On IC 10 X-1, the Most Massive Known Stellar-Mass Black Hole

IC 10 X-1 is a variable X-ray source in the Local Group starburst galaxy IC 10 whose optical counterpart is a Wolf-Rayet (WR) star. Prestwich and coworkers recently proposed that it contains the most massive known stellar-mass black hole (23-34 M_⊙), but their conclusion was based on radial velocities derived from only a few optical spectra, the most important of which was seriously affected by a CCD defect. Here we present new spectra of the WR star, spanning 1 month, obtained with the Keck I 10 m telescope. The spectra show a periodic shift in the He II λ 4686 emission line as compared with IC 10 nebular lines such as [O III] λ 5007. From this, we calculate a period of 34.93 +/- 0.04 hr (consistent with the X-ray period of 34.40 +/- 0.83 hr reported by Prestwich) and a radial velocity semiamplitude of 370 +/- 20 km s^-1. The resulting mass function is 7.64 +/- 1.26 M_⊙, consistent with that of Prestwich (7.8 M_⊙). This, combined with the previously estimated (from spectra) mass of 35 M_⊙ for the WR star, yields a minimum primary mass of 32.7 +/- 2.6 M_⊙. Even if the WR star has a mass of only 17 M_⊙, the minimum primary mass is 23.1 +/- 2.1 M_⊙. Thus, IC 10 X-1 is indeed a WR/black-hole binary containing the most massive known stellar-mass black hole.