tokens: ACKNOWLEDGEMENTS We are grateful for constructive remarks and suggestions from the referee. We also thank Richard Saxton for sharing the Swift data of XMMSL1 J0740, and Suvi Gezari for sharing some of the data in Fig. 10 . TW is funded in part by European Research Council grant 320360 and by European Commission grant 730980. GL was supported by a research grant (19054) from VILLUM FONDEN. JCAM-J is the recipient of an Australian Research Council Future Fellowship (FT 140101082). PGJ and ZKR acknowledge support from European Research Council Consolidator Grant 647208. MG is supported by the Polish NCN MAESTRO grant 2014/14/A/ST9/00121. KM acknowledges support from STFC (ST/M005348/1) and from H2020 through an ERC Starting Grant (758638). MN acknowledges support from a Royal Astronomical Society Research Fellowship. FO acknowledges support of the H2020 Hemera program, grant agreement No 730970. Based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere under ESO programme 199.D-0143. We acknowledge the use of public data from the Swift data archive. The Australia Telescope Compact Array is part of the Australia Telescope National Facility which is funded by the Australian Government for operation as a National Facility managed by CSIRO. This work has made use of data from the European Space Agency (ESA) mission Gaia ( https://www.cosmos.esa.int/gaia ), processed by the Gaia Data Processing and Analysis Consortium (DPAC, https://www.cosmos.esa.int/web/gaia/dpac/consortium ). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. We also acknowledge the Gaia Photometric Science Alerts Team ( http://gsaweb.ast.cam.ac.uk/alerts ).
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