Near IR spectroscopy of the X-ray binary Circinus X-1

We report on new K band spectroscopy of the X-ray binary Cir X-1, which reveals the spectrum to be dominated by emission from H I, He I and low-excitation metals. No features of photospheric origin are identifiable in the spectrum, preventing an unambiguous classification of the mass donor within the system. While the spectrum superficially resembles that of a high-luminosity mid-B supergiant we consider that the magnitude and timescale of the photometric and spectroscopic variability renders such an identification highly unlikely. Instead we favour an explanation where the spectrum is dominated by emission from an accretion disc and/or accretion driven outflow(s), hence obscuring the spectrum of the underlying star.