Continuum reverberation mapping of the quasar PG 2130+099
  We present the results of an intensive six-month optical continuum
reverberation mapping campaign of the Seyfert 1 galaxy PG 2130+099 at redshift
z=0.063. The ground-based photometric monitoring was conducted on a daily basis
with the robotic 46cm telescope of the Wise observatory located in Israel.
Specially designed narrowband filters were used to observe the central engine
of the active galactic nucleus (AGN), avoiding line contamination from the
broad-line region (BLR). We aim to measure inter-band continuum time lags
across the optical range and determine the size-wavelength relation for this
system. We used two methods, the traditional point-spread function (PSF)
photometry and the recently developed proper image subtraction technique, to
independently perform the extraction of the continuum light curves. The
inter-band time lags are measured with several methods, including the
interpolated cross-correlation function, the z-transformed discrete correlation
function, a von Neumann estimator, JAVELIN (in spectroscopic mode), and MICA.
PG 2130+099 displays correlated variability across the optical range, and we
successfully detect significant time lags of up to 3 days between the multiband
light curves. Using the flux variation gradient (FVG) method, we determined the
AGN's host-galaxy-subtracted rest-frame 5100{\AA} luminosity at the time of our
monitoring campaign with an uncertainty of 18%. While a continuum reprocessing
model can fit the data reasonably well, our derived disk sizes are a factor of
2-6 larger than the theoretical disk sizes predicted from the AGN luminosity
estimate of PG 2130+099. This result is in agreement with previous studies of
AGN/quasars and suggests that the standard Shakura-Sunyaev disk theory has
limitations in describing AGN accretion disks.
