A new ultraluminous X-ray source in the galaxy NGC 5907
Salvaterra, R.; Tiengo, A.; Israel, G.; Mereghetti, S.; Wolter, A.; Belfiore, A.; Marelli, M.; Novara, G.; De Luca, A.; Pintore, F.; Walton, D. J.; Zampieri, L.; Rigoselli, M.; Fuerst, F.; Ambrosi, E.; Rodriguez, G.; Salvaggio, C.
Italy, United Kingdom, Spain
Abstract
We report on the serendipitous discovery of a new transient in NGC 5907, at a peak luminosity of 6.4 × 1039 erg s-1. The source was undetected in previous 2012 Chandra observations with a 3σ upper limit on the luminosity of 1.5 × 1038 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 ∼ 1.5 keV). The source might be an ∼30 M⊙ 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.