A Strong Emission Line near 24.8 Å in the X-Ray Binary System MAXI J0556-332: Gravitational Redshift or Unusual Donor?

We report the discovery of a strong emission line near 24.8 Å (0.5 keV) in the newly discovered X-ray binary system MAXI J0556-332 with the reflection grating spectrometer (RGS) on board the XMM-Newton observatory. The X-ray light curve morphology during these observations is complex and shows occasional dipping behavior. Here we present time- and rate-selected spectra from the RGS and show that this strong emission line is unambiguously present in all the XMM observations. The measured line center is consistent with the Lyα transition of N VII in the rest frame. While the spectra contain imprints of absorption lines and edges, there appear to be no other significantly prominent narrow line due to the source itself, thus making the identification of the 24.8 Å line uncertain. We discuss possible physical scenarios, including a gravitationally redshifted O VIII Lyα line originating at the surface of a neutron star or an unusual donor with an extremely high N/O abundance (>57) relative to solar that may have produced this comparatively strong emission line.