Abstract

Ultraluminous X-ray sources (ULXs) are exceptionally bright astronomical objects found throughout the local Universe. Their extreme luminosities, above 10^{39 erg/s, must result from the process of accretion onto a compact object. Viable solutions include accretion onto neutron stars with strong magnetic fields, accretion onto stellar mass black holes (< 100 M_solar) at or in excess of the classical Eddington limit (super-critical accretion) or accretion at more sedate rates onto intermediate mass black holes (IMBHs 10}3-5 M_solar). A major obstacle has been the profound lack of unambiguous atomic features in the X-ray spectra of these sources which can be used to diagnose the nature of the accretion flow around the compact object. As a result, the true nature of the wider population of ULXs has been shrouded in debate. Here we identify strong (> 5-sigma), rest-frame emission lines and narrow, blue-shifted (~0.2-0.3c) absorption lines from a high spectral-energy-resolution study of the archetypal ULX , NGC 1313 X-1 using the XMM -Newton Reflection Grating Spectrometer. The detections confirm the presence of powerful winds in this source and by extension other similar ULXs.