2022MNRAS.516...13T

XMM-Newton observations of the middle-aged radio-quiet γ-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 ∼10¹³ 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_⊙, the radius of ≈13 km, and the redshifted effective temperature of ≈50 eV. The spectrum shows an absorption line of unknown nature at ≈350 eV. Given the extinction-distance relation, the pulsar is located at ≈2 kpc and has the redshifted bolometric thermal luminosity of ≈2 × 10³² 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.