Emission Models for Radio Jets Circular Polarization

Heino Falcke Max-Planck Institut für Radioastronomie, Bonn

The (Standard) Jet Model

• Radio-Plasma freely expanding in a supersonic jet

• superposition of self-absorbed synchrotron spectra

• at each frequency one sees the = 1 surface as the “core”

Circular Polarization

• Circular polarization was found to be rare and rather weak in older AGN surveys c < 0.2%. (Weiler & de Pater 1983)

• Recently circular polarization was found at interesting levels in a number of sources:– Blazars (Homan & Wardle 1999)

– GPS/Baby QSOs (Rayner 2000)

– X-ray binaries (GRS 1915+105, SS 433; Fender et al. 2000, 2001)

– LLAGN (M81*; Sgr A*; Bower, Falcke, Backer 1999, Brunthaler et al. 2001)

Circular Polarization in Blazars

• Blazars are core-jet sources.

• Circular polarization is usually found in the core.

• Locally one can have c< 0.3-1%.

• Still, linear polarization is much higher.

• The sign of CP remains stable between epochs.

3C 2793C 279

Homan & Wardle (1999)Homan & Wardle (1999)

Total IntensityTotal Intensity

Circular Polarization in Blazars

• Blazars are core-jet sources.

• Circular polarization is usually found in the core.

• Locally one can have c< 0.3-1%.

• Still, linear polarization is much higher.

• The sign of CP remains stable between epochs.

3C 2793C 279

Homan & Wardle (1999)Homan & Wardle (1999)

Linear PolarizationLinear Polarization

Circular Polarization in Blazars

• Blazars are core-jet sources.

• Circular polarization is usually found in the core.

• Locally one can have c< 0.3-1%.

• Still, linear polarization is much higher.

• The sign of CP remains stable between epochs.

3C 2793C 279

Homan & Wardle (1999)Homan & Wardle (1999)

Circular PolarizationCircular Polarization

Conversion of Linear to Circular Polarization

• A phase-shift of one linear polarization mode induces circular polarization.

• This is caused by perpendicular B-fields.

• The helicity depends on polarity of the B-field.

• Stochastic field reversals cancel the effect.

Faraday Rotation of Linear Polarization

• A phase-shift of one circular polarization mode induces Faraday rotation.

• This is caused by longitudinal B-fields.

• Stochastic field variations will lead to de-polarization of an extended source.

Circular-to-Linear Polarization Ratio

• Survey of 40 AGN (blazars):

• In all detected blazars circular exceeds linear polarization.

c / l ~ 0.1• This is in agreement with

the theoretical expectation

• Is this generally true?Homan et al. (2001)Homan et al. (2001)

The Galactic Center

• The closest galactic center we can study is in our Galaxy.

• Our Optical view is obscured by the central dust lane of the Galaxy.

• In the radio band we can penetrate the dust screen and approach the very center of the Galaxy.

The Very Center: Sgr A

• Sgr A consists of three parts:

– Sgr A East (Super-/Hypernova Remnant)

– Sgr A East (Super-/Hypernova Remnant)

– Sgr A* (point source)

– Sgr A* (point source)

– Sgr A West (Minispiral)

– Sgr A West (Minispiral)

Sgr A* Radio Emission - A Few Recent Surprises

• Sgr A* was extensively observed for 2 decades

• The more senior guys told us that Sgr A* has no linear polarization .... (so, do not bother to look for it).

Is this true?

(Bower, Falcke, Backer et al. 1999a,b,c)(Bower, Falcke, Backer et al. 1999a,b,c)

Sgr A* Radio Emission - A Few Recent Surprises

• We searched extensively for linear polarization from 1.4 GHz to 110 GHz with the VLA and BIMA and found nothing. It is true!

(Bower, Falcke, Backer et al. 1999a,b,c)(Bower, Falcke, Backer et al. 1999a,b,c)

Sgr A* Radio Emission - A Few Recent Surprises

• ... but variable circular polarization (up to 1%) was found at 15 GHz with an inverted spectrum!

• The circular-to-linear polarization ratio in Sgr A* is c/l > 2-10.

• This is 20-100 times larger than in blazars.

