luminosity of ~3x10^(43) ergs/s/cm-2.
