Testing Accretion Disc Theory: X-ray - UVW1 Lag Measurement in NGC 4593

Recent Swift UV/optical lag measurements show that UV/optical variability inNGC5548 comes from reprocessing of Xrays by the accretion disc. However theimplied disc size is 3x larger than predicted by standard ShakuraSunyaevtheory. Before speculating about the discrepancy we should measure high S/N lagsin AGN with different disc parameters. A very large Swift campaign has beenproposed for NGC4593, mass 7x lower than NGC5548, with guaranteed continuousKepler, daily HST and 3x daily griz monitoring. Swift will measure UVopticallags easily but struggles to measure the estimated 20ks Xray/UV lag. Thistimescale is, however, perfectly suited to XMM, as our recent PN/OM fast modeobservations of NGC4395 show. Here we propose a 130ks observation to measure the crucial Xray/UVW1 lag. truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]