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

Nelson Cruz-Sanchez, Enzo A. Saavedra, Federico A. Fogantini +6

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 pr…