Inner size of a dust torus in the Seyfert 1 galaxy NGC 4151

The most intense monitoring observations yet made were carried out on the Seyfert 1 galaxy NGC 4151 in the optical and near-infrared wave bands. A lag from the optical light curve to the near-infrared light curve was measured. The lag time between the V and K light curves at the flux minimum in 2001 was precisely 48(-3)(+2) days, as determined by a cross-correlation analysis. The correlation between the optical luminosity of an active galactic nucleus (AGN) and the lag time between the UV/optical and the near-infrared light curves is presented for NGC 4151 in combination with previous lag-time measurements of NGC 4151 and other AGNs in the literature. This correlation is interpreted as thermal dust reverberation in an AGN, where the near-infrared emission from an AGN is expected to be the thermal reradiation from hot dust surrounding the central engine at a radius where the temperature equals that of the dust sublimation temperature. We find that the inner radius of the dust torus in NGC 4151 is similar to0.04 pc corresponding to the measured lag time, well outside the broad-line region determined by other reverberation studies of the emission lines.