A Massive Black Hole in I Zw 18?

LIGO recently detected the collision of two usually high mass BHs(66 and 85 solar mass). Similar masses have been suggested for someultraluminous X-ray sources (ULXs). XMM-Newton data on the dwarf galaxy IZw 18 reveal a very high luminosity (near 1E40 erg/s) in a soft spectralstate. Comparison with the low/hard spectrum of the source suggests ablack hole mass of at least 85 solar masses. We propose an observationto either measure the high-state spectrum at a different luminosityor search for timing noise in the low/hard state. Either outcome wouldhelp constrain the compact object mass and help understand whether ornot massive stars can evolve to produce massive BHs.