GeminiFocus January 2014 | Page 7

Figure 2. ly-understood process of accretion, and the conditions which prevail in the strong gravity close to a black hole. Measuring the masses of the components in ULXs is hard, partly because the great luminosity of the accretion disk overwhelms the light from the star. However, rare systems are transient, sometimes entering a state in which they are sufficiently luminous to qualify as ULXs, sometimes returning to a quiescent state in which it might be possible to directly detect the motion of the star. M101 ULX-1 is one such system. It was detected as the brightest X-ray source in the galaxy M101 but since then has regularly been observed in lower-luminosity states. Moreover, M101 ULX-1 is one of the ULXs from which the X-ray spectral energy distribution contains no hint of a Comptonising corona, which reduces the chance that the presently-proposed super-Eddington accretion mechanisms are helping to explain the high luminosity. Furthermore, the X-ray January2014 spectrum is easily fitted by a standard thermal accretion disk with a super-soft temperature of only 100 or 200 electron volts, which implies that the inner disk temperature is exceptionally cool (see Figure 2). This combination of spectral characteristics, combined with the high outburst luminosity, is exactly the set of properties which one would expect an IMBH to display. Quantities derived from fits to the X-ray spectra of a variety of X-ray sources indicate that the accretion disk properties divide naturally into two groups. M101 ULX-1 (shown in red) is a member of the class which apparently maintains very cool inner disk temperatures whilst attaining a high luminosity, as would be expected for IMBHs. Galactic black-hole X-ray binaries (labelled as GBHXRB) and two other known WolfRayet black-hole X-ray binaries lie in a distinctly different region of the parameter space. The dotted lines describe the expected variation in disk luminosity with a fixed inner disk radius (which, naively, would be correlated with black hole mass). For further details, please see Liu et al., (2013). Gemini Observations of a Black Hole Donor Star Based on these arguments, a Gemini proposal was approved to try to detect the motion of the donor star in M101 ULX-1. This resulted in 10 spectra with exposures ranging between 3200 and 9600 seconds, and a combined integration time of 15.6 hours. The first discovery from these observations was the nature of the companion star. Clear helium emission lines indicate that it is a Wolf-Rayet star (an evolved and helium rich GeminiFocus 5