A strong emission line near 24.8 angstrom in the X-ray binary system MAXI J0556--332: gravitational redshift or unusual donor?

TL;DR

This paper reports the detection of a strong emission line at 24.8 angstrom in the X-ray binary MAXI J0556-332, exploring whether it is due to gravitational redshift effects or an unusual donor star composition.

Contribution

It presents the first observation of this emission line in MAXI J0556-332 and discusses potential physical origins, including gravitational redshift or high nitrogen-to-oxygen abundance.

Findings

Line consistent with N VII Ly-alpha transition

No other prominent narrow emission lines detected

Possible origins include neutron star surface or unusual donor star

Abstract

We report the discovery of a strong emission line near 24.8 angstrom (0.5 keV) in the newly discovered X-ray binary system MAXI J0556-332 with the reflection grating spectrometer onboard 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 Ly-alpha 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 angstrom line uncertain. We discuss possible physical scenarios, including a gravitationally redshifted O VIII Ly-alpha 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.