Variation of inner radius of dust torus in NGC4151

Long-term optical and near-infrared monitoring observations for a type 1 active galactic nucleus (AGN) NGC4151 were carried out for six years from 2001 to 2006 by using the MAGNUM telescope, and delayed response of flux variations in the K(2.2 μm) band to those in the V(0.55 μm) band was clearly detected. Based on cross-correlation analysis, we precisely measured a lag time Δt for eight separate periods and we found that Δt is not constant, changing between 30 and 70 d during the monitoring period. Since Δt is the light travel time from the central energy source out to the surrounding dust torus, this is the first convincing evidence that the inner radius of the dust torus did change in an individual AGN. In order to relate such a change of Δt with a change of AGN luminosity L, we present a method of taking an average of the observed V-band fluxes that corresponds to the measured value of Δt, and we find that the time-changing track of NGC 4151 in the Δt versus L diagram during the monitoring period deviates from the relation Δt L ^0.5 expected from dust reverberation. This result, combined with the elapsed time from period to period for which Δt was measured, indicates that the timescale of dust formation is about one year, which should be taken into account as a new constraint in future studies of dust evolution in AGNs.