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Linear and Circular Polarization from Sagittarius A*

Linear and Circular Polarization from Sagittarius A*. Geoffrey C. Bower UC Berkeley. “The Quarter Wave Plates”. Backer, D.C. (UCB) Falcke, H. (MPIfR) Goss, W.M. (NRAO) Mellon, R.R. (UVa) Sault, R.J. (ATNF) Wright, M.C.H. (UCB) Zhao, J.-H. (CfA). Polarization as AGN diagnostic.

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Linear and Circular Polarization from Sagittarius A*

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  1. Linear and Circular Polarization from Sagittarius A* Geoffrey C. Bower UC Berkeley

  2. “The Quarter Wave Plates” • Backer, D.C. (UCB) • Falcke, H. (MPIfR) • Goss, W.M. (NRAO) • Mellon, R.R. (UVa) • Sault, R.J. (ATNF) • Wright, M.C.H. (UCB) • Zhao, J.-H. (CfA)

  3. Polarization as AGN diagnostic • Synchrotron radiation • Shocks and magnetic fields in jets • Interaction with ambient gas • Probe of thermal accretion region/ISM through RM • Electron-positron jets • Strong gravity wave propagation

  4. Rich CM Wavelength Polarization Properties • Linear Polarization Absent • At low frequencies • But Strong Circular Polarization • Inverted Spectrum during Flare • Variable in 1 week • Variable in Hours • But Stable for Decades • Not like AGN!

  5. A Diversity of Models • Homogenous synchrotron • Interstellar propagation • Strong field gravity • LP to CP conversion by low energy electrons • Appears to require field curvature • Stable magnetic field configuration • Driven by black hole spin Q V Broderick & Blandford 2004

  6. Linear Polarization: 1 Year Ago • Confirmed high frequency polarization • Sharp transition ~150 GHz • Upper limit on rotation measure: 2 x 106 rad m-2 • Tight constraints on accretion rate • Eliminate ADAF, high accretion rate models • Rotation measure?

  7. Polarization Angle Variability 1.3 mm

  8. Polarization Fraction Stability

  9. Rotation Measure ≈ 2 +/- 1 x 10^6 rad m^-2

  10. Turbulent Accretion • 1 rad/1 mm/1 year  106 rad m-2 /year • Changing density/B-field in accretion region • Characteristic Length • R ≥ 100 Rg • Characteristic Time • Viscous time scale ~40 (R/100 Rg) days

  11. MHD Simulations of Accretion Proga & Begelman 2003

  12. Accretion Limits for M81* • Beam depolarization •  RM > 4 x 104 rad m-2 •  Mdot > 10-5 M_sun y-1 • Bandwidth depolarization •  RM > 5 x 106 rad m-2 •  Mdot > 10-3 M_sun y-1

  13. Summary • Polarization is a unique probe of Sgr A* • Emission & Accretion Regions • Magnetic field, electron density, electron spectrum • Turbulence • Future --- CARMA, SMA & ALMA • Decoding accretion dynamics/turbulence • Probing fainter sources • How unique is Sgr A*? • MM/Submm VLBI Polarimetry

  14. Sachiko A Star

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