Abstract：Thermal emission from isolated neutron stars (INSs) is expected to originate immediately at the surface, with the bulk of the energy flux peaking in the soft X-ray band. Usually masked by the strong non-thermal processes occurring in the magnetosphere of young pulsars, and heavily absorbed by the interstellar medium, the detection of strong thermal emission in a handful of thermally emitting INSs in our local neighborhood offer the unique opportunity to understand neutron star cooling, derive radii, and constrain the equation of state of matter under extreme physical conditions of gravity and magnetic field. In this talk I will review the properties of the group of seven thermally emitting INSs discovered by the ROSAT All-Sky Survey and dubbed the Magnificent Seven, and their possible evolutionary link with the young and energetic magnetars. I will then report the results of a dedicated XMM-Newton large program on the only source among the seven still lacking a detected spin period, RX J1605.3+3249. The large program allowed us to put stringent constraints on the temperature anisotropy on the surface of the neutron star and the presence of hotspots. Moreover, detailed phase-averaged medium- and high-resolution spectroscopy constrains atmosphere neutron star models and the properties of the cyclotron line in the spectrum of the neutron star with unprecedented statistics.