tokens: We are grateful to the referee S. Johnston for very useful comments, to A. Jessner, D. Schnitzeler and D. Riquelme for discussions, to P. Lazarus for sharing code, and to C. Ng for reading the manuscript. Partly based on observations with the 100-m telescope of the MPIfR at Effelsberg. The Nançay radio observatory is operated by the Paris Observatory, associated with the French CNRS. IRAM is supported by INSU/CNRS (France), MPG (Germany), and IGN (Spain). APEX is a collaboration between the MPIfR, the European Southern Observatory, and the Onsala Space Observatory. Financial support by the European Research Council for the ERC Synergy Grant BlackHoleCam (ERC-2013-SyG, Grant Agreement no. 610058) is gratefully acknowledged. PT is supported for this research through a stipend from the International Max Planck Research School (IMPRS) for Astronomy and Astrophysics at the Universities of Bonn and Cologne.
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