New explanation of opposite position angles in red and blue wings of spectral lines

Abstract

We consider the emission of resonance line radiation from rotating circular accretion disc with the progressive increasing height (the inclined ring). Our theory can also be applied to the rotating spot-like sources of resonance radiation. We assume that the atmosphere of inclined ring is homogeneous. In this case the every part of ring emits the radiation with the wave electric field oscillations perpendicular to plane $(\boldsymbol{nN^{\prime }})$, where ${\boldsymbol{n}}$ is the direction to a telescope and $\boldsymbol{N^{\prime }}$ is the normal to considered local surface of a ring. Geometrical consideration shows that the radiation polarization angle χ (frequently denoted as position angle or PA) depends on the inclination angle α of the ring relative to the central plane of accretion disc, the inclination angle θ of the central plane of accretion disc with the normal $\boldsymbol{N}$ and on azimuthal angle ϕ of radiating part on the ring. The right and left parts of a ring according to the symmetry plane $(\boldsymbol{nN})$ give rise to opposite polarization angles. For rotating accretion disc, due to Doppler’s effect, this means that polarization angles have opposite signs in the red and blue wings of emerging spectral line radiation. Such behaviour is observed in many objects.

Polarization in broad emission lines of active galactic nuclei

“…As it can be seen in Figure 2 (second panel), the PA across line has a swing, showing almost symmetric positive and negative peaks close to the zero velocities, that is very often observed in type 1 AGNs (see Afanasiev et al 2014Afanasiev et al , 2019Savi ć et al 2021;Smith et al 2002Smith et al . 2004) and also theoretically predicted (see Savi ć et al 2018Savi ć et al , 2020Silant'ev et al 2020;Smith et al 2004). Let us assume, as in Figure 2 that at distance R i from the SMBH a group of clouds are orbiting with Keplerian velocity…”