The polarized signal from broad emission lines in AGNs

Abstract

Using the STOKES Monte Carlo radiative transfer code, we revisit the predictions of the spectropolarimetric signal from a disc-like broad emission line region (BLR) in type I active galactic nuclei due to equatorial scattering. We reproduce the findings of previous works, but only for a scatterer that is much more optically and geometrically thick than previously proposed. We also find that when taking into account the polarized emission from all regions of the scatterer, the swing of the polarization position angle (PA) is in the opposite direction to that originally proposed. Furthermore, we find that the presence of outflows in the scattering media can significantly change the observed line profiles, with the PA of the scattering signal being enhanced in the presence of radially outflowing winds. Finally, a characteristically different PA profile, shaped like an 'M', is seen when the scatterer is co-spatial with the BLR and radially outflowing.