Reverberation Measurements of the Inner Radii of the Dust Tori in
  Quasars
  We present the results of a dust-reverberation survey of quasars at redshifts
z<0.6. We found a delayed response of the K-band flux variation after the
optical flux variation in 25 out of 31 targets, and obtained the lag time
between them for 22 targets. Combined with the results for nearby Seyfert
galaxies, we provide the largest homogeneous collection of K-band
dust-reverberation data for 36 type 1 active galactic nuclei (AGNs). This
doubles the sample and includes the most distant AGN and the largest lag so far
measured. We estimated the optical luminosity of the AGN component of each
target using three different methods: spectral decomposition, the
flux-variation-gradient method, and image decomposition. We found a strong
correlation between the reverberation radius for the innermost dust torus and
the optical luminosity over a range of approximately four orders of magnitude
in luminosity, as is already known for Seyfert galaxies. We estimated the
luminosity distances of the AGNs based on their dust-reverberation lags, and
found that the data in the redshift-distance diagram are consistent with the
current standard estimates of the cosmological parameters. We also present the
radius-luminosity relations for isotropic luminosity indicators such as the
hard X-ray (14--195 keV), [OIV] 25.89 um, and mid-infrared (12 um) continuum
luminosities, which are applicable to obscured AGNs.
