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 non-consecutive nights on the source. We construct a radial velocity curve from the spectra, and update the orbital period of the binary to be $P = 5.28388 \pm 0.00046$ 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 $M_1 = 22.7^{+5.2}_{-3.6} M_\odot$ for the primary, and $M_2 = 6.2^{+0.9}_{-0.7} M_\odot$ 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, maybe 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.