Search for Circular Polarization toward Sagittarius A at 100 GHz

M. Tsuboi1, H. Miyahara 1, T. Kasuga2, A. Miyazaki3

1 Institute of Astrophysics and Planetary Sciences, Ibaraki University, Japan
2 Hosei University, Japan
3 Nobeyama Radio Observatory, Japan


We have performed a sensitive search of circular polarization of Sagittarius A, at 100 GHz using the Nobeyama 45-m telescope. The circular polarization properties of Sgr A should be new important information in order to understand this source, although the property has not yet been established. In polarimetry, interferometers have been used because of the advantage of the small beam to observe a point source embedded in extended components. On the other hand, large single-dish telescopes have another advantage that they have lower side lobe level. This is also important to perform precise polarimetry of such a source. Our observation had a small beam of 16" with low side lobe level of leq -13 dB. The upper limit of the fractional circular polarization within a 16" beam toward Sgr A was 0.3 %. This corresponds that the circular polarization of Sgr A was lower than 1.2 % after subtraction of free-free contribution. The observation dates were in a quiescent phase of the 106-days quasi-periodic variation reported by Zhao, Bower and Goss (2001). We have also performed sensitive searches of the linear polarization. The upper limit of the fractional linear polarization within a 16" beam toward Sgr A was 1.7 %. This corresponds that we did not detected a linear polarization of Sgr A itself at a level of 5 %.