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8BA18AAE-4DF1-4357-A772-521A84710EDB

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

Shoichiro Mizukoshi, Takeo Minezaki, Shoichi Tsunetsugu, Atsuhiro Yoshida, Hiroaki Sameshima, Mitsuru Kokubo, Hirofumi Noda

Submitted on 16 September 2022

## 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 $\mathit{\text{Wide-fieldInfrared Survey Explorer (WISE)}}$ for 513 local AGNs catalogued by the $\mathit{Swift}\mathit{/}$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 (${N}_{\mathrm{H}}$) of the BASS catalogue. We found that the ${N}_{\mathrm{H}}/{A}_{V}$ 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 ${N}_{\mathrm{H}}/{A}_{V}$ of obscured AGNs is comparable to the Galactic diffuse ISM. These properties of the ${N}_{\mathrm{H}}/{A}_{V}$ can be explained by increase in the ${N}_{\mathrm{H}}$ attributed to the dust-free gas clouds covering the line of sight in the broad-line region.

## Preprint

Comment: 11 pages, 7 figures, published in MNRAS

Subject: Astrophysics - Astrophysics of Galaxies