Optical and Near-IR long-term monitoring of NGC3783 and MR2251-178: evidence for variable near-IR emission from thin accretion discs

TL;DR

This study presents long-term optical and near-IR observations of two AGN, revealing variable emission sources, with evidence for both accretion disc and torus contributions, and suggests disc flaring as a necessary geometric modification.

Contribution

It provides the first detailed long-term multi-band monitoring of NGC3783 and MR2251-178, analyzing the origins of near-IR emission and proposing disc flaring to explain observed spectral energy distributions.

Findings

Near-IR emission in NGC3783 is partly from the disc and partly from the torus.

In MR2251-178, near-IR flux is mainly from the accretion disc.

Disc flaring may be necessary to explain the optical-IR spectral energy distributions.

Abstract

We present long term near-IR light curves for two nearby AGN: NGC3783 and MR2251-178. The near-IR data are complemented with 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 NIR variations. 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 MR2251-178 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 (SED) 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 MR2251-178 gives some indications for the presence of NIR emission from a torus. Finally, we consider the implications of the standard alpha disc model to explain the different origin of the variable NIR emission in these AGN.