Mid-IR Variability and Dust Reverberation Mapping of Low-$z$ Quasars. I. Data, Methods and Basic Results

TL;DR

This study uses multi-wavelength IR and optical data to map dust structures in low-redshift quasars, revealing size ratios, variability patterns, and potential differences in torus structures, advancing understanding of AGN dust environments.

Contribution

It develops new methods to extract dust reverberation signals from sparse, contaminated datasets and provides the first comprehensive IR variability analysis over several years for a large quasar sample.

Findings

70-95% of quasars show convincing IR time-lags proportional to luminosity

Dust emission size ratios are approximately 0.6:1:1.2 for K, W1, W2 bands

Most AGNs exhibit less than 10% variability at 24 μm over 5 years

Abstract

The continued operation of the Wide-field Infrared Survey Explorer (WISE) combined with several ground-based optical transient surveys (e.g., CRTS, ASAS-SN and PTF) offer an unprecedented opportunity to explore the dust structures in luminous AGNs. We use these data for a mid-IR dust reverberation mapping (RM) study of 87 archetypal Palomar-Green quasars at $z\lesssim0.5$. To cope with various contaminations of the photometry data and the sparse time sampling of the light curves, procedures to combine these datasets and retrieve the dust RM signals have been developed. We find $\sim$70% of the sample (with a completeness correction, up to 95%) have convincing mid-IR time-lags in the WISE W1 ($\sim3.4~\mu m$) and W2 ($\sim4.5~\mu m$) bands and they are proportional to the square root of the AGN luminosity. Combined with previous K-band ($\sim2.2~\mu m$) RM results in the literature, the inferred dust emission size ratios are $R_{K}:R_{W1}:R_{W2}=0.6:1:1.2$. Under simple assumptions, we put preliminary constraints on the projected dust surface density at these bands and reveal the possibly different torus structures among hot-dust-deficient, warm-dust-deficient and normal quasars from the reverberation signals. With multi-epoch Spitzer data and later WISE photometry, we also explore AGN IR variability at 10--24 $\mu m$ over a 5-year time-scale. Except for blazars and flat-spectrum radio sources, the majority of AGNs have typical variation amplitudes at 24 $\mu m$ no more than 10% of that in the W1 band, indicating the dust reverberation signals damp out quickly at longer wavelengths. In particular, steep-spectrum radio quasars also lack strong 24 $\mu m$ variability, consistent with the unification picture of radio-loud AGN.