A deep XMM-Newton observation of the X-Persei-like binary system CXOU J225355.1+624336

Abstract

We report on the follow-up XMM-Newton observation of the persistent X-ray pulsar CXOU J225355.1+624336, which was discovered with the CATS@BAR project on archival Chandra data. The source was detected at f<SUB>X</SUB>(0.5−10 keV) = 3.4 × 10<SUP>−12</SUP> erg cm<SUP>−2</SUP> s<SUP>−1</SUP>, a flux level that is fully consistent with previous observations performed with ROSAT, Swift, and Chandra. When compared with previous measurements, the measured pulse period P = 46.753(3) s implies a constant spin down at an average rate of Ṗ = 5.3 × 10<SUP>−10</SUP> s s<SUP>−1</SUP>. The pulse profile is energy dependent, showing three peaks at low energy and a less structured profile above about 3.5 keV. The pulsed fraction slightly increases with energy. We described the time-averaged EPIC spectrum with four different emission models: a partially covered power law, a cutoff power law, and a power law with an additional thermal component (either a black body or a collisionally ionised gas). In all cases we obtained equally good fits, so it was not possible to prefer or reject any emission model on a statistical basis. However, we disfavour the presence of thermal components since their modeled X-ray flux, resulting from a region larger than the neutron star surface, would largely dominate the X-ray emission from the pulsar. The phase-resolved spectral analysis showed that a simple flux variation cannot explain the source variability and proved that there is a spectral variability along the pulse phase. The results of the XMM-Newton observation confirmed that CXOU J225355.1+624336 is a Be X-ray binary (BeXB) with a low luminosity (L<SUB>X</SUB> ∼ 10<SUP>34−35</SUP> erg s<SUP>−1</SUP>), limited variability, and a constant spin down. Therefore, these results reinforce its source classification as a persistent BeXB. <P />Based on observations obtained with XMM-Newton, an ESA science mission, with instruments and contributions directly funded by ESA Member States and NASA.