A candidate proton cyclotron feature in the ultraluminous X-ray source NGC 4656 ULX-1
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2026
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| author | Cruz-Sanchez, Nelson Saavedra, Enzo A. Fogantini, Federico A. García, Federico Combi, Jorge A. Bachetti, Matteo Imbrogno, Matteo Sidoli, Lara Marino, Alessio |
| author_facet | Cruz-Sanchez, Nelson Saavedra, Enzo A. Fogantini, Federico A. García, Federico Combi, Jorge A. Bachetti, Matteo Imbrogno, Matteo Sidoli, Lara Marino, Alessio |
| contents | Ultraluminous X-ray sources represent extreme super-Eddington accretion regimes, and a subset is now known to host highly magnetized neutron stars. However, direct observational probes of their surface magnetic fields remain scarce. In this Letter, we report the detection of a narrow X-ray absorption feature at $3.29\pm0.02$ keV in the XMM$-$Newton/EPIC-pn spectrum of NGC 4656 ULX-1. The source exhibits a hard-ultraluminous state, while our timing analysis reveals a candidate pulsation at $\sim$0.9736 Hz, with a local significance of $5.5σ$ and a pulsed fraction of $\sim11\%$. The feature is robust against changes in continuum modeling and data-selection criteria, retaining a statistical significance of $\gtrsim3σ$ in Monte Carlo simulations. Interpreting the absorption as a proton cyclotron resonant scattering feature implies a local magnetic field of $B\sim(6-7)\times10^{14}$ G in the line-forming region. This value is consistent with strong magnetic fields anchored near the neutron star surface, even if the large-scale dipole is substantially weaker. Although we discuss electron cyclotron features and atomic transitions as possible alternatives, they appear less consistent with the observed phenomenology. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2603_10331 |
| institution | arXiv |
| publishDate | 2026 |
| record_format | arxiv |
| spellingShingle | A candidate proton cyclotron feature in the ultraluminous X-ray source NGC 4656 ULX-1 Cruz-Sanchez, Nelson Saavedra, Enzo A. Fogantini, Federico A. García, Federico Combi, Jorge A. Bachetti, Matteo Imbrogno, Matteo Sidoli, Lara Marino, Alessio High Energy Astrophysical Phenomena Ultraluminous X-ray sources represent extreme super-Eddington accretion regimes, and a subset is now known to host highly magnetized neutron stars. However, direct observational probes of their surface magnetic fields remain scarce. In this Letter, we report the detection of a narrow X-ray absorption feature at $3.29\pm0.02$ keV in the XMM$-$Newton/EPIC-pn spectrum of NGC 4656 ULX-1. The source exhibits a hard-ultraluminous state, while our timing analysis reveals a candidate pulsation at $\sim$0.9736 Hz, with a local significance of $5.5σ$ and a pulsed fraction of $\sim11\%$. The feature is robust against changes in continuum modeling and data-selection criteria, retaining a statistical significance of $\gtrsim3σ$ in Monte Carlo simulations. Interpreting the absorption as a proton cyclotron resonant scattering feature implies a local magnetic field of $B\sim(6-7)\times10^{14}$ G in the line-forming region. This value is consistent with strong magnetic fields anchored near the neutron star surface, even if the large-scale dipole is substantially weaker. Although we discuss electron cyclotron features and atomic transitions as possible alternatives, they appear less consistent with the observed phenomenology. |
| title | A candidate proton cyclotron feature in the ultraluminous X-ray source NGC 4656 ULX-1 |
| topic | High Energy Astrophysical Phenomena |
| url | https://arxiv.org/abs/2603.10331 |