Linearly polarized millimeter and submillimeter continuum emission of Sgr A* constrained by ALMA
Journal article, 2016
Aims. Our aim is to characterize the polarized continuum emission properties including intensity, polarization position angle, and polarization percentage of Sgr A* at similar to 100 (3.0 mm), similar to 230 (1.3 mm), similar to 345 (0.87 mm), similar to 500 (0.6 mm), and similar to 700 GHz (0.43 mm). 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 similar to 700 GHz than that at similar to 500 GHz, which suggests that emission at greater than or similar to 500 GHz 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 similar to 700 GHz, may vary with time. However, as yet we do not detect variation in the polarization percentage at >500 GHz. The polarization percentage at similar to 700 GHz is likely lower than that at similar to 500 GHz. By comparing the similar to 500 GHz and similar to 700 GHz 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.
radiation mechanisms: non-thermal
quasars: supermassive black holes