The most intense monitoring observations yet made in the optical and near-infrared wave bands were carried out for Seyfert 1 galaxies NGC 5548, NGC 4051, NGC 3227, and NGC 7469 by the MAGNUM telescope, and clear timedelayed responses of the K-band flux variations to the V-band flux variations were found for all of these galaxies. Their H À K color temperatures of 1500-1800 K, estimated from their observed flux variation gradients, support a view that the bulk of the K flux should originate in the thermal radiation of hot dust surrounding the central engine and that the lag time should correspond to light-travel distance between them. Cross-correlation analysis measures their lag times to be 47-53 (NGC 5548), 11-18 (NGC 4051), about 20 (NGC 3227), and 65-87 (NGC 7469) days. The lag times are tightly correlated with the optical luminosities, as expected from dust reverberation (Át / L 0:5 ), while weakly with the central virial masses, which suggests that the inner radii of the dust tori around active nuclei have oneto-one correspondences with their central luminosities. In the lag time versus central luminosity diagram, the K-band lag times place an upper boundary on the similar lag times of broad emission lines in the literature, which not only supports the unified scheme of AGNs but also implies a physical transition from the BLR out to the dust torus that encircles the BLR. Correlated short-term V-band and X-ray flux variations in NGC 5548 are also found with a delay of 1 or 2 days, indicating the thermal reprocessing of X-ray emission by the central accretion flow.

We present the results of a dust reverberation survey for 17 nearby Seyfert 1 galaxies, which provides the largest homogeneous data collection for the radius of the innermost dust torus. A delayed response of the K-band light curve after the V -band light curve was found for all targets, and 49 measurements of lag times between the flux variation of the dust emission in the K band and that of the optical continuum emission in the V band were obtained by the cross-correlation function analysis and also by an alternative method for estimating the maximum likelihood lag. The lag times strongly correlated with the optical luminosity in the luminosity range of M V = −16 to −22 mag, and the regression analysis was performed to obtain the correlation log ∆t (days) = −2.11 − 0.2M V assuming ∆t ∝ L 0.5 , which was theoretically expected. We discuss the possible origins of the intrinsic scatter of the dust lag-luminosity correlation, which was estimated to be approximately 0.13 dex, and we find that the difference of internal extinction and delayed response of changes in lag times to the flux variations could have partly contributed to intrinsic scatter. However, we could not detect any systematic change of the correlation with the subclass of the Seyfert type or the Eddington ratio. Finally, we compare the dust reverberation radius with the near-infrared interferometric radius of the dust torus and the reverberation radius of broad Balmer emission lines. The interferometric radius in the K band was found to be systematically larger than the dust reverberation radius in the same band by about a factor of two, which could be interpreted by the difference between the flux-weighted radius and the response-weighted radius of the innermost dust torus. The reverberation radius of the broad Balmer emission lines was found to be systematically smaller than the dust reverberation radius by about a factor of 4-5, which strongly supports the unified scheme of the Seyfert type of active galactic nuclei (AGNs). Moreover, we examined the radius-luminosity correlations for the hard X-ray (14-195 keV) and the [O IV]λ25.89 µm emission-line luminosities, which would be applicable for obscured AGNs.

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 days, ϩ2 48 Ϫ3 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 ∼0.04 pc corresponding to the measured lag time, well outside the broad-line region determined by other reverberation studies of the emission lines.

The long-term optical and near infrared monitoring observations for a type 1 active galactic nucleus NGC 4151 were carried out for six years from 2001 to 2006 by using the MAGNUM telescope, and delayed response of flux variations in the K(2.2µm) band to those in the V (0.55µm) band was clearly detected.Based on cross correlation analysis, we precisely measured a lag time ∆t for eight separate periods, and we found that ∆t is not constant changing between 30 and 70 d during the monitoring period. Since ∆t 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 dust torus did change in an individual AGN. In order to relate such a change of ∆t with a change of AGN luminosity L, we presented a method of taking an average of the observed V -band fluxes that corresponds to the measured value of ∆t, and we found that the timechanging track of NGC 4151 in the ∆t versus L diagram during the monitoring period deviates from the relation of ∆t ∝ L 0.5 expected from dust reverberation.This result, combined with the elapsed time from period to period for which ∆t 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.