An accreting pulsar with extreme properties drives an ultraluminous x-ray source in NGC 5907

EXTraS is funded by the European Union’s (EU) Seventh Framework Programme under grant agreement no. 607452. This research is based on observations obtained with XMM-Newton, a European Space Agency (ESA) science mission with instruments and contributions directly funded by ESA member states and NASA. This work also made use of data from NuSTAR, a mission led by the California Institute of Technology, managed by the Jet Propulsion Laboratory, and funded by NASA; and from Swift, which is a NASA mission with participation of the Italian Space Agency and the UK Space Agency. G.L.I., P.C., L.Z., and A.W. acknowledge funding from the ASI-INAF contract NuSTAR I/037/12/0. P.E. acknowledges funding from the Netherlands Organization for Scientific Research (NWO) Vidi award A.2320.0076. A.P. acknowledges funding from the EU’s Horizon 2020 Framework Programme for Research and Innovation under the Marie Skłodowska-Curie grant agreement 660657-TMSP-H2020-MSCA-IF-2014. G.L.I. is grateful to T. Tauris for useful comments on an earlier version of the manuscript; P.E. thanks A. Mushtukov for interesting discussions. The EXTraS project acknowledges INAF’s Astronomical Observatory of Catania and Astronomical Observatory of Trieste for the use of computing facilities. The data presented here can be found in the supplementary materials; raw x-ray observations can be retrieved through the ESA–XMM-Newton archive interface (www.cosmos.esa.int/web/xmm-newton/xsa) and the NASA archive interfaces for Chandra (http://cda.harvard.edu /chaser/) and Swift (http://heasarc.gsfc.nasa.gov/cgi-bin/W3Browse/swift.pl); see table S1 for the identification number of the observations.