| Abstract
| - Over the last decade, X-ray observations have revealed 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 fewer 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, $kT=117\pm14$ eV and $N_{\rm H}=(3.5\pm1.1)\times10^{21}$ cm -2 (3 σ), and of the present upper limits in the optical, $m_{\rm B}\ga26$, $m_{\rm V}\ga25.5$ and $m_{\rm R}\ga25$ (98.76% confidence level), which imply a logarithmic X-ray-to-optical flux ratio $\log(F_{\rm X}/F_{\rm V})\ga3.1$, 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 (~10 km s -1) INS accreting at the Bondi-Hoyle rate.
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