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tokens: This research was partially supported by the National Science Foundation under grant NSF PHY-1125915 and by the Polish NCN grants 2014/14/M/ST9/00707, 2015/19/B/ST9/01099, and 2016/22/E/ST9/00037. M.A.A. acknowledges the Czech Science Foundation grant No. 17-16287S, which supported one of his visits to the Paris Observatory. The authors benefited from discussions with Ramesh Narayan, Paweł Haensel, Avi Loeb, Axel Brandenburg, and Krzysztof Belczyński. Some of this work was conducted at the Black Hole Initiative at Harvard University, which is funded through a grant from the John Templeton Foundation.