Measurement of AGN dust extinction based on the near-infrared flux variability of WISE data

TL;DR

This study measures dust extinction in obscured AGNs using near-infrared flux variability from WISE data, revealing diverse dust properties and gas-to-dust ratios that inform AGN structure models.

Contribution

It introduces a novel method to estimate line-of-sight dust extinction in AGNs through NIR flux variability analysis, expanding understanding of AGN dust and gas characteristics.

Findings

Obscured AGNs show redder and broader FVG distributions than unobscured ones.

The N_H/A_V ratios in obscured AGNs are higher and more scattered than in the Galactic ISM.

Lower N_H/A_V values are comparable to the Galactic diffuse ISM, indicating dust-free gas clouds.

Abstract

We present the measurement of the line-of-sight extinction of the dusty torus for a large number of obscured active galactic nuclei (AGNs) based on the reddening of the colour of the variable flux component in near-infrared (NIR) wavelengths. We collected long-term monitoring data by $\textit{Wide-field Infrared Survey Explorer (WISE)}$ for 513 local AGNs catalogued by the $\mathit{Swift/}$BAT AGN Spectroscopic Survey (BASS) and found that the multi-epoch NIR flux data in two different bands (WISE $W1$ and $W2$) are tightly correlated for more than 90% of the targets. The flux variation gradient (FVG) in the $W1$ and $W2$ bands was derived by applying linear regression analysis, and we reported that those for unobscured AGNs fall in a relatively narrow range, whereas those for obscured AGNs are distributed in a redder and broader range. The AGN's line-of-sight dust extinction ($A_V$) is calculated using the amount of the reddening in the FVG and is compared with the neutral hydrogen column density ($N_{\rm{}H}$) of the BASS catalogue. We found that the $N_{\rm{}H}/A_V$ ratios of obscured AGNs are greater than those of the Galactic diffuse interstellar medium (ISM) and are distributed with a large scatter by at most two orders of magnitude. Furthermore, we found that the lower envelope of the $N_{\rm{}H}/A_V$ of obscured AGNs is comparable to the Galactic diffuse ISM. These properties of the $N_{\rm{}H}/A_V$ can be explained by increase in the $N_{\rm{}H}$ attributed to the dust-free gas clouds covering the line of sight in the broad-line region.