The origin of the UV/optical lags in NGC 5548

Understand

The new multi-wavelength monitoring campaign on NGC 5548 shows clearly that the variability of the UV/optical lightcurves lags by progressively longer times at longer wavelengths, as expected from reprocessing of an optically thick disk, but that the timescales are longer than expected for a standard Shakura-Sunyaev accretion disc.

• We build a full spectral-timing reprocessing model to simulate the UV/optical lightcurves of NGC 5548.
• We show that disc reprocessing of the observed hard X-ray lightcurve produces optical lightcurves with too much fast variability as well as too short a lag time.
• Supressing the fast variability requires an intervening structure preventing the hard X-rays from illuminating the disc.