Letter to the Editor

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

¹ Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata, B1900FWA La Plata, Argentina
² Instituto de Astrofísica de Canarias (IAC), Vía Láctea, La Laguna, E-38205 Santa Cruz de Tenerife, Spain
³ Departamento de Astrofísica, Universidad de La Laguna, E-38206 Santa Cruz de Tenerife, Spain
⁴ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁵ Instituto Argentino de Radioastronomía (CCT La Plata, CONICET; CICPBA; UNLP), C.C.5, (1894), Villa Elisa, Argentina
⁶ INAF, Osservatorio Astronomico di Cagliari, via della Scienza 5, I-09047 Selargius (CA), Italy
⁷ Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Can Magrans s/n, E-08193 Barcelona, Spain
⁸ Institut d’Estudis Espacials de Catalunya (IEEC), E-08860 Castelldefels (Barcelona), Spain
⁹ INAF–Osservatorio Astronomico di Roma, via Frascati 33, I-00078 Monteporzio Catone, Italy
¹⁰ INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica Milano, via A. Corti 12, I-20133 Milano, Italy
¹¹ INAF/IASF Palermo, via Ugo La Malfa 153, I-90146 Palermo, Italy

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Received: 6 January 2026
Accepted: 7 March 2026

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 ± 0.02 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 ∼0.9736 Hz, with a local significance of 5.5σ and a pulsed fraction of ∼11%. The feature is robust against changes in continuum modeling and data-selection criteria, retaining a statistical significance of ≳3σ in Monte Carlo simulations. Interpreting the absorption as a proton cyclotron resonant scattering feature implies a local magnetic field of B ∼ (6 − 7)×10¹⁴ 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 to be less consistent with the observed phenomenology.