The persistent nature of the Be X-ray binary pulsar 4U 0728-25

¹ INAF–Istituto di Astrofisica Spaziale e Fisica Cosmica di Milano, Via A. Corti 12, 20133 Milano, Italy
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

^⋆ Corresponding author; nicola.lapalombara@inaf.it

Received: 11 February 2025
Accepted: 23 March 2025

Abstract

We report the results obtained with a XMM–Newton observation, performed in April 2023, of the poorly known Galactic Be X-ray binary pulsar 4U 0728-25. It was revealed at a flux level (not corrected for the absorption) f_X(0.2–12 keV) = 1.7×10⁻¹¹ erg cm⁻² s⁻¹, which implies an unabsorbed source luminosity L_X≃1.3×10³⁵ erg s⁻¹: this is the minimum luminosity ever observed for this source. We measured a pulse period P_spin = 103.301(5) s, a value ≃0.15% longer than that estimated in 2016 with AstroSat. The pulse profile shows a broad single peak at all energies, with a limited energy dependence and a small increase in the pulsed fraction with energy. The time-averaged EPIC spectrum can be described equally well by four different emission models, either with a single non-thermal component (a partially covered power law or a cut-off power law), or with a thermal component in addition to the non-thermal one (a black body plus a power law, or a collisionally ionised gas plus a cut-off power law). All of them provided an equally good fit and, in the case of the power–law plus black–body model, the thermal component is characterized by a high temperature (kT_BB≃1.5 keV) and a small size (R_BB≃240 m), comparable with that of the neutron-star polar caps. A spectral variability along the pulse phase is present, which suggests a flux variation of the black-body component. These results show that, for its luminosity level, flux variabilty over long time scales, and spectral properties, 4U 0728-25 is very similar to most of the persistent Be X-ray binaries. Therefore, it can be considered a member of this class of sources.