Astronomers using ESO's Very Large Telescope have detected, in another galaxy, a stellar-mass black hole much farther away than any other previously known.

With a mass above fifteen times that of the Sun, this is also the second most massive stellar-mass black hole ever found. It is entwined with a star that will soon become a black hole itself.

The newly announced black hole lies in a spiral galaxy called NGC 300, six million light-years from Earth. "This is the most distant stellar-mass black hole ever weighed, and it's the first one we've seen outside our own galactic neighbourhood, the Local Group," says Paul Crowther, Professor of Astrophysics at the University of Sheffield and lead author of the paper reporting the study.

The black hole's curious partner is a Wolf-Rayet star, which also has a mass of about twenty times as much as the Sun. Wolf-Rayet stars are near the end of their lives and expel most of their outer layers into their surroundings before exploding as supernovae, with their cores imploding to form black holes.

In 2007, an X-ray instrument aboard NASA's Swift observatory scrutinised the surroundings of the brightest X-ray source in NGC 300 discovered earlier with the European Space Agency's XMM-Newton X-ray observatory. "We recorded periodic, extremely intense X-ray emission, a clue that a black hole might be lurking in the area," explains team member Stefania Carpano from ESA.

Thanks to new observations performed with the FORS2 instrument mounted on ESO's Very Large Telescope, astronomers have confirmed their earlier hunch. The new data show that the black hole and the Wolf-Rayet star dance around each other in a diabolic waltz, with a period of about 32 hours. The astronomers also found that the black hole is stripping matter away from the star as they orbit each other.

"This is indeed a very 'intimate' couple," notes collaborator Robin Barnard. "How such a tightly bound system has been formed is still a mystery."

"This artist's impression depicts the newly discovered stellar-mass black hole in the spiral galaxy NGC 300. The black hole has a mass of about twenty times the mass of the Sun and is associated with a Wolf - Rayet star; a star that will become a black hole itself. Thanks to the observations performed with the FORS2 instrument mounted on ESO's Very Large Telescope, astronomers have confirmed an earlier hunch that the black hole and the Wolf - Rayet star dance around each other in a diabolic waltz, with a period of about 32 hours. The astronomers also found that the black hole is stripping matter away from the star as they orbit each other. How such a tightly bound system has survived the tumultuous phases that preceded the formation of the black hole is still a mystery. (Credit: ESO/L. Calçada)"

Source: ESO