Measuring the AGN Sublimation Radius with a New Approach: Reverberation Mapping of Broad Line Polarization

Abstract

Here we give an observational method for measurements of the equatorial scattering region radius using variability in the polarized broad lines in Type 1 active galactic nuclei (AGNs). The polarization in broad lines of Type 1 AGNs is mostly caused by equatorial scattering, where specific features allow one to separate its contribution from the total polarized flux. We propose to monitor variability in the polarized line flux and find the time lag between the nonpolarized continuum and polarized broad line variability. The distance to the scattering screen can then be determined from the time delay.The method was, for the first time, applied to the observations of the Type 1 AGN Mrk 6, and we found that the size of the scattering region in this AGN is around 100 light days. That is significantly smaller than the dusty region size estimated by the infrared interferometric observations and also larger than known broad line region (BLR) size. This indicates that the scattering region lies between the BLR and the dusty region and could be used as a probe of the dust sublimation radius.

Estimating supermassive black hole masses in active galactic nuclei using polarization of broad Mg ii, H α, and H β lines

“…An additional difficulty of the AP15 method is the need to to estimate the inner radius of dusty torus where equatorial scattering is probably starting (see Fig. 1 in Shablovinskaya et al 2020). Since there are no estimates of the radius of the dust torus in the IR range for the quasar SBS 1419+538 in the literature, and the results of the SDSS RM campaign have not been published yet.…”

“…lies within the accuracy of the AP15 method for the Mg II line (Savić et al 2020). In the future, the data of the SDSS-RM project or the results of the application of a new method for estimating the scattering region by reverberation mapping in polarized light (Shablovinskaya et al 2020) will help to improve the accuracy of the mass estimate.…”

Polarization in broad emission lines of active galactic nuclei

“…3. The reverberation in the polarized broad lines seems to be very perspective for determination the scattering radius (see Shablovinskaya et al 2020), since the polarization in the broad lines of type 1 AGNs is mostly coming from equatorial dust scattering. This region is probably coincidences with dust sublimation region and innermost dusty torus radius.…”

“…Here we gave an overview of polarization in the broad line emission of type 1 AGNs, with an emphasis on up‐to‐data work. Ours and other recent investigations led us to following conclusions:Polarization in the broad emission lines of type 1 AGNs can be very useful for investigation of the BLR characteristics, especially the BLR inclination and dimensions. The method for SMBH mass estimate given in more details in Afanasiev & Popović (2015) seems to be promising, but require high‐quality polarization observations. The reverberation in the polarized broad lines seems to be very perspective for determination the scattering radius (see Shablovinskaya et al 2020), since the polarization in the broad lines of type 1 AGNs is mostly coming from equatorial dust scattering. This region is probably coincidences with dust sublimation region and innermost dusty torus radius. …”

“…The method given in Shablovinskaya et al (2020) proposes to use the reverberation in polarized broad line, that represents a new method which will be able to trace the dust‐forming region in the inner part of AGNs. Note here, that in the case of Mrk 6, the reverberation in polarized broad Hα line gives significantly smaller (around 100 light days), that is two times smaller than obtained from the infrared reverberation (measured in the K‐band Kishimoto et al 2011).…”