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A hot, dusty torus located around the outer edge of the broad-line region ofAGNs is a fundamental ingredient in unified AGN models. While the existence ofcircumnuclear dust around AGNs at pc-scale radii is now widely accepted,questions about the origin, evolution and long-term stability of these dusttori remain unsettled.\\ We used reverberation mapping of the hot circumnucleardust in the Seyfert 1 galaxy NGC 4151, to monitor its temperature andreverberation lag as a function of the varying accretion disk brightness. Wecarried out multiband, multiepoch photometric observations of the nucleus ofNGC 4151 in the z,Y,J,H, and K bands for 29 epochs from 2010 January to 2014June, supported by new near-infrared and optical spectroscopic observations,and archived WISE data.\\ We see no signatures of dust destruction due tosublimation in our data, since they show no increase in the hot dustreverberation delay directly correlated with substantial accretion disk fluxincreases in the observed period. Instead, we find that the hot dust in NGC4151 appears to merely heat up, and the hot dust temperature closely tracks theaccretion disk luminosity variations. We find indications of a decreasedreverberation delay within the observed period from t = 42.5 +/- 4.0 days in2010 to t = 29.6 +/- 1.7 days in 2013-2014. Such a varying reverberation radiuson longer timescales would explain the intrinsic scatter observed in theradius-luminosity relation of dust around AGNs.\\ Our observations rule outthat a second, larger dust component within a 100-light-day radius from thesource contributes significantly to the observed near-infrared flux in thisgalaxy.

Monitoring the temperature and reverberation delay of the circumnuclear hot dust in NGC 4151