X-ray - UV/optical lag measurement in the very low mass AGN NGC4395 using the OM in sub-second readout mode: Implications for disc models

The lag between the X-ray and UV/optical variations in AGN is a strong diagnostic of the emission processes in those bands; eg an X-ray lag means that seed photon variations drive X-ray variability but an X-ray lead means that the UV/optical variations result from reprocessing of X-rays by a surrounding accretion disc. Previous RXTE-based observations suggest that the UV/optical usually lags, although with large uncertainty. Recent Swift observations of NGC5548 (McHardy et al 2014; Edelson et al 2015), mass 4e7 solar, confirm a definite UV/optical lag, but the lag is larger than expected from a standard Shakura-Sunyaev disc model. To properly test our understanding of disc models it is vital to make similar lag observations of an AGN of very different mass. Here we report XMM-Newton observations of the low mass (3e5 solar) AGN, NGC4395. Using the OM in sub-second readout mode, we find that UVW1 lags the X-rays (EPIC) by ∼ 450 s. We consider the implications of this result and of parallel ground based g-band observations, together with our previous Swift observations, for our understanding of accretion discs. To the best of our knowledge, these observations represent the first successful use of the OM fast-readout mode for AGN lag measurements.