Optical and near-IR long-term monitoring of NGC 3783 and MR 2251-178: evidence for variable near-IR emission from thin accretion discs

We present the long-term near-IR light curves for two nearby active galactic nuclei (AGN), NGC 3783 and MR 2251-178. The near-IR data are complemented by optical photometry obtained over the same period of time. The light curves in all bands are highly variable and good correlations can be seen between optical and near-IR variations. A cross-correlation analysis for NGC 3783 suggests that some disc near-IR emission is present in the J-band flux, while the H and K bands are dominated by emission from a torus located at the dust sublimation radius. For MR 2251-178 the cross-correlation analysis and the optical-near-IR flux-flux plots suggest that the near-IR flux is dominated by disc emission. We model the optical to near-IR spectral energy distributions (SEDs) of both sources and find that disc flaring might be a necessary modification to the geometry of a thin disc in order to explain the observations. The SED of MR 2251-178 gives some indications of the presence of near-IR emission from a torus. Finally, we consider the implications of the standard alpha-disc model to explain the different origin of the variable near-IR emission in these AGN.