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

Abstract

The continued operation of the Wide-field Infrared Survey Explorer (WISE) combined with several groundbased 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 0.5. 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 that ∼70% of the sample (with a completeness correction, up to 95%) has convincing mid-IR time lags in the WISE W 1 (∼ 3.4 µm) and W 2 (∼ 4.5 µm) bands and they are proportional to the square root of the AGN luminosity. Combined with previous K-band (∼ 2.2 µm) RM results in the literature, the inferred dust emission size ratios are R K : R W 1 : R W 2 = 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 µm over a 5 yr time-scale. Except for blazars and flat-spectrum radio sources, the majority of AGNs have typical variation amplitudes at 24 µm of no more than 10% of that in the W 1 band, indicating that the dust reverberation signals damp out quickly at longer wavelengths. In particular, steep-spectrum radio quasars also lack strong 24 µm variability, consistent with the unification picture of radio-loud AGNs.

Mid-infrared Outbursts in Nearby Galaxies (MIRONG). I. Sample Selection and Characterization

“…The torus will unavoidably absorb part of the UV-optical photons from the accretion disk and reprocess them into the IR. The picture has been widely accepted through SED fitting (e.g., Fritz et al 2006;Nenkova et al 2008;Stalevski et al 2012) and IR reverberation mapping (e.g., Suganuma et al 2006;Koshida et al 2014;Lyu et al 2019). It is plausible that our discovered MIR flares are due to dust echoes of turn-on AGNs.…”

Reverberation mapping of active galactic nuclei: From X-ray corona to dusty torus

“…In the last 20 years, a number of dedicated dust reverberation campaigns have substantially increased the sample of lags (e.g., Suganuma et al., 2006 ; Koshida et al., 2014 ; Minezaki et al., 2019 ), as have surveys with repeated imaging that search for transient phenomena ( Lyu et al., 2019 ; Yang et al., 2020 ), firmly establishing the dust relation over 4 orders of magnitude in luminosity. On average, the dust lags are 4 times longer than Hβ lags (e.g., Koshida et al., 2014 ; Netzer 2015 ; see Figure 10 for a comparison of dust and BLR radii), demonstrating that the bulk of the BLR is enclosed within the dusty torus, as expected from AGN unification arguments.…”

AGN Light Echoes and the Accretion Disc Self-Gravity Limit

“…In addition, Suganuma et al (2006) search for time delays between the optical and NIR (J, H, K) bands in three additional sources, also finding much longer time delays: NGC 5548: 47-53 d; NGC 3227: ∼20 d; NGC 7469: 65-87 d. Meanwhile, further dedicated dust-reverberation campaigns and imaging surveys (e.g. Koshida et al 2014;Lyu, Rieke & Smith 2019;Minezaki et al 2019;Yang et al 2020) continue to build up a large sample of long, correlated time delays from the distant, toroidal structures that surround AGN (also see Netzer 2015). Such results help to build up evidence for a picture in which accretion-disc gas may extend out to ∼12 light-days, while continuum-band lags may be observed with significantly longer time delays, relative to the black-hole mass of the source.…”