Logo (Main page)

A new approach to measuring the dust sublimation radius in active nuclei

Russian version

    Active galactic nuclei (AGN) are unresolvable with modern optical telescopes since the apparent angular size of the central parsec is <<0.1 arcsec even for the closest active galaxies. This leads to difficulties with methods for estimating distances within the AGN. In particular, determining the size of the dusty torus surrounding the core is critical for estimating the dust sublimation radius and, consequently, the physical state of matter in the circumnuclear region, as well as for determining the masses of central supermassive black holes by the spectropolarimetric method.
    The authors propose a method for estimating the size of a dusty torus based on reverberation mapping of broad emission lines in polarized light. In type 1 AGN, the polarization of the broad lines is caused by the equatorial scattering on the dusty torus; the profile of the broad lines in polarized light thus acquires specific features. This allows us to uniquely determine by spectropolarimetry tools whether the equatorial scattering is the dominant polarization mechanism in the observed object. The time delay between the signal in the unpolarized continuum and the polarized broad emission line determines the distance to the scattering region, where the medium becomes optically thick and the temperature is low enough for the dust to survive. This method was first applied to the Seyfert galaxy Mrk 6, whose spectropolarimetric observations were carried out in 2010-2013 using the universal spectrograph SCORPIO-2 in the primary focus of the BTA. The resulting delay was ~100 days, which clarified the size of the scattering region by a factor of 2 relative to the estimates given by infrared interferometric observations obtained at the Keck Observatory telescopes (Hawaii, USA).

Shablovinskaya E.S., Afanasiev V.L., PopoviÓ L.ó. Measuring the AGN Sublimation Radius with a New Approach: Reverberation Mapping of Broad Line Polarization, 2020, ApJ, 892, 118. doi:10.3847/1538-4357/ab7849.

Contact person - Shablovinskaya E.S., Trainee researcher at the Laboratory of Spectroscopy and Photometry of Extragalactic Objects, SAO RAS
Fig.1. Illustration of the equatorial scattering of radiation from the AGN central parts at the inner boundary of the dusty torus. The green arrow shows the unpolarized radiation in the broad emission lines, and the blue arrow shows the polarized radiation in the lines reflected from the scattering region.
Fig.2. A diagram of the distances inside the AGN. The upper part of the figure illustrates the distribution of dust inside the AGN. t1, t2, and t3 correspond to the moments of signal registration from the central source, the broad line region (BLR), and the scattering region, respectively. The lower part corresponds to the scale of distances in the AGN, which are determined in different ways. It can be seen that the distance Rsc, measured using the proposed method of polarimetric reverberation mapping, is the closest to the radius of the dust sublimation, in contrast to the methods of IR reverberation mapping ("K-band RM") and IR interferometry ("IR IF").