Measurement of AGN dust extinction based on the near-infrared flux variability of WISE data. Issue 2 (15th September 2022) - Mizukoshi, Shoichiro
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 W 1 and W 2) are tightly correlated for more than 90 per cent of the targets. The flux variation gradient (FVG) in the W 1 and W 2 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 ( N H ) of the BASS catalogue. We found that the N H / 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 N H / AV of obscured AGNs is comparable to the Galactic diffuse ISM. These properties of the N H / AV can be explained by increase in the N H attributed to the dust-free gas clouds covering the line of sight in the broad-line region.