GeminiFocus 2013 Year in Review | Page 8

Figure 3. Panel (a) presents the radial velocities measured from the 4686 Angstrom helium emission line in the Gemini spectra of M101 ULX-1. Panel (b) illustrates the chisquared value obtained when fitting circular orbits of different periods to the data, demonstrating that the best-fitting orbital period is 8.2 days. Panel (c) again shows the measured radial velocities, now folded over the inferred orbital period. For further details, please see Liu et al., 2013. massive star that burns brightly, fiercely, and erratically). More detailed analysis indicated a star with spectral type WN8, with a mass somewhere between 17.5 and 19 solar masses. For details, see the journal paper (Liu et al., 2013). The motion of those emission lines showed clear radial velocity variations, indicating that the observations had successfully detected the orbital motion of the donor star about the black hole (see Figure 3). Assuming a circular orbit, the best-fitting orbital period was 8.2 days (which achieved a chisquared value of 1.6). Clearly substantial uncertainties remain, given moderately large error bars for each of the radial-velocity measurements and imperfect phase coverage (remember that this is in a galaxy about 20 million light-years away!) In reality, the orbit might also have non-zero eccentricity (as often observed in the windaccreting Galactic high-mass X-ray binaries), although the good fit to the data using a pure sine curve suggests that any eccentric- 6 ity in this case would be small. The best-fit model radial velocity curve indicates a minimum mass for the compact object in M101 ULX-1 of five solar masses, which confirms that it is a black hole. Despite those uncertainties in the precise properties of the binary, two conclusions are very hard to escape: the black hole in M101 ULX-1 is not an IMBH, and it accretes from the wind of the Wolf-Rayet star. For any binary system whose inclination is unknown, radial velocity measurements can only ever lead to a lower limit on the component masses — since the binary could, in principle, be arbitrarily close to face-on to the line of sight. However, the chance of detecting such a system is small. For M101 ULX-1, the combination of our best-fitting orbital period and Wolf-Rayet mass would require an orbital inclination within 5 degrees of face-on to contain a black hole of 300 solar masses or greater. T