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 \tau = 42.5 \pm 4.0 days in 2010 to \tau = 29.6 \pm 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 .