Taking a Long Look: A Two-Decade Reverberation Mapping Study of
  High-Luminosity Quasars
  Reverberation mapping (RM) of active galactic nuclei (AGNs) has been used
over the past three decades to determine AGN broad-line region (BLR) sizes and
central black-hole masses, and their relations with the AGN's luminosity. Until
recently the sample of objects with RM data was limited to low-luminosity AGNs
($L_{\rm opt} \lesssim 10^{46}$ ergs s$^{-1}$) and low redshifts ($z \lesssim
0.5$). Here we present results from a reverberation-mapping project of some of
the most luminous and highest redshift quasars that have been mapped to date.
The study is based on almost twenty years of photometric monitoring of 11
quasars, six of which were monitored spectrophotometrically for 13 years. This
is the longest reverberation-mapping project carried out so far on this type of
AGNs. We successfully measure a time lag between the CIV$\lambda$1549 broad
emission line and the quasar continuum in three objects, and measure a
CIII$\lambda$1909 lag in one quasar. Together with recently published data on
CIV reverberation mapping, the BLR size is found to scale as the square root of
the UV luminosity over eight orders of magnitude in AGN luminosity. There is a
significant scatter in the relation, part of which may be intrinsic to the
AGNs. Although the CIV line is probably less well suited than Balmer lines for
determination of the mass of the black hole, virial masses are tentatively
computed and in spite of a large scatter we find that the mass of the black
hole scales as the square root of the UV luminosity.
