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Linear and Circular Polarization from Sagittarius A*

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Presentation on theme: "Linear and Circular Polarization from Sagittarius A*"— Presentation transcript:

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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