Diffuse X-ray emission around an ultraluminous X-ray pulsar

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Abstract

Ultraluminous X-ray sources (ULXs) are extragalactic X-ray emitters located off-centre of their host galaxy and with a luminosity in excess of a few 1039 erg s−1, if emitted isotropically1,2. The discovery of periodic modulation revealed that in some ULXs the accreting compact object is a neutron star3,4,5,6,7, indicating luminosities substantially above their Eddington limit. The most extreme object in this respect is NGC 5907 ULX-1 (ULX1), with a peak luminosity that is 500 times its Eddington limit. During a Chandra observation to probe a low state of ULX1, we detected diffuse X-ray emission at the position of ULX1. Its diameter is 2.7 ± 1.0 arcsec and contains 25 photons, none below 0.8 keV. We interpret this extended structure as an expanding nebula powered by the wind of ULX1. Its diameter of about 200 pc, characteristic energy of ~1.9 keV and luminosity of ~2 × 1038 erg s−1 imply a mechanical power of 1.3 × 1041 erg s−1 and an age of ~7 × 104 yr. This interpretation suggests that a genuinely super-Eddington regime can be sustained for timescales much longer than the spin-up time of the neutron star powering the system. As the mechanical power from a single ULX nebula can rival the injection rate of cosmic rays of an entire galaxy8, ULX nebulae could be important cosmic ray accelerators9.

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Fig. 1: Multi-instrument soft X-ray light curve of NGC 5907 ULX-1 since April 2017, when the Swift monitoring resumed.
Fig. 2: X-ray sky map between 0.3 and 7.0 keV of the region around the direction of NGC 5907 ULX-1 as observed by Chandra in November 2017.

Data availability

The datasets analysed in this work (XMM-Newton OBSIDs: 0804090301, 0804090401, 0804090501, 0804090601, 0804090701; Chandra OBSIDs: 12987, 20830, 20994, 20995) are available for download from the HEASARC archive at https://heasarc.gsfc.nasa.gov. The data that support the plots within this paper and other findings of this study are available from the corresponding author on reasonable request.

Code availability

The code used in this work is available from the corresponding author on reasonable request.

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Acknowledgements

This research is based on observations made with the Chandra X-ray Observatory and has made use of software provided by the Chandra X-ray Center (CXC) in the application packages CIAO, ChIPS and Sherpa. This research also made use of data obtained with the Neil Gehrels Swift Observatory and XMM-Newton. Swift is a NASA mission with participation of the Italian Space Agency and the UK Space Agency. XMM-Newton is an ESA science mission with instruments and contributions directly funded by ESA Member States and NASA. A.B. is grateful to A. Fabian for an interesting discussion and to S. Covino for help in optical data reduction. A.B. and G.N. are supported by EXTraS, a project funded by the European Union’s Seventh Framework Programme under grant agreement no. 607452. We acknowledge funding in the framework of the project ULTraS (ASI-INAF contract no. 2017-14-H.0). M.M. acknowledges funding from ASI-INAF contract no. 2015-023-R.0. D.J.W. acknowledges financial support from an STFC Ernest Rutherford Fellowship.

Author information

A.B., A.T., F.P., G.N. and P.E. processed and analysed the data. A.B. and D.M. performed the statistical analysis. Theoretical interpretation was mainly provided by A.B. with contributions and inputs by A.D.L., A.T., F.P., P.E., R.S. and other co-authors. A.B. and P.E. composed the text. All authors discussed the results and commented on the manuscript.

Correspondence to Andrea Belfiore.

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Supplementary Figs. 1–4, Table 1 and refs. 1–21

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Belfiore, A., Esposito, P., Pintore, F. et al. Diffuse X-ray emission around an ultraluminous X-ray pulsar. Nat Astron (2019) doi:10.1038/s41550-019-0903-z

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