Ultraluminous X-ray sources ( ULXs ) are extragalactic X-ray emitters located off-center of their host galaxy and with a luminosity in excess of a few { 10 ^ { 39 } \text { \mbox { 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 , 4 , 57 , 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 \pm 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 \text { pc } } , characteristic energy of { \sim 1.9 \text { keV } } , and luminosity of { \sim 2 \times 10 ^ { 38 } \text { \mbox { erg s$ { } ^ { -1 } $ } } } imply a mechanical power of { 1.3 \times 10 ^ { 41 } \text { \mbox { erg s$ { } ^ { -1 } $ } } } and an age { \sim 7 \times 10 ^ { 4 } \text { yr } } .
This interpretation suggests that a genuinely super-Eddington regime can be sustained for time scales 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 ]