Ultraluminous X-ray Sources (ULXs) are off-nuclear X-ray sources in nearby galaxies not associated with the central black hole (BH) and exceeding a luminosity of few 10^39 erg/s (well above the Eddington luminosity, LEdd, for a neutron star). since their discovery in the 70′, these sources have been considered stellar-mass BHs (10-100 solar masses) accreting at very high rate or intermediate-mass BHs (10^3-10^5 solar masses), the fundamental blocks for the super-massive BH formation at z~4. The recent discovery, also thanks to XMM archival data analysis in the framework of the EXTraS project, of ULXs showing fast and rapidly evolving pulsations (PULXs) unambiguously associated these sources to accreting neutron stars (NSs) exceeding up to more than 500 times their LEdd. These sources challenge our understanding of accretion physics and pose a key question about the nature of the ULXs as a class. A number of accretion models have been proposed in order to account for the observational properties of PULXs which will be briefly discussed, as well as the possible reason(s) for which a relatively small number of PULXs have been discovered so far. Preliminary results from a search for new PULXs in both archival XMM data and those from the UNSEEN XMM-Newton project will be also shown.
Taking the beat of the UNSEEN: a new Ultraluminous Neutron Star Extragalactic populatioN