linear and circular polarization from sagittarius a*
DESCRIPTION
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. - PowerPoint PPT PresentationTRANSCRIPT
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
Broderick & Blandford 2004
Q
V
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