Middle-aged gamma-ray pulsar J0554+3107 in X-rays

We present some results of X-ray observations of the middle-aged & gamma;-ray pulsar J0554+3107 with XMM-Newton. For the first time, we detected X-ray pulsations with the J0554+3107 spin period from the presumed X-ray counterpart, thus confirming its pulsar nature. The pulsed fraction in the 0.2-2 keV band is 25 & PLUSMN;6%. The pulsar spectrum can be fitted by the model consisting of thermal and non-thermal components. To describe the former, we created and applied hydrogen atmosphere models for neutron stars with dipole magnetic fields. In addition, an absorption feature at 0.34 keV is required to fit the spectrum. The spectral analysis implies that J0554+3107 has the effective temperature of -47 & PLUSMN;2 eV. The analysis also indicates that J0554+3107 may be a rather heavy neutron star with the mass of -1.9 & PLUSMN;0.2 M & ODOT;. Implementing the relation between the interstellar absorption and the distance in the pulsar direction, we obtained the distance to the pulsar to be about 2 kpc. Implications of the results for cooling scenarios of neutron stars and the equation of state of supra-dense matter in their cores are briefly discussed.