Issue |
A&A
Volume 696, April 2025
|
|
---|---|---|
Article Number | A197 | |
Number of page(s) | 8 | |
Section | Stellar structure and evolution | |
DOI | https://doi.org/10.1051/0004-6361/202554103 | |
Published online | 21 April 2025 |
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
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) fX(0.2–12 keV) = 1.7×10−11 erg cm−2 s−1, which implies an unabsorbed source luminosity LX≃1.3×1035 erg s−1: this is the minimum luminosity ever observed for this source. We measured a pulse period Pspin = 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 (kTBB≃1.5 keV) and a small size (RBB≃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.
Key words: accretion, accretion disks / binaries: general / stars: emission-line, Be / stars: neutron / pulsars: individual: 4U 0728-25
© The Authors
Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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