Issue |
A&A
Volume 401, Number 2, April II 2003
|
|
---|---|---|
Page(s) | 499 - 504 | |
Section | Extragalactic astronomy | |
DOI | https://doi.org/10.1051/0004-6361:20030162 | |
Published online | 21 March 2003 |
Does circular polarisation reveal the rotation of quasar engines?
Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, Postfach 1317, 85741 Garching, Germany
Corresponding author: ensslin@mpa-garching.mpg.de
Received:
6
November
2002
Accepted:
30
January
2003
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
advection dominated accretion flow (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 () as well as electron-proton (
) 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,
or
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-
(
) are expected to rotate clockwise (counter-clockwise) in this
scenario.
Key words: radiation mechanisms: non-thermal / radio continuum: general / polarization / galaxies: active / galaxies: jets / galaxies: magnetic fields
© ESO, 2003
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