THE ORIGIN OF OPTICAL VARIABILITY IN AGN: XMM AND GROUND OBSERVATIONS OF NGC4395

The underlying driver behind the optical variability of AGN, and its link to theXray variability, is currently in dispute. Are the optical variations dominatedby reprocessing of Xray photons (ie the optical lags), or by inwardlypropagating accretion rate fluctuations which alter the intrinsic thermalemission from the disc (ie the optical leads)? Current observations favouroptical lags there are no unambiguous examples. In NGC4395, a well known highlyvariable Xray AGN, we have previously found a tentative 400s UV lag, atimescale well suited to observations with XMM. Here, with a combination of XMMand groundbased observations we aim to measure the lag accurately and henceshow whether optical variability in AGN is driven by disc variations or by Xray reprocessing. truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]