Dust physics in the nucleus of NGC 4151

The unified model of active galactic nuclei (AGNs) presumes the existence of a so-called dusty torus around the outer edge of the broad-line region. Despite the solid observational evidence for the existence of dust around AGNs and our growing information on the characteristic scales of these obscuring tori, the origin of this dust and its morphology are not yet well understood. Using dust reverberation mapping, we monitor the effects of AGN continuum variability to determine the temperature and covering factor of the circumnuclear dust, in order to constrain the physical conditions for dust survival and formation in the radiation field of the AGN. Multi-band photometry observations in the z, Y, J, H, and K bands were carried out on the nucleus of the prototypical Seyfert 1 galaxy NGC 4151 over six epochs from 2010 January to June, supported by spectroscopic observations, in order to investigate the response of the hot dust to varying accretion disk emission. Our data confirm that most of the hot dust reacts to increased radiation from the central source with a delayed brightening of similar to 50 days. In accretion disk brightening, we see no signatures of dust destruction in our data. The innermost dust appears to increase in temperature rather than sublimate, suggesting that it is cooler than sublimation temperature and located beyond the current sublimation radius. We characterize the dust geometry by interpreting the wavelength-dependent reverberation response with a simplified torus model, pointing to a static radially extended distribution of the central (similar to 0.1 pc) hot dust.