tokens: Acknowledgements We thank an anonymous referee for very useful comments and suggestions, and Valerie Connaughton for providing us with the details of the Fermi GBM follow ups to the BBH merger events in runs O1 and O2. RP was partly supported by NSF award AST-1616157. KB acknowledges support from the Polish National Science Center (NCN) grant: Sonata Bis 2 (DEC-2012/07/E/ST9/01360). AC acknowledges support from the NSF CAREER award #1455090 and also partial support from the Swift Cycle 12 GI programme (Grant No. NNX17AF93G).
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