Original article can be found at: http://www.nature.com/nature/index.html--Copyright Nature Publishing Group --DOI : 10.1038/nature0345

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 have recently completed a 64-night spectroscopic monitoring campaign at the Lick Observatory 3-m Shane telescope with the aim of measuring the masses of the black holes in 12 nearby (z < 0.05) Seyfert 1 galaxies with expected masses in the range ∼ 10 6 -10 7 M ⊙ and also the well-studied nearby active galactic nucleus (AGN) NGC 5548. Nine of the objects in the sample (including NGC 5548) showed optical variability of sufficient strength during the monitoring campaign to allow for a time lag to be measured between the continuum fluctuations and the response to these fluctuations in the broad Hβ emission. We present here the light curves for all the objects in this sample and the subsequent Hβ time lags for the nine objects where these measurements were possible. The Hβ lag time is directly related to the size of the broad-line region in AGNs, and by combining the Hβ lag time with the measured width of the Hβ emission line in the variable part of the spectrum, we determine the virial mass of the central supermassive black hole in these nine AGNs. The absolute calibration of the black hole masses is based on the normalization derived by Onken et al., which brings the masses determined by reverberation mapping into agreement with the local M BH − σ ⋆ relationship for quiescent galaxies. We also examine the time lag response as a function of velocity across the Hβ line profile for six of the AGNs. The analysis of four leads to rather ambiguous results with relatively flat time lags as a function of velocity. However, SBS 1116+583A exhibits a symmetric time lag response around the line center reminiscent of simple models for circularly orbiting broad-line region (BLR) clouds, and Arp 151 shows an asymmetric profile that is most easily explained by a simple gravitational infall model. Further investigation will be necessary to fully understand the constraints placed on physical models of the BLR by the velocityresolved response in these objects.

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.

To investigate the black hole mass vs. stellar velocity dispersion (M BH − σ * ) relation of active galaxies, we measured the velocity dispersions of a sample of local Seyfert 1 galaxies, for which we have recently determined black hole masses using reverberation mapping. For most objects, stellar velocity dispersions were measured from high signal-to-noise ratio optical spectra centered on the Ca II triplet region (∼ 8500Å), obtained at the Keck, Palomar, and Lick Observatories. For two objects, in which the Ca II triplet region was contaminated by nuclear emission, the measurement was based on high-quality H-band spectra obtained with the OH-Suppressing Infrared Imaging Spectrograph at the Keck-II Telescope. Combining our new measurements with data from the literature, we assemble a sample of 24 active galaxies with stellar velocity dispersions and reverberation-based black hole mass measurements in the range of black hole mass 10 6 < M BH /M ⊙ < 10 9 . We use this sample to obtain reverberation mapping constraints on the slope and intrinsic scatter of the M BH − σ * relation of active galaxies. Assuming a constant virial coefficient f for the reverberation mapping black hole masses, we find a slope β = 3.55 ± 0.60 and the intrinsic scatter σ int = 0.43 ± 0.08 dex in the relation log(M BH /M ⊙ ) = α + β log(σ * /200 km s −1 ), which are consistent with those found for quiescent galaxies. We derive an updated value of the virial coefficient f by finding the value which places the reverberation masses in best agreement with the M BH − σ * relation of quiescent galaxies; using the quiescent M BH − σ * relation determined by Gültekin et al. we find log f = 0.72 +0.09 −0.10 with an intrinsic scatter of 0.44±0.07 dex. No strong correlations between f and parameters connected to the physics of accretion (such as the Eddington ratio or line-shape measurements) are found. The uncertainty of the virial coefficient remains one of the main sources of the uncertainty in black hole mass determinations using reverberation mapping, and therefore also in single-epoch spectroscopic estimates of black hole masses in active galaxies.