Backstory to detecting a possible middleweight black hole

A European team's announcement this week that it's discovered a missing link among black holes is another case of looking for one kind of object, and stumbling upon the unexpected.

The object's discovery came as one of Farrel's students combed through a catalog of objects assembled from observations by the European Space Agency's XMM-Newton x-ray observatory -- a 10-year-old satellite that's still going strong.

The catalog -- somewhat whimsically dubbed the XMM-Newton Serendipitous Source 2 catalog -- contains all of the x-ray sources the observatory has eyed between February 2000 and March 2007.

Initially, the team was interested in white dwarf stars and neutron stars.  Then the student stumbled across some unusual x-ray sources, including HLX-1. Last year the team conducted follow-up observations with XMM-Newton, and that sealed the deal.

At first, the researchers thought HLX-1 was just another supermassive black hole at a galactic center.

"But then we realized it was too far away" from its host-galaxy's core, Farrell explains. "When we calculated the luminosity, we found that it was about 400 times brighter than what we would expect for a 20 solar-mass black hole [the stellar variety]. This is when we realized we had a unique and very interesting object."

The making of IMBH

So how might such mid-range black holes form? Harvard University astrophysicist Avi Loeb explained during a phone chat that such ultra-luminous x-ray sources often are found in galactic regions undergoing intense bouts of star formation.

The most massive stars form in dense groups or clusters that tend to remain clustered -- bound by their mutual gravity.

"You can imagine a situation where very massive stars collide with each other, which is quite possible because they are bigger. It may become a run-away process. So instead of making 10 100-solar-mass-stars, you make one 1,000-solar-mass star," he says.

Or, he adds, the initial seed of a single star could set it on a course to become one of these humongous stars by its lonesome.

Either way, at the end of its life, a star that massive would collapse into a black hole. Do not form supernova. Do not pass neutron-star stage. There is no way for anything that massive to end up in any other form, Dr. Loeb continues.

The idea that intermediate-mass black holes exist has been around for decades, and others have claimed to have detected such objects. But earlier evidence was far more shaky.

The European team's provides "the most suggestive evidence yet for intermediate-mass black holes," Loeb says.