Polarization of AGN Jets

1. Polarization of AGN Jets Dan Homan National Radio Astronomy Observatory

2. Polarization of AGN Jets • Introduction • Probing Jet Physics • Progress + Future • Field Structures in Jets • Faraday Rotation • Circular Polarization

3. 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?

4. 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?

5. Quasar 1055+018,  = 6 cm Attridge 1998; Attridge, Roberts, & Wardle 1999 z = 0.889

6. A Toroidal Field A Toroidal Field A Helical Field A Helical Field Possible Field Order in Jets Shock Shear

7. 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°

8. Time Evolution of Polarization:Magnetic Movies! • 3C 120, 16 monthly epochs at 43 and 22 GHz (Gomez et al. 2000, 2001)

9. 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!

10. Faraday Rotation Zavala & Taylor 2001

11. 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

12. 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

13. 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°

14. (Homan & Wardle 1999) Circular Polarization 3C 84 3C 279 Intrinsic CP Or Faraday Conversion? (Wardle et al. 1998)

15. 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

16. CP vs. LP at 5 GHz Homan, Attridge, & Wardle 2001

17. 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

18. 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?

19. 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?

20. 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!