Frequency-resolved lags in UV/optical continuum reverberation mapping
  In recent years, continuum reverberation mapping involving high cadence
UV/optical monitoring campaigns of nearby Active Galactic Nuclei has been used
to infer the size of their accretion disks. One of the main results from these
campaigns has been that in many cases the accretion disks appear too large, by
a factor of 2 - 3, compared to standard models. Part of this may be due to
diffuse continuum emission from the broad line region (BLR), which is indicated
by excess lags around the Balmer jump. Standard cross correlation lag analysis
techniques are usually used to just recover the peak or centroid lag and can
not easily distinguish between reprocessing from the disk and BLR. However,
frequency-resolved lag analysis, where the lag is determined at each Fourier
frequency, has the potential to separate out reprocessing on different size
scales. Here we present simulations to demonstrate the potential of this method
and then apply a maximum likelihood approach to determine frequency-resolved
lags in NGC 5548. We find that the lags in NGC 5548 generally decrease smoothly
with increasing frequency, and are not easily described by accretion disk
reprocessing alone. The standard cross correlation lags are consistent with
lags at frequencies lower than 0.1 per day, indicating they are dominated from
reprocessing at size scales greater than about 10 light days. A combination of
a more distant reprocessor, consistent with the BLR, along with a
standard-sized accretion disk is more consistent with the observed lags than a
larger disk alone.
