Astronomy and Astrophysics – Astronomy
Scientific paper
May 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009aas...21431507m&link_type=abstract
American Astronomical Society, AAS Meeting #214, #315.07; Bulletin of the American Astronomical Society, Vol. 41, p.761
Astronomy and Astrophysics
Astronomy
Scientific paper
We report the detection of circularly polarized (CP) emission from the compact radio source Sagittarius A* at a level of 1.5% at a frequency of 235 GHz (1.4 mm). Sgr A* is associated with the supermassive black hole (SMBH) in the Galactic Center. The observations, taken with the Submillimeter Array (SMA) on 03/31/2007, also show a linearly polarized (LP) component of 7%. The snr of our detection of CP is about 14. Before our measurements, CP had only been detected at frequencies between 1.4 and 15 GHz (21 and 2 cm) at levels <1%. In the same frequency range, LP has never been detected to a limit of 0.3%. Throughout our track, the sign of CP remains the same, indicating a consistent coherent production of CP. Previous polarization and variability studies of Sgr A* have supported a two-layer scenario of source plus cold plasma screen, in which Faraday rotation in the stationary screen (constant RM)acts on a time variable background source. A cold, optically thin plasma screen cannot be responsible for both a constant RM and Faraday conversion from LP to CP, therefore the observed amounts of CP are likely to be originated close to the central source. Sgr A* shows a flat-to-inverted radio spectrum and a submillimeter excess referred to as the "submillimeter bump". This excess it thought to come from the closest regions to the SMBH. In such a scenario, millimeter wavelength data is associated with regions in which the material is likely to be relativistic and the magnetic field ordered. We have carried out polarized radiative transfer calculations exploring different combinations of ordered and stochastic magnetic fields looking for a favored scenario that can explain the apparent constant increase of CP with frequency as well as the sudden jump in LP between 40 and 80 GHz.
Marrone Dan
Moran J. J.
Muñoz Diego
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