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 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
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!
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
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?
Polarization Angle Variability 1.3 mm
Polarization Fraction Stability
Rotation Measure ≈ 2 +/- 1 x 10^6 rad m^-2
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
MHD Simulations of Accretion Proga & Begelman 2003
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
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
Sachiko A Star