Variability in spectropolarimetric properties of Sy 1.5 galaxy Mrk 6

Abstract

Here, we present an analysis of spectropolarimetric observations of type 1.5 AGN Mrk 6, performed with 6 m telescope of the Special Astrophysical Observatory of Russian Academy of Science in 12 epochs (2010–2013). Additionally, the interstellar matter (ISM) polarization has been observed and its contribution to the AGN spectral polarization is taken into account. We measured Stokes parameters and determined the polarization parameters in 12 spectra with and without correction for the ISM polarization. We estimated the time lag between the unpolarized and polarized continuum flux variation of about ∼2 d, which indicates a compact scattering region which contributes to the polarized continuum variability. The polarization in Hα is complex, showing three prominent components in the broad-line region (BLR), one redshifted around +3000 km s−1 that corresponds to the red shoulder in Hα and two blueshifted around −2000 km s−1 and −6000 km s−1. We found that the ISM polarization has a very significant influence on the measured AGN polarization parameters. After correcting the observations for the ISM polarization, we were able to detect the Keplerian motion in the BLR. We give a new method for the black hole mass estimation using spectropolarimetric observation in the line profile, finding the black hole mass in Mrk 6 of MBH ∼ 1.53 × 108 M⊙, which is in good agreement with reverberation estimates.