Studying X-ray reprocessing and continuum variability in quasars: PG
  1211+143
  We present the results from a monitoring campaign of the Narrow-Line
Seyfert~1 galaxy PG 1211+143. The object was monitored with ground-based
facilities (UBVRI photometry; from February to July, 2007) and with Swift
(X-ray photometry/spectroscopy and UV/Optical photometry; between March and
May, 2007). We found PG 1211+143 in a historical low X-ray flux state at the
beginning of the Swift monitoring campaign in March 2007. It is seen from the
light curves that while violently variable in X-rays, the quasar shows little
variations in optical/UV bands. The X-ray spectrum in the low state is similar
to other Narrow-Line Seyfert 1 galaxies during their low-states and can be
explained by a strong partial covering absorber or by X-ray reflection onto the
disk. With the current data set, however, it is not possible to distinguish
between both scenarios. The interband cross-correlation functions indicate a
possible reprocessing of the X-rays into the longer wavelengths, consistent
with the idea of a thin accretion disk, powering the quasar. The time lags
between the X-ray and the optical/UV light curves, ranging from ~2 to ~18 days
for the different wavebands, scale approximately as ~lambda^(4/3), but appear
to be somewhat larger than expected for this object, taking into account its
accretion disk parameters. Possible implications for the location of the X-ray
irradiating source are discussed.
