Polarization of AGN Jets

1
Polarization of AGN Jets

• Dan Homan

National Radio Astronomy Observatory
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Polarization of AGN Jets

• Introduction
• Probing Jet Physics
• Progress Future
• Field Structures in Jets
• Faraday Rotation
• Circular Polarization

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Polarization as a Probe of Jet Physics

• Jet Structure and Composition
• 3-D Magnetic Field Structure of Jets
• Connection with SMBH/Accretion Disk System
• Low energy end of particle spectrum
• Dominates Kinetic Luminosity of Jets
• Important for constraining
• particle accel. mechanisms
• Particle Composition of Jets
• Electron-Proton?
• Electron-Positron?

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Polarization as a Probe of Jet Physics

• Magneto-Hydrodynamics of Jets
• Field signatures of Oblique Shocks
• Time evolution of Field Structures
• Compared to simulations
• Dependence on Optical Class
• Jet Environment
• Jet Polarization as Backlighting
• Nature of Faraday Screen on Parsec Scales
• Scale Height
• Relation to Jet Magnetic Field
• Are we seeing Narrow Line Clouds?

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Quasar 1055018, ? 6 cm
Attridge 1998 Attridge, Roberts, Wardle 1999
z 0.889
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Possible Field Order in Jets
Shock
Shear
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Observed Linear Polarization in AGN

• Fractional Polarization
• Cores few percent up to 10
• Jet features 5-10 up to a few tens of percent
• Orientation relative to jet ? ?
• 6 cm Cawthorne et al. (1993), Gabuzda et al.
  (2000), Pollack et al. (2003)
• 1.3/0.7 cm Lister Smith (2000), Lister
  (2001), Marscher et al. (2002)
• Quasar Jets
• no clear relation at 6 cm
• excess near 0 at 1.3/0.7 cm with a broad tail
• Oblique Shocks? (Marscher et al. 2002)
• BL Lac Jets
• both 6 cm and 1.3/0.7 cm have an excess near 0

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Time Evolution of PolarizationMagnetic Movies!

• 3C 120, 16 monthly epochs at 43 and 22 GHz
  (Gomez et al. 2000, 2001)

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Time Evolution of PolarizationMagnetic Movies!

• Brandeis Monitoring Program, 12 sources at 15 and
  22 GHz for 6 epochs separated at 2 month
  intervals. (Homan et al. 2001, 2002 Ojha et al.
  2003)
• Polarization changes not related to Faraday
  Rotation
• Jet features increased in fractional polarization
  
• Tendency for Jet ? to rotate toward 90
• Fluctuations in ? larger for smaller fractional
  polarization
• BL Lac, 17 epochs over 3 years (Stirling et al.
  2003)
• Precessing Jet Nozzle!

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Faraday Rotation
Zavala Taylor 2001
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Parsec Scale Faraday Screens

• Quasars (Taylor 1998,2000 Zavala Taylor 2003)
• 1000 to a few thousand rad/m2 in core
• CSS quasar OQ172 has 40,000 rad/m² in core
• (Udomprasert et al. 1997)
• 100 rad/m2 in jet
• BL Lacs (Gabuzda et al. 2001,2003 Reynolds et
  al. 2001 Zavala Taylor 2003)
• comparable to quasars, perhaps a bit weaker in
  core
• Galaxies (Taylor et al. 2001 Zavala Taylor
  2002)
• FR stronger than quasars
• Often have depolarized cores

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Nature of the Screen

• How much of the screen is local to the source?
• Are we seeing narrow line clouds?
• ne 102-3 cm-3, B 10 ?G
• Alternatives inter-cloud gas, boundary layer
  of the jet
• Large rotation measures observed at bends
• 3C120 (Gomez et al. 2000), 0820225 (Gabuzda et
  al. 2001), 0548165 (Mantovani et al. 2002)
• Direct evidence for jet-cloud interactions

