Correlated X-ray/Ultraviolet/Optical Variability in NGC 6814
  We present results of a 3-month combined X-ray/UV/optical monitoring campaign
of the Seyfert 1 galaxy NGC 6814. The object was monitored by Swift from June
through August 2012 in the X-ray and UV bands and by the Liverpool Telescope
from May through July 2012 in B and V. The light curves are variable and
significantly correlated between wavebands. Using cross-correlation analysis,
we compute the time lag between the X-ray and lower energy bands. These lags
are thought to be associated with the light travel time between the central
X-ray emitting region and areas further out on the accretion disc. The computed
lags support a thermal reprocessing scenario in which X-ray photons heat the
disc and are reprocessed into lower energy photons. Additionally, we fit the
lightcurves using CREAM, a Markov Chain Monte Carlo code for a standard disc.
The best-fitting standard disc model yields unreasonably high super-Eddington
accretion rates. Assuming more reasonable accretion rates would result in
significantly under-predicted lags. If the majority of the reprocessing
originates in the disc, then this implies the UV/optical emitting regions of
the accretion disc are farther out than predicted by the standard thin disc
model. Accounting for contributions from broad emission lines reduces the lags
in B and V by approximately 25% (less than the uncertainty in the lag
measurements), though additional contamination from the Balmer continuum may
also contribute to the larger than expected lags. This discrepancy between the
predicted and measured interband delays is now becoming common in AGN where
wavelength-dependent lags are measured.
