Polarimetric properties of Event Horizon Telescope targets from ALMA

Understand

We present the results from a full polarization study carried out with ALMA during the first VLBI campaign, which was conducted in Apr 2017 in the $\lambda$3mm and $\lambda$1.3mm bands, in concert with the Global mm-VLBI Array (GMVA) and the Event Horizon Telescope (EHT), respectively.

• We determine the polarization and Faraday properties of all VLBI targets, including Sgr A*, M87, and a dozen radio-loud AGN.
• We detect high linear polarization fractions (2-15%) and large rotation measures (RM $>10^{3.3}-10^{5.5}$ rad m$^{-2}$).
• For Sgr A* we report a mean RM of $(-4.2\pm0.3) \times10^5$ rad m$^{-2}$ at 1.3 mm, consistent with measurements over the past decade, and, for the first time, an RM of $(-2.1\pm0.1) \times10^5$ rad m$^{-2}$ at 3 mm, suggesting that about half of the Faraday rotation at 1.3 mm may occur between the 3 mm photosphere and the 1.3 mm source.