National Geographic - October 17, 2007
A strange black hole locked in a tight orbit with a huge star in a nearby galaxy could be the most massive stellar black hole known, astronomers say.
...
Astronomers soon realized that the companion star passes directly in front of the black hole on its three-day orbit, eclipsing the black hole's x-ray emissions.
This arrangement allowed the team to combine data from NASAs orbiting Chandra X-Ray Observatory and the Gemini North Telescope on Mauna Kea, Hawaii, to calculate the two object's masses more accurately than usual.
...