Swift monitoring of NGC 5548: X-ray reprocessing and short term UV/optical variability

TL;DR

This study analyzes the relationship between X-ray, UV, and optical variability in NGC 5548, showing that UV/optical variability is largely driven by X-ray reprocessing, with some additional variability from accretion rate changes.

Contribution

It presents the largest dataset to date on AGN variability, confirming the reprocessing model and exploring deviations from standard accretion disc predictions.

Findings

UV/optical lags scale with wavelength as ~wavelength^{1.23}

Lags are longer than standard disc models predict, suggesting hotter or more massive discs

Some UV/optical variability is due to accretion rate perturbations, not X-ray reprocessing

Abstract

Lags measured from correlated X-ray/UV/optical monitoring of AGN allow us to determine whether UV/optical variability is driven by reprocessing of X-rays or X-ray variability is driven by UV/optical seed photon variations. We present the results of the largest study to date of the relationship between the X-ray, UV and optical variability in an AGN with 554 observations, over a 750d period, of the Seyfert 1 galaxy NGC 5548 with Swift. There is a good overall correlation between the X-ray and UV/optical bands, particularly on short timescales (tens of days). These bands lag the X-ray band with lags which are proportional to wavelength to the power 1.23+/-0.31. This power is very close to the power (4/3) expected if short timescale UV/optical variability is driven by reprocessing of X-rays by a surrounding accretion disc. The observed lags, however, are longer than expected from a standard Shakura-Sunyaev accretion disc with X-ray heating, given the currently accepted black hole mass and accretion rate values, but can be explained with a slightly larger mass and accretion rate, and a generally hotter disc. Some long term UV/optical variations are not paralleled exactly in the X-rays, suggesting an additional component to the UV/optical variability arising perhaps from accretion rate perturbations propagating inwards through the disc.