Bower et al. (1998-2001)Bower et al. (1998-2001)

Long-Term Stability of Circular Polarization

• There are 20 years of circular polarization data of Sgr A* in the archive.

• One can see weekly fluctuations.

• The average level and the sign of CP has remained constant over 20 years!

Bower et al. (2001)Bower et al. (2001)

Sgr A*: 20 years of CPSgr A*: 20 years of CP

Is Sgr A* unusual?Comparison with M81*

• Like the Milky Way,M81 has a very compact, variable radio nucleus.

• Multi-Epoch VBLI observations finally resolved the source into core-jet structure.

• The radio spectrum of M81* is also very similar to Sgr A*.

Reuter & Lesch (1996)Reuter & Lesch (1996)

M81* - VLBIM81* - VLBI

Is Sgr A* unusual?Comparison with M81*

• Like the Milky Way,M81 has a very compact, variable radio nucleus.

• Multi-Epoch VBLI observations finally resolved the source into core-jet structure.

• The radio spectrum of M81* is also very similar to Sgr A*.

Bietenholz, Bartel, Rupen (2000)Bietenholz, Bartel, Rupen (2000)

M81* - VLBIM81* - VLBI

Detection of Circular Polarization in M81*

• We found M81* to be circularly polarized at a level of c~0.5% at 8 GHz.

• We found no linear polarization, l<0.1%

c/l > 2-5

• The sign of CP remained stable over one year.

Brunthaler, Bower, Falcke, Melon (2001)Brunthaler, Bower, Falcke, Melon (2001)

M81* - VLA: CPM81* - VLA: CP

Anomalous Polarization: Why are Sgr A* and M81* different?

• Modeling of radiative transfer (synchrotron) with all Stokes vectors in a compact radio jet.

• Kolmogorov turbulence spectrum

• Powerlaw distribution of electrons or pairs

• Arbitrary large scale magnetic field configuration

• Jet described as conical Blandford & Königl jet adapted to give Falcke (1996) adiabatic jet model.

Anomalous Polarization:Applying a Blazar Model

• Power-law with low and high-energy electrons

• Helical plus turbulent magnetic field

CPCP

LPLP

Total intensityTotal intensity

d

dN

6min

4max 10

2p

Beckert & Falcke (2001)Beckert & Falcke (2001)10

hot

cold

n

n

Anomalous Polarization:Applying a Sgr A* Jet Model

• Steeper power-law with only few high-energy electrons

• Helical plus turbulent magnetic field

CPCP

LPLP

Total intensityTotal intensity

d

dN

6min

5.2max 10

3p

Beckert & Falcke (2001)Beckert & Falcke (2001)100hot

cold

n

n

Anomalous Polarization:Role of Longitudinal B-field

• Steeper power-law with only few high-energy electrons

• Only azimuthal plus turbulent magnetic field

CPCP

LPLP

Total intensityTotal intensity

Beckert & Falcke (2001)Beckert & Falcke (2001)

d

dN

6min

5.2max 10

3p

Anomalous Polarization:What does one need?

• Needs lots of `cold‘ electrons, ncold/nhot ~ 102-3

– Sgr A* and M81* carry a lot of hidden matter!?– Needed accretion rate increases by a factor of 100

• Needs longitudinal B-field with non-vanishing magnetic flux – helix+outflow, toroidal B-fields don‘t work

• Electrons are preferred over e pairs to reduce linear polarization through Faraday rotation.

• Long-term stability of sign of CP, indicates a stability of the magnetic pole of the black hole/accretion disk.– We can measure the magnetic north pole of black holes ...

How Sgr A* Works ... ... as of Sep. 19, 2001

• The accretion rate is 10-8..-6 M/yr (hot X-ray bubble).• Accretion has to proceed through an optically thin

accretion flow (ADAF-like).• A flat radio spectrum is the signature of a freely

expanding outflow.• The Sgr A* emission is jet-dominated: radio is from the

jet, sub-mm and X-rays come from the nozzle.• The X-ray emission is synchrotron self-Compton emission

from T > 1011 K electrons.• Sgr A* will become a paradigm for AGN and black hole

physics, possibly revealing the event horizon soon.