Optical Analysis and Modeling of HD96670, a New Black Hole X-Ray Binary Candidate

We report on optical observations and modeling of HD96670, a single-line spectroscopic binary in the Carina OB2 association. We collected 10 epochs of optical spectroscopy, and optical photometry on 17 nonconsecutive nights on the source. We construct a radial velocity curve from the spectra, and update the orbital period of the binary to be P = 5.28388 +/- 0.00046 days. The spectra show oxygen and helium absorption, consistent with an O-type primary. We see no evidence for spectral lines from the secondary star in the binary. We model the optical light curve and radial velocity curve simultaneously using the Wilson-Devinney code and find a best-fit mass of M-1 = 22.7(-3.6)(+5.2) M-circle dot for the primary, and M-2 = 6.2(-0.7)(+0.9) M-circle dot for the secondary. An object of this mass is consistent with either a B-type star, or a black hole. Given that we see no absorption lines from the secondary, in combination with an observed hard power-law X-ray spectrum with Gamma = 2.6 detected past 10 keV that may have been produced by wind accretion onto the secondary, we conclude that the secondary is most likely a black hole. We see asymmetrical helium lines with a shape consistent with the presence of a third star. If the secondary is indeed a black hole, this system would add to the small sample of only four possible black hole high mass X-ray binaries in the galaxy.

Automated summary

The study presents optical observations and modeling of HD96670 in the Carina OB2 association, revealing it as a single-line spectroscopic binary system with an O-type primary star and a secondary star likely to be a black hole. By simulating optical light curves and radial velocity curves, estimates of the masses of the primary and secondary stars were obtained.