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Polarization of AGN Jets. Dan Homan. National Radio Astronomy Observatory. Polarization of AGN Jets. Introduction Probing Jet Physics Progress + Future Field Structures in Jets Faraday Rotation Circular Polarization. Polarization as a Probe of Jet Physics. Jet Structure and Composition - PowerPoint PPT Presentation
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Polarization of AGN Jets
Dan Homan
National Radio Astronomy Observatory
Polarization of AGN Jets
• Introduction– Probing Jet Physics
• Progress + Future– Field Structures in Jets– Faraday Rotation– Circular Polarization
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?
2min/1totalN
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?
Quasar 1055+018, = 6 cm
z = 0.889Attridge 1998; Attridge, Roberts, & Wardle 1999
Possible Field Order in Jets
Shock ShearA Helical Field
A Toroidal FieldA Toroidal Field
A Helical Field
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°
Time Evolution of Polarization:Magnetic Movies!
• 3C 120, 16 monthly epochs at 43 and 22 GHz (Gomez et al. 2000, 2001)
Time Evolution of Polarization:Magnetic 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!
Faraday Rotation
20 RM
dlBnRM e ||
Zavala & Taylor 2001
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
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), 0820+225 (Gabuzda et al. 2001), 0548+165 (Mantovani et al. 2002)
• Direct evidence for jet-cloud interactions
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°
Circular Polarization
(Wardle et al. 1998)
(Homan & Wardle 1999)
3C 279
3C 84
Intrinsic CP
Or
Faraday Conversion?
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 > 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
CP vs. LP at 5 GHz
Homan, Attridge, & Wardle 2001
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
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?
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?
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!