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 ~492 GHz.
This frequency, well into the submillimeter bump where the emission is supposed to become
optically thin, allows us to see down to the event horizon. Hence the reported
observations contain potentially vital information on black hole properties. We have
compared our measurements with previous, lower frequency observations, which provides
information in the time domain.
Methods. We report continuum emission properties of Sgr A* at
~492 GHz, based on
Atacama Large Millimeter Array (ALMA) observations. We measured flux densities of Sgr A*
from the central fields of our ALMA mosaic observations. We used calibration observations
of the likely unpolarized continuum emission of Titan and the observations of Ci
line emission, to gauge the degree of spurious polarization.
Results. The flux density of 3.6 ± 0.72 Jy which we measured during our run
is consistent with extrapolations from previous, lower frequency observations. We found
that the continuum emission of Sgr A* at ~492 GHz shows large amplitude differences between the
XX and the
YY
correlations. The observed intensity ratio between the XX and YY correlations as a
function of parallactic angle can be explained by a constant polarization position angle
of ~158°± 3°. The
fitted polarization percentage of Sgr A* during our observational period is 14%
± 1.2%. The calibrator
quasar J1744-3116 we observed on the same night can be fitted to Stokes I = 252 mJy, with 7.9%
± 0.9% polarization at
position angle PA = 14°±
4.2°.
Conclusions. The observed polarization percentage and polarization
position angle in the present work appear consistent with those expected from longer
wavelength observations in the period of 1999−2005. In particular, the polarization position angle at 492 GHz
expected from the previously fitted 167°±
7° intrinsic polarization
position angle and (−5.6
± 0.7) × 105 rotation measure is 155\hbox{$^{+9}_{-8}$}°, which is consistent with our new measurement of polarization
position angle within 1σ. The polarization percentage and the polarization
position angle may be varying over the period of our ALMA 12 m Array observations, which
demands further investigation with future polarization observations.
