A new ultraluminous X-ray source in the galaxy NGC 5907

We report on the serendipitous discovery of a new transient in NGC 5907, at a peak luminosity of 6.4 x 10(39) erg s(-1). The source was undetected in previous 2012 Chandra observations with a 3 sigma upper limit on the luminosity of 1.5 x 10(38) erg s(-1), implying a flux increase of a factor of >35. We analysed three recent 60 ks/50 ks Chandra and 50 ks XMM-Newton observations, as well as all the available Swift/XRT observations performed between 2017 August and 2018 March. Until the first half of 2017 October, Swift/XRT observations did not show any emission from the source. The transient entered the ultraluminous X-ray source (ULX) regime in less than two weeks and its outburst was still on-going at the end of 2018 February. The 0.3-10 keV spectrum is consistent with a single multicolour blackbody disc (kT similar to 1.5 keV). The source might be an similar to 30 M-circle dot black hole accreting at the Eddington limit. However, although we did not find evidence of pulsations, we cannot rule out the possibility that this ULX hosts an accreting NS.