The millimeter/submillimeter wavelength polarization of Sgr A* is known to be variable in both magnitude and position angle on time scales down to a few hours. The unstable polarization has prevented measurements made at different frequencies and different epochs from yielding convincing measurements of Faraday rotation in this source. Here we present observations made with the Submillimeter Array polarimeter at 227 and 343 GHz with sufficient sensitivity to determine the rotation measure at each band without comparing position angles measured at separate epochs. We find the 10-epoch mean rotation measure to be (-5.6+/-0.7)x10^5 rad/m^2; the measurements are consistent with a constant value. We conservatively assign a 3sigma upper limit of 2x10^5 rad/m^2 to rotation measure changes, which limits accretion rate fluctuations to 25%. This rotation measure detection limits the accretion rate to less than 2x10^-7 M_sun/yr if the magnetic field is near equipartition, ordered, and largely radial, while a lower limit of 2x10^-9 M_sun/yr holds even for a sub-equipartition, disordered, or toroidal field. The mean intrinsic position angle is 167+/-7 degrees and we detect variations of 31(+18/-9) degrees. These variations must originate in the submillimeter photosphere, rather than arising from rotation measure changes.Comment: 5 pages, 1 figure. To appear in the Astrophysical Journal Letter
Using the Submillimeter Array, we have made the first high angular resolution measurements of the linear polarization of Sagittarius A Ã at submillimeter wavelengths and the first detection of intraday variability in its linear polarization. We detected linear polarization at 340 GHz (880 m) at several epochs. At the typical resolution of 1B4 ; 2B2, the expected contamination from the surrounding ( partially polarized) dust emission is negligible. We found that both the polarization fraction and the position angle are variable, with the polarization fraction dropping from 8.5% to 2.3% over 3 days. This is the first significant measurement of variability in the linear polarization fraction in this source. We also found variability in the polarization and total intensity within single nights, although the relationship between the two is not clear from these data. The simultaneous 332 and 342 GHz position angles are the same, setting a 1 rotation measure (RM) upper limit of 7 ; 10 5 rad m À2 . From position angle variations and comparison of ''quiescent'' position angles observed here and at 230 GHz, we infer that the RM is a few times 10 5 rad m À2 , a factor of a few below our direct detection limit. A generalized model of the RM produced in the accretion flow suggests that the accretion rate at small radii must be low, below 10 À6 -10 À7 M yr À1 depending on the radial density and temperature profiles, but in all cases below the gas capture rate inferred from X-ray observations.
We report on the submillimeter properties of Sgr A* derived from observations with the Submillimeter Array and its polarimeter. We find that the spectrum of Sgr A* between 230 and 690 GHz is slightly decreasing when measured simultaneously, indicating a transition to optically thin emission around 300 − 400 GHz. We also present very sensitive and well calibrated measurements of the polarization of Sgr A* at 230 and 345 GHz. With these data we are able to show for the first time that the polarization of Sgr A* varies on hour timescales, as has been observed for the total intensity. On one night we find variability that may arise from a polarized "blob" orbiting the black hole. Finally, we use the ensemble of observations to determine the rotation measure. This represents the first statistically significant rotation measure determination and the only one made without resorting to comparing position angles measured at separate epochs. We find a rotation measure of (−5.6 ± 0.7) × 10 5 rad m −2 , with no evidence for variability on inter-day timescales at the level of the measurement error. The stability constrains interday fluctuations in the accretion rate to 8%. The mean intrinsic polarization position angle is 167 • ±7 • and we detect variations of 31 +18 −9 degrees. This separation of intrinsic polarization changes and possible rotation measure fluctuations is now possible because of the frequency coverage and sensitivity of our data. The observable rotation measure restricts the accretion rate to the range 2 × 10 −7 M⊙ yr −1 to 2 × 10 −9 M⊙ yr −1 , if the magnetic field is near equipartition and ordered.
At radio wavelengths, images of the compact radio source Sagittarius A* (Sgr A*) in the Galactic Center are scatter broadened with a λ 2 dependence due to an intervening ionized medium. We present VLBI observations of Sgr A* at 86 GHz using a six station array including the VLBA antennas at Pie Town, Fort Davis and Los Alamos, the 12 m antenna at Kitt Peak and the millimeter arrays at Hat Creek and Owens Valley. To avoid systematic errors due to imperfect antenna calibration, the data were modeled using interferometric closure information. The data are best modeled by a circular Gaussian brightness distribution of FWHM 0.18 ± 0.02 mas. The data are also shown to be consistent with an elliptical model corresponding to the scattering of a point source. The source structure in the N-S direction, which is less well determined than in the E-W direction due to the limited N-S (u, v) coverage of the array, is constrained to be less than 0.27 mas by these measurements. These results are consistent with extrapolations of intrinsic structure estimates obtained with VLBI at 7 mm wavelength assuming the intrinsic size of Sgr A* has a greater dependence than λ 0.9 with wavelength.
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