The Extreme ULX in NGC 5907 is a Pulsar

An exciting result from EXTraS: the extreme ULX in NGC 5907, one of the best intermediate mass black hole candidates, is truly an accreting neutron star. Other properties of this system must also be extreme to sustain its apparent luminosity, 1000 times beyond its Eddington limit.
The paper reporting the discovery has been accepted for publication by the journal Science and is now in press. You can read it here, on arXiv or here, on Science.
You might also want to read the ESA press release (in English) or the Media INAF press release (in Italian).