Dust Reverberation of 3C 273: Torus Structure and Lag–Luminosity Relation

Abstract

We monitored the z = 0.158 quasar 3C 273 between 2015 and 2019 in the optical (BVrz) and near-infrared (JHK) with the aim to perform dust reverberation mapping. Accounting for host galaxy and accretion disk contributions, we obtained pure dust light curves in JHK. Cross correlations between the V band and the dust light curves yield an average rest-frame delay for the hot dust of τ cent ∼ 410 days. This is a factor of two shorter than that expected from the the dust ring radius R x ∼ 900 lt-day reported from interferometric studies. The dust covering factor (CF) is about 8%, much smaller than that predicted from the half covering angle of 45° found for active galactic nuclei (AGNs). We analyze the asymmetric shape of the correlation functions and explore whether an inclined biconical bowl-shaped dust torus geometry could bring these findings (τ cent, R x and CF) into a consistent picture. The hot varying dust emission originates from the edge of the bowl rim with a small covering angle 40° < θ < 45°, and we see only the near side of the biconus. Such a dust gloriole with R x = 900 ± 200 lt-day and an inclination 12° matches the data remarkably well. Comparing the results of 3C 273 with literature for less luminous AGN, we find a lag–luminosity relation τ ∝ L α with α = 0.33–0.40, flatter than the widely adopted relation with α ∼ 0.5. We address several explanations for the new lag–luminosity relation.

The picture of BLR in 2.5-D FRADO: Dynamics & Geometry

“…Our new sophisticated 3-D FRADO model predicts the launch of dusty material from the disk due to AD radiation pressure as the mechanism of formation of BLR whose trajectories can extend to large radii and high altitudes that can be responsible for the formation of torus. The results of complex flying material above the disk surface are consistent with the findings of studies by Goad et al (2012); Figaredo et al (2020). It also shows that the emission of BLR and dusty region are interrelated as confirmed by Wang et al (2013).…”

“…As our results for high Eddington rate, and others' findings (e.g. Kawaguchi & Mori 2010Goad et al 2012;Hönig 2019;Figaredo et al 2020) imply, a claim can be taken into account that the dusty torus is a posterior to BLR and is part of it indeed, however it can not be yet widely and firmly accepted.…”

“…The bowl-like structure of the stream (when viewed in 3-D) is also hinted by others (e.g. Kawaguchi & Mori 2010Goad et al 2012;Oknyansky et al 2015;Figaredo et al 2020). Our ratio of the stream width to the distance is indeed very small, ∼ 0.01 for the adopted parameters, but if the bowl-like structure is filled with the BLR clouds densely, and the cloud's gas number-density is ∼ 10 12 [cm −3 ], as argued in BL18 model and a number of papers (e.g.…”

“…We later assume the fixed specific value of the dust evaporation temperature of 1500 [K] (e.g. Barvainis 1987;Li 2007;Figaredo et al 2020), the same for all species and sizes, and we assume that the process happens instantaneously. The evaporation is indeed fast, of order of 1 day (see BL18).…”

A numerical study of long-term multi-wavelength blazar variability

“…In what follows, we will not include in our calculations the infrared radiation from the torus, since for typical radii (e.g. Hönig & Beckert 2007;Kishimoto et al 2011;Sobrino Figaredo et al 2020) its energy density will be much smaller than that of the BLR. We will also neglect the direct irradiation from the accretion disc (Dermer & Schlickeiser 2002).…”