XMM-Newton observation of the persistent Be/neutron star system X Persei at a high-luminosity level

We report on the XMM-Newton observation of the HMXRB X Persei, the prototype of the persistent and low-luminosity Be/neutron star pulsars, which was performed in February 2003. The source was detected at a luminosity level of ~1.4×10³⁵ erg s^-1, which is the highest level of the latest three decades. The pulsation period has increased to 839.3 s, thus confirming the overall spin-down of the neutron star detected in the previous observations. The folded light-curve has a complex structure, with features not observed at lower luminosities, and shows a significant energy dependence. The spectral analysis reveals the presence of a significant excess, at low energies, over the main power-law spectral component, which can be described by a black-body spectrum of high temperature (kT_BB ∼ 1.5 keV) and small emitting region (R_BB∼ 360 m); its properties are consistent with a polar-cap origin. Phase-resolved spectroscopy shows that the emission spectrum varies along the pulse period, but it is not possible to prove whether the thermal component is pulsed or not.