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

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) $f_{\rm X}$(0.2-12 keV) = 1.7$\times 10^{-11}$ erg cm$^{-2}$ s$^{-1}$, which implies an unabsorbed source luminosity $L_{\rm X} \simeq 1.3 \times 10^{35}$ erg s$^{-1}$: this is the minimum luminosity ever observed for this source. We measured a pulse period $P_{\rm spin}$ = 103.301(5) s, a value $\simeq$ 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 ($kT_{\rm BB} \simeq$ 1.5 keV) and a small size ($R_{\rm BB} \simeq$ 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.