Detection of the radial velocity curve of the B5–A0 supergiant companion star of Cir X‐1?

In this paper, we report on phase‐resolved I ‐band optical spectroscopic and photometric observations of Cir X‐1 obtained with the Very Large Telescope. The spectra are dominated by Paschen absorption lines at nearly all orbital phases except near phase zero (coinciding with the X‐ray dip) when the absorption lines are filled in by broad Paschen emission lines. The radial velocity curve of the absorption lines corresponds to an eccentric orbit ( e = 0.45) whose period and time of periastron passage are consistent with the period and phase predicted by the most recent X‐ray dip ephemeris. We found that the I ‐band magnitude decreases from 17.6 to ∼16.8 near phase 0.9–1.0; this brightening coincides in phase with the X‐ray dip. Even though it is likely that the absorption‐line spectrum is associated with the companion star of Cir X‐1, we cannot exclude the possibility that the spectrum originates in the accretion disc. However, if the spectrum belongs to the companion star, it must be a supergiant of spectral type B5–A0. If we assume that the compact object does not move through the companion star at periastron, the companion star mass is constrained to ≲10 M ⊙ for a 1.4‐M ⊙ neutron star, whereas the inclination has to be . Alternatively, the measured absorption lines and their radial velocity curve can be associated with the accretion disc surrounding a 1.4‐M ⊙ neutron star and its motion around the centre of mass. An absorption‐line spectrum from an accretion disc is typically found when our line of sight passes through the accretion disc rim implying a high inclination. In this scenario, the companion star mass is found to be ∼0.4 M ⊙ . However, from radio observations it was found that the angle between the line of sight and the jet axis is smaller than 5°. This would mean that the jet ploughs through the accretion disc in this scenario, making this solution less probable.