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

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 10(39) erg s(-1), if emitted isotropically(1,2). The discovery of periodic modulation revealed that in some ULXs the accreting compact object is a neutron star(3-7), 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 similar to 1.9 keV and luminosity of similar to 2 x 10(38) erg s(-1) imply a mechanical power of 1.3 x 10(41) erg s(-1) and an age of similar to 7 x 10(4) 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 galaxy(8), ULX nebulae could be important cosmic ray accelerators(9).