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

Abstract

Ultraluminous X-ray sources are extragalactic, off-nucleus, point sources in galaxies, and have X-ray luminosities in excess of 3 × 1039 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 fields1, onto stellar-mass black holes (of up to 20 solar masses) at or in excess of the classical Eddington limit2,3,4, or onto intermediate-mass black holes (103–105 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 flow6,7,8. 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 around the source. We conclude that the compact object in each source is surrounded by powerful winds with an outflow velocity of about 0.2 times that of light, as predicted by models of accreting supermassive black holes and hyper-accreting stellar-mass black holes9,10.

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Figure 1: Simultaneous spectral fits to the stacked XMM-Newton RGS and EPIC/PN spectra of NGC 1313 X-1.
Figure 2: Ratios between the individual RGS spectra of NGC 1313 X-1.
Figure 3: Significance of the features in the NGC 1313 X-1 RGS stacked spectrum.
Figure 4: Best fit to the stacked XMM-Newton RGS spectrum of NGC 5408 X-1.

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Acknowledgements

We thank A. Lohfink for help with the use of photoionization emission codes. A.C.F. acknowledges support from the European Research Council through Advanced Grant on Feedback 340492. M.J.M. appreciates support from an STFC advanced fellowship. This work is based on observations with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and NASA. This research has also made use of data obtained from NASA’s Chandra satellite. All codes used are publicly available.

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Contributions

C.P. wrote the manuscript with comments from all authors, and analysed the XMM-Newton data. Both M.J.M. and A.C.F. made substantial contributions to the overall science case and manuscript.

Corresponding author

Correspondence to Ciro Pinto.

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The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 EPIC MOS+PN stacked image of NGC 1313.

The circular source extraction regions (large white circles) have a diameter of 1 arcmin. The small region to the south of X-1 (small white circle) is a star-forming region near the galactic centre, orders of magnitude fainter across the 0.3–10 keV bandpass than X-1. The strip enclosed within dashed yellow lines is the RGS extraction region. Counts per pixel are colour coded (key at bottom).

Extended Data Figure 2 ACIS image of NGC 1313 X-1 and the nearby star-forming region, SFR.

The ultraluminous X-ray source is the brightest object. The small circles have 6 arcsec radii, that is, 0.1 arcmin; the larger circle has 0.5 arcmin radius. Counts per pixel are colour coded (key at bottom).

Extended Data Figure 3 EPIC MOS+PN stacked images of NGC 5408 and NGC 6946.

a, NGC 5408; b, NGC 6946. The ultraluminous X-ray sources are the brightest objects in both images. Additional, nearby X-ray bright sources—mostly high-mass X-ray binaries and background active galactic nuclei—can be seen. The white circular source extraction regions have a diameter of 1 arcmin. Counts per pixel are colour coded (key at bottom).

Extended Data Figure 4 XMM-Newton/RGS stacked spectra of the brightest ULXs (X-1) in NGC 1313, NGC 5408 and NGC 6946.

Spectra are in flux units. The rest-frame wavelengths of relevant transitions are given as vertical red dashed lines, labelled with the transition. The spectra have been re-binned for display purposes. Error bars, ±1σ.

Extended Data Figure 5 XMM-Newton RGS spectra and best-fitting model to each observation of NGC 1313 X-1.

Spectra are in flux units. The rest-frame wavelengths of the most relevant emission lines (green vertical dashed lines) are shown, labelled with the transition in red, and the rest-frame wavelengths of the blueshifted absorption lines are shown by the position of the transition in blue. Obs 1 shows both absorption and emission lines. Obs 2 is dominated by absorption, while obs 3 shows mostly emission features. Error bars, ±1σ.

Extended Data Figure 6 Significance of the features in the NGC 5408 X-1 RGS stacked spectrum.

Negative values refer to absorption lines (see also Fig. 3 for NGC 1313 X-1). The solid and dashed curves show the line significance obtained with 500 km s−1 and 10,000 km s−1 widths, respectively. The dark and light grey regions enclose points within 2σ and 3σ confidence levels, respectively. Relevant transitions are labelled in red.

Extended Data Figure 7 Constraints on the location of the extreme absorber, XABS 3, and mass of the compact object.

The white area shows the acceptable values between the Schwarzschild radius (RS, bottom oblique line), the relation R = f(Lion, NH, ξ), which is given by ξ = L/nHR2 (dotted horizontal line), and the radius assuming the escape velocity to be equal to the wind speed (vw = 0.2c, top oblique line, where M and Msun are the compact object and solar masses, respectively). The red arrows show the maximum radius and the upper limit for a compact object with a 100Msun mass.

Extended Data Table 1 Summary of the XMM-Newton observations
Extended Data Table 2 XMM-Newton EPIC-RGS spectral modelling

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Pinto, C., Middleton, M. & Fabian, A. Resolved atomic lines reveal outflows in two ultraluminous X-ray sources. Nature 533, 64–67 (2016). https://doi.org/10.1038/nature17417

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