The origin of UV‐optical variability in AGN and test of disc models: XMM‐Newton and ground‐based observations of NGC 4395

Abstract

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 ∼ 3× 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 (< 10 6 M⊙) AGN, the combination of XMM-Newton EPIC (X-rays) with the optical monitor in fast readout mode allows an X-ray/UVoptical lag to be measured within a single long observation. Here we summarise previous related observations and report on XMM-Newton observations of NGC4395 (mass 100× lower, accretion rate ∼ 20× 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. Models for UV/Optical Variability and relationship to X-ray VariabilityThe origin of UV/optical variability in AGN and its relationship to X-ray variability has been a puzzle for some time and there are two main possibilities for the origin of the UV/optical variability. The UV/optical variability could result from reprocessing of X-ray emission by the accretion disc or it could simply be the result of intrinsic variability of the thermal emission from the disc. These two models can, in principle, be distinguished simply by measuring the lag between the X-ray and UV/optical wavebands. In the reprocessing model, the UV/optical variations will lag behind the X-ray variations by the light travel time between the two emission regions. For a typical AGN this time will be a few hours. If the UV/optical variations are produced by intrinsic ⋆ Corresponding author: e-mail: imh@soton.ac.uk disc variations there are two possible lag timescales. If the UV/optical photons are the seed photons for the X-ray emission, being Compton up-scattered in the central corona, then if the X-ray variations are driven by seed photon variations, the X-ray emission will lag behind the UV/optical variations by the light travel time between the two emission regions, ie a few hours. Alternatively, if the UV/optical variations are caused by inwardly propagating accretion rate variations (Arévalo & Uttley 2006), these variations will ev...

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