Diego J. Muñoz, Daniel P. Marrone, James M. Moran, Ramprasad Rao
We report the first detections of circularly polarized emission at
submillimeter wavelengths from the compact radio source and supermassive black
hole candidate Sgr A* at a level of 1.2\pm0.3% at 1.3 mm wavelength (230 GHz)
and 1.6\pm0.3% at 860 microns (345 GHz) with the same handedness as observed at
lower frequencies (1.4-15 GHz). The observations, taken with the Submillimeter
Array in multiple epochs, also show simultaneous linear polarization (LP) at
both wavelengths of about 6%. These properties differ sharply from those at
wavelengths longer than 1 cm (frequencies below 30 GHz), where weak circular
polarization (CP) (~ 0.5%) dominates over LP, which is not detected at similar
fractional limits. We describe an extensive set of tests to ensure the accuracy
of our measurements. We find no CP in any other source, including the bright
quasar 1924-292, which traces the same path on the sky as Sgr A* and therefore
should be subject to identical systematic errors originating in the instrument
frame. Since a relativistic synchrotron plasma is expected to produce little
CP, the observed CP is probably generated close to the event horizon by the
Faraday conversion process. We use a simple model to show that the phase shift
associated with Faraday conversion can be nearly independent of frequency, a
sufficient condition to make the handedness of CP independent of frequency.
Because the size of the tau=1-surface changes by more than an order of
magnitude between 1.4 and 345 GHz, the magnetic field must be coherent over
such scales to consistently produce left CP.
View original:
http://arxiv.org/abs/1105.0427
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