Linear and Circular Polarization from Sagittarius A*

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Linear and Circular Polarization from Sagittarius
A

• Geoffrey C. Bower
• UC Berkeley

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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)

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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

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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!

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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
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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?

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Polarization Angle Variability
1.3 mm
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Polarization Fraction Stability
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Rotation Measure
2 /- 1 x 106 rad m-2
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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

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MHD Simulations of Accretion
Proga Begelman 2003
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Accretion Limits for M81

• Beam depolarization
• ? RM gt 4 x 104 rad m-2
• ? Mdot gt 10-5 M_sun y-1
• Bandwidth depolarization
• ? RM gt 5 x 106 rad m-2
• ? Mdot gt 10-3 M_sun y-1

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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

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Sachiko A Star