Reverberation Measurements of the Inner Radius of the Dust Torus in Nearby Seyfert 1 Galaxies

The most intense monitoring observations yet made in the optical (UBV) andnear-infrared (JHK) wave bands were carried out for nearby Seyfert1 galaxies ofNGC 5548, NGC 4051, NGC 3227, and NGC 7469. Over three years of observationswith MAGNUM telescope since early 2001, clear time-delayed response of theK-band flux variations to the V-band flux variations was detected for all ofthese galaxies. Their H-K color temperature was estimated to be 1500-1800 Kfrom the observed flux variation gradients, which supports a view that the bulkof the K flux should originate in the thermal radiation of hot dust thatsurrounds the central engine. Cross-correlation analysis was performed toquantify the lag time corresponding to the light-travel distance of the hotdust region from the central engine. The measured lag time is 47-53 days forNGC 5548, 11-18 days for NGC 4051, about 20 days for NGC 3227, and 65-87 daysfor NGC 7469. We found that the lag time is tightly correlated with the opticalluminosity as expected from dust reverberation ($\Delta t \propto L^{0.5}$),while only weakly with the central virial mass, which suggests that an innerradius of the dust torus around the active nucleus has a one-to-onecorrespondence to central luminosity. In the lag time versus central luminositydiagram, the K-band lag times place an upper boundary on the similar lag timesof broad-emission lines in the literature. This not only supports the unifiedscheme of AGNs, but also implies a physical transition from the BLR out to thedust torus that encircles the BLR. Furthermore, our V-band flux variations ofNGC 5548 on timescales of up to 10 days are found to correlate with X-rayvariations and delay behind them by one or two days, indicating the thermalreprocessing of X-ray emission by the central accretion flow.Comment: ApJ, March 2006, v639 issue, 24 pages, 33 figures, 10 table