http://www.astronomy.com/news/2013/12/extreme-black-hole-is-more-luminous-than-astronomers-thought-possible
December 2, 2013 - Astronomy.com
For decades, astronomers have puzzled over an odd source of X-ray light
in an arm of the galaxy M101, just off the handle of the Big Dipper. The
object, like other so-called ultraluminous X-ray sources, is a system
made of a star and a black hole that orbit each other. But its
brightness and estimated mass haven’t seemed to add up.
...
To measure the mass of this system, known as ULX-1, Liu and colleagues
first looked at the system through the Gemini telescope. Then they
conducted a spectroscopic analysis of the images, which involved
analyzing the color of the light to figure out which elements the star
in the system contained. They found that there wasn’t much hydrogen in
the star. That helped them determine it’s a type known as a Wolf-Rayet.
Such stars are rare, large, and hot. And they’re shrinking fast due to
their strong stellar winds.
...