Orientation Effects on the Inner Region of Dusty Torus of Active Galactic Nuclei

TL;DR

This paper demonstrates that the anisotropic emission of accretion disks explains why the observed inner radius of the dusty torus in active galactic nuclei is smaller than expected, affecting the torus structure and infrared flux delays.

Contribution

It introduces a model considering anisotropic disk emission to resolve the discrepancy in the inner radius of the dusty torus in AGNs.

Findings

Anisotropic emission reduces the observed inner radius by about 1/3.

The inner edge of the torus can be connected to the disk's outer edge.

Infrared flux delay is 37% of the isotropic case for a 25° observer.

Abstract

A sublimation process governs the innermost region of the dusty torus of active galactic nuclei. However, the observed inner radius of the torus is systematically smaller than the expected radius by a factor of ~ 1/3. We show that the anisotropy of the emission from accretion disks resolves this conflict naturally and quantitatively. An accretion disk emits lesser radiation in the direction closer to its equatorial plane (i.e., to the torus). We find that the anisotropy makes the torus inner region closer to the central black hole and concave. Moreover, the innermost edge of the torus may connect with the outermost edge of the disk continuously. Considering the anisotropic emission of each clump in the torus, we calculate the near-infrared flux variation in response to a UV flash. For an observer at the polar angle theta_obs = 25 deg, the centroid of the time delay is found to be 37% of the delay expected in the case of isotropic illumination, which explains the observed systematic deviation.