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The Reverberation Radius of the Central Dust Hole in NGC 5548

The most intensive multicolor monitoring observations in the optical and near-infrared wave bands were carried out for the Seyfert 1 galaxy NGC 5548. During a monitoring period from 2001 March to 2003 July, the V(0.55 μm) and K(2.2 μm) fluxes separately reached a minimum state twice. A delayed response of light variations in the K band to those in the V band was detected, and the lag time Δt was measured by a cross-correlation analysis based on a new technique of simulating the light curves to fill in the sampling gaps. The measured lag time is Δt = 48 days in the first minimum state and Δt = 47 days in the second minimum state. The lag time is interpreted as the light-travel time from the central energy source to the surrounding dust torus, because the K light is emitted from the hot dust heated by absorption of ultraviolet and optical light from the central energy source. Compared with lag measurements of the broad emission lines in the literature, the lag time for such dust reverberation is found to be longer, indicating that the inner radius of the dust torus corresponds to an outer edge of the broad emission line region in NGC 5548. Furthermore, the V light curve exhibits short-timescale variations superposed on its global variation. These variations on timescales of 10 days or less are found to correlate with X-ray variations and delay behind them by 1 or 2 days, indicating the optical reprocessing of X-ray emission in the central accretion flow in NGC 5548.