摘要：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 τcent ~ 410 days.This is a factor of two shorter than that expected from the the dust ring radius Rx ~ 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 (τcent, Rx 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 Rx = 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 ∝ Lα 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.