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Nature of the Screen

• Is there a contribution from FR Internal to the
  Jet?
• Expected from CP observations theory
• Important for constraining low-energy end of
  particle distribution in the jet line of sight
  B-field in jet
• Cannot be a large contribution or we would see
• Deviations from ?² for ?? ? 45
• Significant depolarization for ?? ? 30

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Circular Polarization
(Homan Wardle 1999)
3C 84
3C 279
Intrinsic CP Or Faraday Conversion?
(Wardle et al. 1998)
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Parsec-Scale Circular Polarization in AGN

• CP almost always detected in VLBI cores
  (Homan Wardle 1999 Homan, Attridge, Wardle
  2001)
• 3C84 clear exception (0.15 pc linear resolution)
• Sensitive function of opacity
• Local CP ? 0.3 is rare!
• 2/36 sources at 5 GHz (Homan, Attridge Wardle
  2001)
• 6/50 sources at 15 GHz (MOJAVE result)
• LP gt CP in most AGN
• LLAGN an exception Sgr A (Bower et al. 1999)
  M81 (Brunthaler et al.
  2001)
• 3C84, 3C273, and M87 (MOJAVE result) also
  exceptions

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CP vs. LP at 5 GHz
Homan, Attridge, Wardle 2001
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Mechanism for CP Production?

• Intrinsic CP implausible
• High field B-strengths and a large (dominant)
  component of uni-directional field required
• Faraday Conversion linear circular
• Easier to generate large amounts of CP
• Direct or driven by Faraday Rotation
• Probes field order and low energy particles in
  the jet
• Difficulties
• Poor spectral coverage
• Coincidence of CP with the inhomogeneous core

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Sign Consistency of CP

• Short term sign consistency
• 3-5 years, but not perfect (Komessaroff et al.
  1984)
• 1 year, during an outburst (Homan Wardle
  1999)
• Longer term sign consistency suggested
• 20 years (Homan, Attridge, Wardle 2001)
• 20 years demonstrated for Sgr A (Bower et al.
  2002)
• 7 years for 3C273 and 3C279 (1996-2003)
• A Persistent B-field Order?
• Net magnetic flux?
• Consistent twist to a helix?
• Related to SMBH/Accretion Disk?

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The Future

• Field Order in Jets
• Faraday corrected maps
• Greater sensitivity
• Time evolution to study hydro-dynamics
• Information from Faraday Rotation and CP
• Faraday Rotation
• Higher resolution studies to probe the nature of
  the high rotation measure region
• RM distributions transverse to the jet
• Jet-Cloud interactions
• Can we study internal rotation?

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The Future

• Circular Polarization
• Variability studies to explore the
  sign consistency
• Better spectral studies to constrain emission
  mechanism and implied physics
• Requires high sensitivity
• Higher resolution studies, so we will be less
  confounded by the inhomogeneous VLBI core.
• Improved Calibration!

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Linear Polarization as a Probe

• Direct Polarization
• Stokes Q
• 70 Q for optically thin radiation, uniform
  B-field
• 10 Q for optically thick radiation, uniform
  B-field
• Sensitive to net field order in plane of sky
• Bi-refringence Faraday Rotation
• Stokes Q ? U (??????²)
• Sensitive to field order along the line of sight
• Sensitive to charge sign of rotating particles
• Stronger for lower energy particles
• Significant (Dominant ?) contribution by external
  thermal matter

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Circular Polarization as a Probe

• Direct Polarization Intrinsic CP
• Stokes V
• 1 for optically thin radiation, uniform
  B-field
• mc ? ?0.5 for an optically thin, homogeneous
  source
• Sensitive to net field order along the line of
  sight
• Sensitive to charge sign of radiating particles
• Bi-refringence Faraday Conversion
• Stokes U ? V (??????³)
• Requires field order in the plane of the sky
• Charge sign of the converting particles
  unimportant
• Stronger for lower energy relativistic particles
• No significant contribution by external thermal
  matter