A candidate proton cyclotron feature in the ultraluminous X-ray source NGC 4656 ULX-1

Nelson Cruz-Sanchez; Enzo A. Saavedra; Federico A. Fogantini; Federico García; Jorge A. Combi; Matteo Bachetti; Matteo Imbrogno; Lara Sidoli; Alessio Marino

arXiv · 2603.10331

A candidate proton cyclotron feature in the ultraluminous X-ray source NGC 4656 ULX-1

Nelson Cruz-Sanchez·Enzo A. Saavedra·Federico A. Fogantini·Federico García·Jorge A. Combi·Matteo Bachetti·Matteo Imbrogno·Lara Sidoli·Alessio Marino

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Abstract

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 $5.5\sigma$ and a pulsed fraction of $\sim11\%$. The feature is robust against changes in continuum modeling and data-selection criteria, retaining a statistical significance of $\gtrsim3\sigma$ 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.

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Nelson Cruz-Sanchez, Enzo A. Saavedra, Federico A. Fogantini, Federico García, Jorge A. Combi, Matteo Bachetti, Matteo Imbrogno, Lara Sidoli, Alessio Marino. 2026-03-11. A candidate proton cyclotron feature in the ultraluminous X-ray source NGC 4656 ULX-1. https://arxiv.org/abs/2603.10331

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