Resolved atomic lines reveal outflows in two ultraluminous X-ray sources

Ultraluminous X-ray sources are thought to be powered by accretion onto a compact object; now the discovery of X-ray emission lines and blueshifted absorption lines in the high-resolution spectra of ultraluminous X-ray sources NGC 1313 X-1 and NGC 5408 X-1 shows that in each case the compact object is surrounded by powerful winds with an outflow velocity of about 0.2 times that of light. Ultraluminous X-ray sources characterized The lack of sufficiently high energy-resolution spectra has previously prevented the unambiguous identification of emission or absorption lines in the X-ray band of ultraluminous X-ray sources (ULXs), so it has not been possible to test the widely held theory that they are powered by accretion onto a compact object. Here Ciro Pinto et al . report the use of the high energy-resolution reflection grating spectrometer on board ESA's XMM-Newton observatory to identify X-ray emission lines and blueshifted absorption lines in the high-resolution spectra of the ULXs NGC 1313 X-1 and NGC 5408 X-1. The lines indicate that in each case the compact object is surrounded by powerful winds with an outflow velocity of about 0.2 times the speed of light. This agrees with predictions of extreme matter accretion onto compact objects. Ultraluminous X-ray sources are extragalactic, off-nucleus, point sources in galaxies, and have X-ray luminosities in excess of 3 × 10 39 ergs per second. They are thought to be powered by accretion onto a compact object. Possible explanations include accretion onto neutron stars with strong magnetic fields 1 , onto stellar-mass black holes (of up to 20 solar masses) at or in excess of the classical Eddington limit 2 , 3 , 4 , or onto intermediate-mass black holes (10 3 –10 5 solar masses) 5 . The lack of sufficient energy resolution in previous analyses has prevented an unambiguous identification of any emission or absorption lines in the X-ray band, thereby precluding a detailed analysis of the accretion flow 6 , 7 , 8 . Here we report the presence of X-ray emission lines arising from highly ionized iron, oxygen and neon with a cumulative significance in excess of five standard deviations, together with blueshifted (about 0.2 times light velocity) absorption lines of similar significance, in the high-resolution X-ray spectra of the ultraluminous X-ray sources NGC 1313 X-1 and NGC 5408 X-1. The blueshifted absorption lines must occur in a fast-outflowing gas, whereas the emission lines originate in slow-moving gas arou