The Circular Polarization of Sagittarius A at Submillimeter Wavelengths

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% + or - 0.3% at 1.3 mm wavelength (230 GHz) and 1.6% + or - 0.3% at 860 mu m (345 GHz) with the same handedness, l...

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Bibliographic Details
Published in:The Astrophysical journal Vol. 745; no. 2; pp. 115 - 14
Main Authors: Muñoz, D. J, Marrone, D. P, Moran, J. M, Rao, R
Format: Journal Article
Language:English
Published: Bristol IOP Publishing 01.02.2012
IOP
Subjects:
APPROXIMATIONS
Arrays
Astronomy
ASTROPHYSICS, COSMOLOGY AND ASTRONOMY
BLACK HOLES
Circular polarization
Circularity
Conversion
Earth, ocean, space
Exact sciences and technology
FARADAY EFFECT
FLUX DENSITY
GALAXIES
Handedness
Liquid crystal polymers
MAGNETIC FIELDS
PHASE SHIFT
POLARIZATION
QUASARS
RELATIVISTIC RANGE
Wavelengths
Quasars
plasmas
Galaxies
Circular polarization
Black holes
Compact radiosource
Supermassive black hole
Galaxy: center
Relativistic plasma
Sufficient condition
Flux density
black hole physics
submillimeter: general
Faraday effect
Cosmic radio sources
Cosmology
Linear polarization
polarization
Event horizon
Magnetic fields
techniques: interferometric
ISSN:0004-637X, 1538-4357
Online Access:Get full text
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Summary: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% + or - 0.3% at 1.3 mm wavelength (230 GHz) and 1.6% + or - 0.3% at 860 mu m (345 GHz) with the same handedness, left circular polarization (LCP), as observed at all 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 approximation 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 LCP. To improve our understanding of the environment of SgrA* critical future measurements includes determining whether the Faraday rotation deviates from a [lambda] super(2) dependence in wavelength and whether the circular and linear components of the flux density are correlated.
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ISSN:0004-637X
1538-4357
DOI:10.1088/0004-637X/745/2/115