摘要：Aims. Our aim is to characterize the polarized continuum emission
properties including intensity, polarization position angle, and polarization percentage
of Sgr A* at ~100 (3.0
mm), ~230 (1.3 mm),
~345 (0.87 mm),
~500 (0.6 mm), and
~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
~700 GHz than that at
~500 GHz, which
suggests that emission at ≳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 ~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
~700 GHz is likely
lower than that at ~500
GHz. By comparing the ~500 GHz and ~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.
关键词：polarization;radiation mechanisms: non-thermal;relativistic processes;techniques: interferometric;techniques: polarimetric;quasars: supermassive black holes
