Quasi-periodic oscillations around compact objects within the Sen spacetime
Keywords:
accretion discs, QPOs, black holes, neutron star, X-ray binariesAbstract
We investigate kilohertz (kHz) quasi-periodic oscillations (QPOs) observed in eight neutron-star low-mass X-ray binaries within the framework of the relativistic precession model (RPM). Fundamental (epicyclic) frequencies of test particles on circular orbits are computed in the static Sen spacetime. By fitting the Keplerian and epicyclic frequencies to the observed lower and upper QPO frequency pairs, (fU, fL) we infer the masses and electric charges of the compact objects and compare the results with those obtained in the Schwarzschild spacetime using the Akaike and Bayesian information criteria (AIC/BIC). We find that the Schwarzschild geometry provides physically consistent fits for four sources, while for GX 5–1 and GX 340+0 the Sen spacetime becomes effectively indistinguishable from Schwarzschild, indicating that no electric charge is required. Although the Sen metric yields statistically improved fits for the remaining four sources, the inferred large masses as well as large electric charges are incompatible with neutron-star physics. We therefore conclude that the static Sen spacetime does not provide a physically viable description of kHz QPOs in neutron-star systems.
References
J. W. T. Hessels, S. M. Ransom, I. H. Stairs, P. C. C. Freire, V. M. Kaspi and F. Camilo, Science 311, 1901–1904 (2006). https://doi.org/10.1126/science.1123430
J. Patterson, E. L. Robinson and R. E. Nather, Astrophysical Journal 214, 144–151 (1977). https://doi.org/10.1086/155305
M. van der Klis, Annual Review of Astronomy and Astrophysics 38, 717–760 (2000). https://doi.org/10.1146/annurev.astro.38.1.717
R. A. Remillard and J. E. McClintock, Annual Review of Astronomy and Astrophysics 44, 49–92 (2006). https://doi.org/10.1146/annurev.astro.44.051905.092532
T. E. Strohmayer and R. F. Mushotzky, Astrophysical Journal Letters 586, L61–L64 (2003). https://doi.org/10.1086/374637
M. Bachetti et al., Nature 514, 202–204 (2014). https://doi.org/10.1038/nature13791
M. Gierliński, M. Middleton, M. Ward and C. Done, Nature 455, 369–371 (2008). https://doi.org/10.1038/nature07237
M. Middleton and C. Done, Monthly Notices of the Royal Astronomical Society 403, 9–16 (2010). https://doi.org/10.1111/j.1365-2966.2009.16005.x
W. Lewin and M. van der Klis, Accreting Neutron Stars in Low-Mass X-Ray Binary Systems, Cambridge University Press (2006).
G. Hasinger and M. van der Klis, Astronomy and Astrophysics 225, 79–96 (1989). https://doi.org/10.1051/0004-6361:19891208
J. Middleditch and W. C. Priedhorsky, Astrophysical Journal 306, 230–236 (1986). https://doi.org/10.1086/164368
M. van der Klis et al., Nature 316, 225–230 (1985). https://doi.org/10.1038/316225a0
M. van der Klis, Annual Review of Astronomy and Astrophysics 27, 517–553 (1989). https://doi.org/10.1146/annurev.aa.27.090189.002505
S. E. Motta et al., Monthly Notices of the Royal Astronomical Society 468, 2311–2324 (2017). https://doi.org/10.1093/mnras/stx570
K. Boshkayev et al., Physical Review D 108, 044063 (2023). https://doi.org/10.1103/PhysRevD.108.044063
K. Boshkayev, O. Luongo and M. Muccino, Physical Review D 108, 124034 (2023). https://doi.org/10.1103/PhysRevD.108.124034
K. Boshkayev et al., Monthly Notices of the Royal Astronomical Society 531, 3876–3887 (2024). https://doi.org/10.1093/mnras/stae1388
K. Boshkayev et al., Gravitation and Cosmology 20, 233–239 (2014). https://doi.org/10.1134/S0202289314040033
K. Boshkayev and M. Muccino, arXiv:2508.15682 (2025). https://doi.org/10.48550/arXiv.2508.15682
W. Kluźniak and M. A. Abramowicz, Astrophysics and Space Science 300, 143–148 (2005). https://doi.org/10.1007/s10509-005-0020-6
L. Stella and M. Vietri, Physical Review Letters 82, 17–20 (1999). https://doi.org/10.1103/PhysRevLett.82.17
H. Ardavan et al., Monthly Notices of the Royal Astronomical Society 529, 3744–3757 (2024).
S. P. O’Sullivan et al., Monthly Notices of the Royal Astronomical Society 421, 3300–3315 (2012). https://doi.org/10.1111/j.1365-2966.2012.20563.x
T. J. Mozdzen et al., Monthly Notices of the Royal Astronomical Society 483, 4411–4425 (2019). https://doi.org/10.1093/mnras/sty3331
A. Mohammadi et al., Monthly Notices of the Royal Astronomical Society 525, 3274–3287 (2023).
G. Srinivasan, Bulletin of the Astronomical Society of India 30, 523–547 (2002).
K. Boshkayev et al., arXiv:2506.11581 (2025). https://doi.org/10.48550/arXiv.2506.11581
S. Boutloukos et al., Astrophysical Journal 653, 1435–1444 (2006). https://doi.org/10.1086/508934
R. Wijnands et al., Astrophysical Journal Letters 504, L35–L38 (1998). https://doi.org/10.1086/311564
P. G. Jonker et al., Monthly Notices of the Royal Astronomical Society 333, 665–678 (2002). https://doi.org/10.1046/j.1365-8711.2002.05442.x
J. Homan et al., Astrophysical Journal 568, 878–900 (2002). https://doi.org/10.1086/339057
P. G. Jonker et al., Astrophysical Journal 537, 374–386 (2000). https://doi.org/10.1086/309029
M. Méndez and M. van der Klis, Monthly Notices of the Royal Astronomical Society 318, 938–942 (2000). https://doi.org/10.1046/j.1365-8711.2000.03788.x
M. Méndez et al., Astrophysical Journal Letters 505, L23–L26 (1998). https://doi.org/10.1086/311600
M. Méndez and M. van der Klis, Astrophysical Journal Letters 517, L51–L54 (1999). https://doi.org/10.1086/312025
E. C. Ford et al., Astrophysical Journal Letters 486, L47–L50 (1997). https://doi.org/10.1086/310827
