XMM-Newton observations of PSR J0554+3107: pulsing thermal emission from a cooling high-mass neutron star

XMM-Newton observations of the middle-aged radio-quiet gamma-ray pulsar J0554+3107 allowed us, for the first time, firmly identify it in X-rays by detection of pulsations with the pulsar period. In the 0.2-2 keV band, the pulse profile shows two peaks separated by about a half of the rotation phase with the pulsed fraction of 25 +/- 6 per cent. The profile and spectrum in this band can be mainly described by thermal emission from the neutron star with the hydrogen atmosphere, dipole magnetic field of similar to 10(13) G, and non-uniform surface temperature. Non-thermal emission from the pulsar magnetosphere is marginally detected at higher photon energies. The spectral fit with the atmosphere+power-law model implies that J0554+3107 is a rather heavy and cool neutron star with the mass of 1.6-2.1 M-circle dot, the radius of approximate to 13 km, and the redshifted effective temperature of approximate to 50 eV. The spectrum shows an absorption line of unknown nature at approximate to 350 eV. Given the extinction-distance relation, the pulsar is located at approximate to 2 kpc and has the redshifted bolometric thermal luminosity of approximate to 2 x 10(32) erg s(-1). We discuss cooling scenarios for J0554+3107 considering plausible equations of state of superdense matter inside the star, different compositions of the heat-blanketing envelope, and various ages.