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

¹ INAF–Istituto di Astrofisica Spaziale e Fisica Cosmica di Milano, Via A. Corti 12, 20133 Milano, Italy
e-mail: nicola.lapalombara@inaf.it
² Scuola Universitaria Superiore IUSS Pavia, Palazzo del Broletto, piazza della Vittoria 15, 27100 Pavia, Italy
³ INAF–Osservatorio Astronomico di Roma, Via Frascati 33, 00040 Monteporzio Catone, Italy
⁴ INAF – Istituto di Astrofisica Spaziale e Fisica Cosmica di Palermo, Via U. La Malfa 153, 90146 Palermo, Italy

Received: 9 March 2021
Accepted: 19 April 2021

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_X(0.5−10 keV) = 3.4 × 10⁻¹² erg cm⁻² s⁻¹, 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 × 10⁻¹⁰ s s⁻¹. 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_X ∼ 10³⁴⁻³⁵ erg s⁻¹), limited variability, and a constant spin down. Therefore, these results reinforce its source classification as a persistent BeXB.