ИСТИНА

As seen from the central source, the dusty torus of AGNs has a puzzlingly high covering factor. If the torus consists of clouds of dust, each with a relatively unobscured view of the higher energy photons from nearer the center of the AGN, the temperature of each dust cloud will fall off as roughly the inverse square root of the radius. Since the dust is heated by the central radiation, in such a model the Near and Mid IR lag would increase with the wavelength to a power of 2 to 2.8. We show that, contrary to this simple prediction, for a significant fraction of AGNs the lags of the J, H, K, and L bands with respect to the optical show at best only a small difference. This means that rather than there being an extended radial temperature gradient, the hot dust reprocessing the central radiation is effectively in a relatively thin shell. We show that this can be explained by the hot dust being on the surface of a cone that is approximately tangential to the paraboloidal isodelay surface. We note that a number of the AGNs showing similar J, H, K, and L lags are also Seyferts that have changed between type 1 and type 2. It is not clear whether this is related or is merely a consequence of these objects being well studied for a long time.