Origins of X-Ray Line Emissions in Circinus X-1 at Very Low X-Ray Flux

Accretion conditions and morphologies of X-ray transients containing neutron stars are still poorly understood. Circinus X-1 is an enigmatic case where we observe X-ray flux changes covering four orders of magnitude. We observed Circinus X-1 several times at its very lowest X-ray flux using the high-energy transmission grating spectrometer on board the Chandra X-ray Observatory. At a flux of $1.8\times {10}^{-11}$ erg cm⁻² s⁻¹ we observed a single 1.6 keV blackbody spectrum. The observed continuum luminosity of 10³⁵ erg s⁻¹ is about two orders of magnitude too low to explain the observed photoionized luminosity, suggesting a much more complex structure of the X-ray source that is partially or entirely obscured, as had been previously suggested. This affects most emissions from the accretion disk, including previously observed accretion-disk coronal line emissions. Instead, the strongest observed photoionized lines are blueshifted by about ∼400 km s⁻¹, and we suggest that they originate in the ionized wind of a B5Ia supergiant companion, supporting a previous identification. The neutron star in Cir X-1 is very young and should have a high magnetic field. At the observed luminosity, the emission radius of the blackbody is small enough to be associated with the accretion hot spot as the X-ray-emitting region. The small emission radius then points to a field strength below 10¹² G, which would be consistent with the observation of occasional type I X-ray bursts at high magnetic fields. We discuss Cir X-1 in the context of being a high-mass X-ray binary, with some emphasis on a possible Be-star X-ray binary nature.