  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 and of the present upper limits in the optical,
which imply a logarithmic X-ray-to-optical flux ratio greater than 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 (~10 km/s) INS accreting at the Bondi-Hoyle rate.
