Linearly polarized millimeter and submillimeter continuum emission of Sgr A* constrained by ALMA
Abstract
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.
- Publication:
-
Astronomy and Astrophysics
- Pub Date:
- September 2016
- DOI:
- 10.1051/0004-6361/201628731
- arXiv:
- arXiv:1605.05544
- Bibcode:
- 2016A&A...593A.107L
- Keywords:
-
- polarization;
- radiation mechanisms: non-thermal;
- relativistic processes;
- techniques: interferometric;
- techniques: polarimetric;
- quasars: supermassive black holes;
- Astrophysics - High Energy Astrophysical Phenomena;
- Astrophysics - Astrophysics of Galaxies
- E-Print:
- 11 pages, 10 figures, submitted to A&