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 days, as determined by a cross-correlation analysis. The correlation between the optical luminosity of an active galactic nucleus (AGN) and the lagtime 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 re-radiation from hot dust surrounding the central Institute of Astronomy, School of Science, University of Tokyo, 2-21-1 Osawa, Mitaka, Tokyo 181-0015, Japan e-mail: [email protected] Research Center for the Early Universe, School of Science, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan National Astronomical Observatory, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 3-1-1, Yoshinodai, Sagamihara, Kanagawa, 229-8510, Japan Department of Astronomy, School of Science, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0013, Japan Kiso Observatory, Institute of Astronomy, School of Science, University of Tokyo, 10762-30 Mitake, Kiso, Nagano 397-0101, Japan Mount Stromlo Observatory, Research School of Astronomy and Astrophysics, The Australian National University, Weston Creek P. O., A.C.T. 2611, Australia