The Origin of UV-optical Variability in AGN and Test of Disc Models:
  XMM-Newton and ground based observations of NGC4395
  The origin of short timescale (weeks/months) variability of AGN, whether due
to intrinsic disc variations or reprocessing of X-ray emission by a surrounding
accretion disc, has been a puzzle for many years. However recently a number of
observational programmes, particularly of NGC5548 with Swift, have shown that
the UV/optical variations lag behind the X-ray variations in a manner strongly
supportive of X-ray reprocessing. Somewhat surprisingly the implied size of the
accretion disc is ~3x greater than expected from a standard, smooth,
Shakura-Sunyaev thin disc model. Although the difference may be explained by a
clumpy accretion disc, it is not clear whether the difference will occur in all
AGN or whether it may change as, eg, a function of black hole mass, accretion
rate or disc temperature. Measurements of interband lags for most AGN require
long timescale monitoring, which is hard to arrange. However for low mass (<1
million solar mass) AGN, the combination of XMM-Newton EPIC (X-rays) with the
optical monitor in fast readout mode allows an X-ray/UV-optical lag to be
measured within a single long observation. Here we summarise previous related
observations and report on XMM-Newton observations of NGC4395 (mass ~100x lower
and accretion rate ~20x lower than for NGC5548). We find that the UVW1 lags the
X-rays by ~470s. Simultaneous observations at 6 different ground based
observatories also allowed the g-band lag (~800s) to be measured. These
observations are in agreement with X-ray reprocessing but initial analysis
suggests that, for NGC4395, they do not differ markedly from the predictions of
the standard thin disc model.
