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

Abstract

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 Wide-field Infrared Survey Explorer (WISE) for 513 local AGNs catalogued by the 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 per cent 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 (AV) is calculated using the amount of the reddening in the FVG and is compared with the neutral hydrogen column density (NH) of the BASS catalogue. We found that the NH/AV 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 NH/AV of obscured AGNs is comparable to the Galactic diffuse ISM. These properties of the NH/AV can be explained by increase in the NH attributed to the dust-free gas clouds covering the line of sight in the broad-line region.