2016A&A...593A.107L

Aims. Our aim is to characterize the polarized continuum emission properties including intensity, polarization position angle, and polarization percentage of Sgr A* at ∼100 (3.0mm), ∼230 (1.3mm), ∼345 (0.87mm), ∼500 (0.6mm), and ∼700GHz (0.43mm).
Methods. We report continuum emission properties of Sgr A* at the above frequency bands, based on the Atacama Large Millimeter Array (ALMA) observations. We measured flux densities of Sgr A* from ALMA single pointing and mosaic observations. We performed sinusoidal fittings to the observed (XX-YY)/I intensity ratios, to derive the polarization position angles and polarization percentages.
Results. We successfully detect polarized continuum emission from all observed frequency bands. We observed lower Stokes I intensity at ∼700GHz than that at ∼500GHz, which suggests that emission at ≥500GHz is from the optically thin part of a synchrotron emission spectrum. Both the Stokes I intensity and the polarization position angle at our highest observing frequency of ∼700GHz, may vary with time. However, as yet we do not detect variation in the polarization percentage at >500GHz. The polarization percentage at ∼700GHz is likely lower than that at ∼500GHz. By comparing the ∼500GHz and ∼700GHz observations with the observations at lower frequency bands, we suggest that the intrinsic polarization position angle of Sgr A* varies with time. This paper also reports the measurable polarization properties from the observed calibration quasars.
Conclusions. Future simultaneous multi-frequency polarization observations are required to clarify the time and frequency variation of the polarization position angle and polarization percentage.