Torus skin outflow in a near-Eddington quasar revealed by spectropolarimetry

Abstract

ABSTRACT Even when the direct view towards the active nucleus is obscured, nuclear emission propagating along other directions can scatter off surrounding material, become polarized and reach the observer. Spectropolarimetry can thus be an important tool in investigating the circumnuclear geometry and kinematics of quasars on scales that cannot yet be probed via direct observations. Here, we discuss an intriguing class of quasars where the polarization position angle swings by large amounts ($\sim 90\deg$) within an emission line. We investigate a kinematic model in which the scattering dust or electrons are in an axisymmetric outflow. We propagate Stokes parameters in a variety of geometries of emitter, scatterer, and observer. We use these models to predict polarization fraction, line profiles, and polarization position angles and compare them to observations. We demonstrate that the swinging polarization angle can be a result of the geometry of the outflow and the orientation of the observer. Polarization properties of a near-Eddington extremely red quasar SDSS J1652 can be successfully explained by a model in which the quasar is surrounded by a geometrically thick disc, whose ‘skin’ is outflowing at ∼1000 km s−1 and acts as the scatterer on scales of a few tens of pc. The line of sight to the observer in this source is within or close to the skin of the torus, in agreement with multiwavelength data. Spectropolarimetric data and models presented here strongly support the thick-disc geometry of circumnuclear material suggested by recent numerical simulations of high-rate accretion flows on to black holes.