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 CirX-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 CirX-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 Msun for a 1.4 Msun neutron star, whereas the inclination has to be ~> 13.7 degrees. Alternatively, the measured absorption lines and their radial velocity curve can be associated with the accretion disc surrounding a 1.4 Msun 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. However, from radio observations it was found that the angle between the line-of-sight and the jet axis is smaller than 5 degrees implying that the jet ploughs through the accretion disc in this scenario.Comment: 8 pages, 4 figures, 3 tables, accepted by MNRA

Optical spectroscopy of (candidate) ultra-compact X-ray binaries: constraints on the composition of the donor stars

We present optical spectroscopy of several (candidate) ultra-compact X-ray binaries (UCXBs) obtained with the ESO VLT and Gemini-North telescopes. In only one of five observed UCXB candidates did we find evidence for H in its spectrum (4U 1556-60). For XB 1905+00 the optical counterpart is not detected. For the known UCXBs 4U 1626-67 and XB 1916-05 we find spectra consistent with a C/O and a He/N accretion disc respectively, the latter is the first optical spectrum of a He-rich donor in an UCXB. Interestingly, the C/O spectrum of 4U 1626-67 shows both similarities as well as marked differences from the optical C/O spectrum of 4U 0614+09. We obtained phase resolved spectroscopy of 4U 0614+09 and the 44 min transient XTE J0929-314. In neither object were we able to detect clear orbital periodicities, highlighting the difficulties of period determinations in UCXBs. We reanalysed the spectra of XTE J0929-314 that were taken close to the peak of its 2003 X-ray outburst and do not confirm the detection of Halpha emission as was claimed in the literature. The peak spectra do show strong C or N emission around 4640A, as has also been detected in other UCXBs. We discuss the implications of our findings for our understanding of the formation of UCXBs and the Galactic population of UCXBs. At the moment all studied systems are consistent with having white dwarf donors, the majority being C/O rich.Comment: Accepted for publication in MNRA

The mass of the neutron star in the low-mass X-ray binary 2A 1822-371

Using phase resolved spectroscopic observations obtained with the Ultraviolet and Visual Echelle Spectrograph on ESO's Kueyen Very Large Telescope supplemented by spectroscopic observations obtained with the Boller and Chivens Spectrograph on the Walter Baade Magellan telescope, we found sinusoidal radial-velocity variations with a semi-amplitude 327+-17 km/s. From previous observations and from the fact that the epoch of minimum velocity arrived early with respect to the epoch calculated from pulse timing we know that the companion star is suffering from irradiation. Since we most likely observed primarily the side of the companion star facing the observer at phase ~0.75 the velocity quoted above is not the true radial velocity semi-amplitude of the companion star. Assuming a uniform contribution to the line profile from this hemisphere yields a radial velocity semi-amplitude of 280+-26 km/s for a systemic velocity of 54+-24 km/s; if the contribution is instead weighted somewhat more towards the side of the companion facing the X-ray source then the true semi-amplitude is larger than this value. Together with the well constrained inclination (81<i<84 degrees) and the mass-function determined from pulse-timing analysis (2.03+-0.03 x 10^-2 Msun), we derive a lower limit to the mass of the neutron star and to that of the companion star of 0.97+-0.24 Msun and 0.33+-0.05 Msun, respectively (1 sigma; including uncertainties in the inclination). We briefly discuss other aspects of the spectrum and the implications of our findings.Comment: 6 pages, 2 figures, accepted for publication by MNRA