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?

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 , = 6 cm z = Attridge 1998; Attridge, Roberts, & Wardle 1999

Possible Field Order in Jets ShockShear A Helical Field A 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 Zavala & Taylor 2001

Parsec Scale Faraday Screens Quasars (Taylor 1998,2000; Zavala & Taylor 2003) –~ 1000 to a few thousand rad/m 2 in core CSS quasar OQ172 has 40,000 rad/m² in core (Udomprasert et al. 1997) –~ 100 rad/m 2 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? –n e ~ 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), (Gabuzda et al. 2001), (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 ( ) 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!

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

Circular Polarization as a Probe Direct Polarization: Intrinsic CP –Stokes V ~ 1% for optically thin radiation, uniform B-field m c  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