Emission Models for Radio Jets Circular Polarization

1
Emission Models for Radio Jets Circular
Polarization

• Heino Falcke
• Max-Planck Institut für Radioastronomie, Bonn

2
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

3
Circular Polarization

• Circular polarization was found to be rare and
  rather weak in older AGN surveys ?c lt 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 1915105, SS 433 Fender et
  al. 2000, 2001)
• LLAGN (M81 Sgr A Bower, Falcke, Backer 1999,
  Brunthaler et al. 2001)

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Circular Polarization in Blazars
3C 279

• Blazars are core-jet sources.
• Circular polarization is usually found in the
  core.
• Locally one can have ?clt 0.3-1.
• Still, linear polarization is much higher.
• The sign of CP remains stable between epochs.

Total Intensity
Homan Wardle (1999)
5
Circular Polarization in Blazars
3C 279

• Blazars are core-jet sources.
• Circular polarization is usually found in the
  core.
• Locally one can have ?clt 0.3-1.
• Still, linear polarization is much higher.
• The sign of CP remains stable between epochs.

Linear Polarization
Homan Wardle (1999)
6
Circular Polarization in Blazars
3C 279

• Blazars are core-jet sources.
• Circular polarization is usually found in the
  core.
• Locally one can have ?clt 0.3-1.
• Still, linear polarization is much higher.
• The sign of CP remains stable between epochs.

Circular Polarization
Homan Wardle (1999)
7
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.

8
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.

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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)
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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.

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The Very Center Sgr A

• Sgr A consists of three parts

• Sgr A East (Super-/Hypernova Remnant)

• Sgr A West (Minispiral)

• Sgr A (point source)

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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)
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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)
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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 gt 2-10.
• This is 20-100 times larger than in blazars.

Bower et al. (1998-2001)
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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!

Sgr A 20 years of CP
Bower et al. (2001)
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Is Sgr A unusual?Comparison with M81
M81 - VLBI

• 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)
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Is Sgr A unusual?Comparison with M81
M81 - VLBI

• 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)
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Detection of Circular Polarization in M81
M81 - VLA CP

• We found M81 to be circularly polarized at a
  level of ?c0.5 at 8 GHz.
• We found no linear polarization, ?llt0.1
• ?c/l gt 2-5
• The sign of CP remained stable over one year.

Brunthaler, Bower, Falcke, Melon (2001)
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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.

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Anomalous PolarizationApplying a Blazar Model

• Power-law with low and high-energy electrons
• Helical plus turbulent magnetic field

Total intensity
CP
LP
Beckert Falcke (2001)
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Anomalous PolarizationApplying a Sgr A Jet
Model

• Steeper power-law with only few high-energy
  electrons
• Helical plus turbulent magnetic field

Total intensity
CP
LP
Beckert Falcke (2001)
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Anomalous PolarizationRole of Longitudinal
B-field

• Steeper power-law with only few high-energy
  electrons
• Only azimuthal plus turbulent magnetic field

Total intensity
CP
LP
Beckert Falcke (2001)
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Anomalous PolarizationWhat 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
• helixoutflow, toroidal B-fields dont 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 ...

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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 gt 1011 K electrons.
• Sgr A will become a paradigm for AGN and black
  hole physics, possibly revealing the event
  horizon soon.