ar X iv : a st ro - p h / 98 11 21 4 v 1 1 3 N ov 1 99 8 VLBA Imaging at 7 mm and Linear Polarimetric

We summarize the results of 7 mm VLBA imaging of Sgr A* and discuss some of the difficulties 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 find an upper limit to the polarization of 0.1%. At 86 GHz, we report a marginal detection of 1±1% linear polarization. We discuss the effects of interstellar propagation on the linear polarization and consider the significance of very low intrinsic linear polarization in Sgr A*. 1. 7 mm VLBA Imaging of 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 λ. 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 0.762± 0.038 mas in a position angle of 77 •. 0± 7 •. 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 fits. 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 significant 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 difficult to prove.