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

TL;DR

This paper reports the detection of a potential proton cyclotron line in the ultraluminous X-ray source NGC 4656 ULX-1, indicating a very strong magnetic field near the neutron star surface, supported by spectral and timing analysis.

Contribution

It presents the first evidence of a proton cyclotron feature in an ULX, suggesting extremely strong magnetic fields in neutron star accretors within these sources.

Findings

Detection of a 3.29 keV absorption feature in X-ray spectrum.

Candidate pulsation at ~0.9736 Hz with 5.5σ significance.

Estimated magnetic field of (6-7)×10^{14} G near the neutron star surface.

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.