Monitoring the temperature and reverberation delay of the circumnuclear
  hot dust in NGC 4151
  A hot, dusty torus located around the outer edge of the broad-line region of
AGNs is a fundamental ingredient in unified AGN models. While the existence of
circumnuclear dust around AGNs at pc-scale radii is now widely accepted,
questions about the origin, evolution and long-term stability of these dust
tori remain unsettled.\\ We used reverberation mapping of the hot circumnuclear
dust in the Seyfert 1 galaxy NGC 4151, to monitor its temperature and
reverberation lag as a function of the varying accretion disk brightness. We
carried out multiband, multiepoch photometric observations of the nucleus of
NGC 4151 in the z,Y,J,H, and K bands for 29 epochs from 2010 January to 2014
June, supported by new near-infrared and optical spectroscopic observations,
and archived WISE data.\\ We see no signatures of dust destruction due to
sublimation in our data, since they show no increase in the hot dust
reverberation delay directly correlated with substantial accretion disk flux
increases in the observed period. Instead, we find that the hot dust in NGC
4151 appears to merely heat up, and the hot dust temperature closely tracks the
accretion disk luminosity variations. We find indications of a decreased
reverberation delay within the observed period from t = 42.5 +/- 4.0 days in
2010 to t = 29.6 +/- 1.7 days in 2013-2014. Such a varying reverberation radius
on longer timescales would explain the intrinsic scatter observed in the
radius-luminosity relation of dust around AGNs.\\ Our observations rule out
that a second, larger dust component within a 100-light-day radius from the
source contributes significantly to the observed near-infrared flux in this
galaxy.
