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

Abstract

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<SUB>solar</SUB> for a 1.4-M<SUB>solar</SUB> 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<SUB>solar</SUB> 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<SUB>solar</SUB>. 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.