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

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