The internal Faraday screen of Sagittarius A*

This study of high-cadence full-Stokes data provides new measurements of the fractional linear polarization at a 1-2% level resolved in 4 s time segments, and stringent upper limits on the fractional circular polarization at 0.3%.

We compare these findings to ALMA light curves of Sgr A* at 212-230GHz observed three days later, characterizing a steep depolarization of the source at frequencies below about 150GHz. We obtain time-dependent rotation measure (RM) values, with the mean RM at 85-101GHz being a factor of two lower than that at 212-230GHz. Together with the rapid temporal variability of the RM and its different statistical characteristics in both frequency bands, these results indicate that the Faraday screen in Sgr A* is largely internal, with about half of the Faraday rotation taking place inside the inner 10 gravitational radii, contrary to the common external Faraday screen assumption.