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

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 5.5 σ and a pulsed fraction of ∼11%. The feature is robust against changes in continuum modeling and data-selection criteria, retaining a statistical significance of ≳3 σ in Monte Carlo simulations. Interpreting the absorption as a proton cyclotron resonant scattering feature implies a local magnetic field of B ∼ (6 − 7)×10 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 to be less consistent with the observed phenomenology.