The Origin of UV-optical Variability in AGN and Test of Disc Models: XMM-Newton and ground based observations of NGC4395

TL;DR

This study investigates the origin of UV-optical variability in AGN, using XMM-Newton and ground-based observations of NGC4395, to test if reprocessing or intrinsic disc variations explain observed lags, and compares findings with standard disc models.

Contribution

It provides the first simultaneous X-ray and UV/optical lag measurements for a low-mass AGN, testing disc models against observational data.

Findings

UVW1 lags X-rays by ~470s in NGC4395.

Ground-based g-band lag is ~800s, consistent with reprocessing.

Results align with standard thin disc predictions for this low-mass AGN.

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 ~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.