Over the last decade , X-ray observations unveiled the existence of several classes of isolated neutron stars ( INSs ) which are radio-quiet or exhibit radio emission with properties much at variance with those of ordinary radio pulsars .
The identification of new sources is crucial in order to understand the relations among the different classes and to compare observational constraints with theoretical expectations .
A recent analysis of the 2XMMp catalogue provided less than 30 new thermally emitting INS candidates .
Among these , the source 2XMM J104608.7-594306 appears particularly interesting because of the softness of its X-ray spectrum , kT = 117 \pm 14 eV and N _ { H } = ( 3.5 \pm 1.1 ) \times 10 ^ { 21 } cm ^ { -2 } ( 3 \sigma ) , and of the present upper limits in the optical , m _ { B } \ga 26 , m _ { V } \ga 25.5 and m _ { R } \ga 25 ( 98.76 % confidence level ) , which imply a logarithmic X-ray-to-optical flux ratio \log ( F _ { X } / F _ { V } ) \ga 3.1 , corrected for absorption .
We present the X-ray and optical properties of 2XMM J104608.7-594306 and discuss its nature in the light of two possible scenarios invoked to explain the X-ray thermal emission from INSs : the release of residual heat in a cooling neutron star , as in the seven radio-quiet ROSAT -discovered INSs , and accretion from the interstellar medium .
We find that the present observational picture of 2XMM J104608.7-594306 is consistent with a distant cooling INS with properties in agreement with the most up-to-date expectations of population synthesis models : it is fainter , hotter and more absorbed than the seven ROSAT sources and possibly located in the Carina Nebula , a region likely to harbour unidentified cooling neutron stars .
The accretion scenario , although not entirely ruled out by observations , would require a very slow ( \sim 10 km s ^ { -1 } ) INS accreting at the Bondi-Hoyle rate .