B and JHKL light curves of NGC 4151

At 1-4{mu}m, the hot dust reverberation signals come from two distinct dust populations at separate radii (~0.033pc and ~0.076pc), with temperatures of ~1500-2500K and ~900-1000K, consistent with the expected properties of sublimating graphite and silicate dust grains.

The domination of the torus infrared output by carbon and silicate grains near their sublimation temperatures and radii may account for the general similarity of AGN near-IR spectral energy distributions. The torus inner edge defined by the hottest dust remains at roughly the same radius independent of the AGN optical luminosity over ~25yr. The emission by hot dust warmed directly by the optical/UV AGN output has increased gradually by ~4% yr-1, indicating a possibly growing torus.