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

Vol. 707
7. Stellar structure and evolution

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

UltraLuminous X-ray (ULX) sources are bright X-ray sources in the outskirts of local galaxies, with luminosities in excess of the Eddington rate for an ~10 solar mass black hole. Their nature is puzzling, and initially, they were considered intermediate (100-1000 solar mass) black holes. Even more puzzling was the discovery of X-ray pulsations in several ULXs, indicating the presence of a neutron star. To explain this, beaming and high-magnetic-field models were proposed, which are not necessarily mutually exclusive. In this Letter, Cruz-Sanchez et al. report signs of a high magnetic field B∼(6–7) ×10^14 G in ULX-1 in the barred galaxy NGC4657. This high magnetic field is inferred thanks to a narrow absorption feature in the X-ray spectrum around 3 keV, whose statistical significance has been robustly assessed. This absorption feature is then interpreted as a proton cyclotron resonant scattering feature. This is the second neutron star ULX in which such a feature is found, supporting the connection between magnetars and ULXs.