Astronomy and Astrophysics – Astronomy
Scientific paper
Mar 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008aas...21116106s&link_type=abstract
American Astronomical Society, AAS Meeting #211, #161.06
Astronomy and Astrophysics
Astronomy
1
Scientific paper
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 et al. (2007, ApJ, 669, 21) recently proposed that it contains the most massive known stellar-mass black hole (24-33 solar masses), 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 5 nights, obtained with the Keck-I 10 m telescope (+LRIS). The spectra show a periodic shift in the He II emission line as compared with IC 10 nebular lines such as [O III] λ5007. From the measured centroids we derive the radial velocity of the WR star at each of our epochs. After performing a nonlinear least-squares fit to our velocity curve, we calculate a period of 35.6±0.305 hours (consistent with the X-ray period of 34.40±0.83 hours reported by Prestwich et al.) and a radial velocity semi-amplitude of 364䔸 km/s. The resulting mass function is 7.41±1.25 solar masses, consistent with that of Prestwich et al. (7.8). This, combined with the previously estimated (from spectra) mass of 35 solar masses for the WR star and assuming an edge-on system, yields a minimum primary mass of 32.2±2.66 solar masses. Even if the WR star has only 17 solar masses, the minimum primary mass is 22.7±2.05 solar masses. Thus, IC 10 X-1 is indeed a WR/black-hole binary containing the most massive known stellar-mass black hole. We acknowledge NSF grant AST-0607485 for support.
Filippenko Alexei V.
Silverman Jeffrey
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