While fewer in number than the dominant rotation-powered radio pulsar population , peculiar classes of isolated neutron stars ( INSs ) – which include magnetars , the ROSAT-discovered “ Magnificent Seven ” ( M7 ) , rotating radio transients ( RRATs ) , and central compact objects in supernova remnants ( CCOs ) – represent a key element in understanding the neutron star phenomenology . We report the results of an observational campaign to study the properties of the source 2XMM~J104608.7-594306 , a newly discovered thermally emitting INS . The evolutionary state of the neutron star is investigated by means of deep dedicated observations obtained with the XMM-Newton Observatory , the ESO Very Large Telescope , as well as publicly available \gamma -ray data from the Fermi Space Telescope and the AGILE Mission . The observations confirm previous expectations and reveal a unique type of object . The source , which is likely within the Carina Nebula ( N _ { H } = 2.6 \times 10 ^ { 21 } cm ^ { -2 } ) , has a spectrum that is both thermal and soft , with kT _ { \infty } = 135 eV . Non-thermal ( magnetospheric ) emission is not detected down to 1 % ( 3 \sigma , 0.1 - 12 keV ) of the source luminosity . Significant deviations ( absorption features ) from a simple blackbody model are identified in the spectrum of the source around energies 0.6 keV and 1.35 keV . While the former deviation is likely related to a local oxygen overabundance in the Carina Nebula , the latter can only be accounted for by an additional spectral component , which is modelled as a Gaussian line in absorption with \textrm { EW } = 91 eV and \sigma = 0.14 keV ( 1 \sigma ) . Furthermore , the optical counterpart is fainter than m _ { V } = 27 ( 2 \sigma ) and no \gamma -ray emission is significantly detected by either the Fermi or AGILE missions . Very interestingly , while these characteristics are remarkably similar to those of the M7 or the only RRAT so far detected in X-rays , which all have spin periods of a few seconds , we found intriguing evidence of very rapid rotation , P = 18.6 ms , at the 4 \sigma confidence level . We interpret these new results in the light of the observed properties of the currently known neutron star population , in particular those of standard rotation-powered pulsars , recycled objects , and CCOs . We find that none of these scenarios can satisfactorily explain the collective properties of 2XMM~J104608.7-594306 , although it may be related to the still poorly known class of Galactic anti-magnetars . Future XMM-Newton data , granted for the next cycle of observations ( AO11 ) , will help us to improve our current observational interpretation of the source , enabling us to significantly constrain the rate of pulsar spin down .