PI: Jifeng Liu, SAO, firstname.lastname@example.org
Address: 60 Garden St MS-51, Cambridge, MA 02138, US
CoI: Jeff E. McClintock, SAO
CoI: Joel N. Bregman, University of Michigan
CoI: Luis Ho, Carnegie
Title: The first direct mass measurement for the black hole in an ultraluminous X-ray source
Abstract: The ultraluminous X-ray sources (ULXs) are off-nuclear X-ray point sources in nearby galaxies with luminosities exceeding the Eddington luminosity for stellar mass black holes. It has been hotly debated whether they are beamed sources with stellar mass black holes or sub- Eddington emitter with long sought intermediate mass black holes (IMBHs). Deciding this question hinges on achieving dynamical mass measurements through photometric and spectroscopic monitoring of the secondaries of these systems. The secondaries in two ULXs, M101-ULX1 and M81-ULX1, were recently identified as Wolf-Rayet stars with strong emission lines, and here we propose a spectroscopic campaign of 15 Gemini/GMOS-N spectra for M101-ULX1 to measure both the radial velocity amplitude and the orbital period with the HeII (lambda)4686 and HeI (lambda)5876 emission lines that have been already detected in a GMOS spectrum. This allows us to compute its mass function and set a lower limit for the black hole mass, and to compute the black hole mass itself once the period and inclination angle have been obtained from a well- sampled light curve. We also propose to obtain three Gemini/GMOS spectra for M81-ULX1 to confirm the Wolf-Rayet nature of its secondary (as suggested by HST photometry and prism spectrum) and to detect possible emission line shifts.
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