Vlba Imaging at 7 Mm and Linear Polarimetric Observations at 6 Cm and 3 Mm of Sgr A* 1. 7 Mm Vlba Imaging of Sgr A*

We summarize the results of 7 mm VLBA imaging of Sgr A* and discuss some of the diiculties of accurately constraining the size of Sgr A* with VLBI observations. Our imaging results are fully consistent with the hypothesis that the VLBA image of Sgr A* is a resolved elliptical Gaussian caused by the scattering of an intervening thermal plasma. We show that determination of the minor axis size at 7 mm with the VLBA is very unreliable. We also present new polarimetric observations from the VLA and from the BIMA array of Sgr A*. At 4.8 GHz, we nd an upper limit to the polarization of 0.1%. At 86 GHz, we report a marginal detection of 11% linear polarization. We discuss the eeects of interstellar propagation on the linear polarization and consider the signiicance of very low intrinsic linear polarization in Sgr A*. We summarize previously-published results of 7 mm VLBA imaging of Sgr A* (Bower & Backer 1998). Observations of Sgr A* at 43 GHz were performed on 29 September 1994 with the VLBA. Fringes on Sgr A* were detected to a maximum (u; v)-distance of 250 M. Fringes to the compact calibrator NRAO 530 were detected on all baselines throughout the experiment. Imaging and modeling revealed a single elliptical Gaussian component with a major axis FWHM of 0:7620:038 mas in a position angle of 77 : 07 : 4. The axial ratio was measured to be 0:73 0:10. We show in Figure 1 the visibility amplitudes as a function of (u; v)-distance along with model ts. No secondary structure was detected in the vicinity of Sgr A* to a limit of 35 mJy. The measured size is fully consistent with the expected major and minor axes predicted by lower frequency observation (Yusef-Zadeh et al. 1994, Lo et al. 1998). However, as Lo et al. note, this result combined with their new result implies a signiicant deviation in the axial ratio from that expected due to scattering. While this is true, poor North-South resolution and systematic errors in calibration make this claim uncertain and diicult to prove.