Does circular polarisation reveal the rotation of quasar engines?

Abstract

Many radio sources like quasars, blazars, radio galaxies, and micro-quasars exhibit circular polarisation (CP) with surprising temporal persistent handedness. As a possible explanation we propose that the CP is due to Faraday conversion (FC) of linear polarisation (LP) synchrotron light which propagates along a line-of-sight (LOS) through twisted magnetic fields. The rotational nature of accretion flows onto black holes naturally generates the required magnetic twist in the emission region, independent of whether it is a jet or an ADAF. The expected twist in both types of flows is of the order of what is required for optimal CP generation. This scenario requires that Faraday rotation (FR) is insignificant in the emission region. Although this is an assumption, it relaxes constraints on the plasma parameters, that were given in scenarios which rely on FR, since there the strength of FR can not be too far from the optimum. The proposed mechanism works in electron-positron (e+-) as well as electron-proton (e/p) plasma. In the latter case, the emission region should consist of individual flux tubes with independent polarities in order to suppress too strong FR. The predominant CP is expected to mostly counter-rotate (rotation is measured here in sky-projection) with respect to the central engine in all cases (jet or ADAF, e+- or e/p plasma). If the proposed mechanism is indeed operating, it will allow to measure the sense of rotation of quasar engines. The engine of SgrA* is then expected to rotate clockwise and therefore counter-Galactic, as do the young hot stars in its vicinity, which are thought to feed SgrA* by their winds. Similarly, we expect the microquasars SS 443 and GRS 1915+105 to rotate clockwise. Generally, sources with Stokes-V<0 (V>0) are expected to rotate clockwise (counter-clockwise) in this scenario.