Puzzling Accretion onto a Black Hole in M101

Gemini observations support an unexpected discovery in the galaxy M101. The target, M101 ULX-1 is an ultraluminous X-ray source—ultraluminous in the sense of excessive accretion-powered X-ray emission for a stellar-mass black hole. Jifeng Liu (National Astronomical Observatories of China) and collaborators measured the black hole’s mass, finding that it is 20–30 times the mass of the Sun. Thus, this is not an example of an intermediate mass black hole (as many researchers in the field expected), which would have a mass roughly 100–1000 MSun. Such objects remain elusive, leaving an observational gap between stellar-mass and supermassive black holes. A further unexpected result is that the X-ray emission is relatively soft, dominated by low-energy X-ray emission, which tends to be characteristic of much more massive black holes, not lower-mass cases like M101 ULX-1. A Gemini press release is posted, and the complete results are published in Nature.