Polarization of AGN Jets

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!