Astronomers may have found a new way to search for intermediate black holes, the long sought after missing link between stellar black holes and their supermassive cousins at the heart of galaxies.

A combination of temperature studies and X-ray observations led a team of researchers to a number of X-ray objects from four different, and distant, galaxies that could be intermediate black holes. The objects are apparently too cool to be neutron stars or stellar black holes, but emit X-rays with more vigor than those objects.

"There's no direct evidence for intermediate black holes," said study leader Rosanne Di Stefano, an astrophysicist with the Harvard-Smithsonian Center for Astrophysics. "And it will be very difficult to establish them in the same way that we have for supermassive black holes, at the center of galaxies, or stellar black holes. "

Di Stefano's study used the space-based Chandra X-ray Observatory to detect objects dubbed "quasisoft" X-ray sources because of their low temperatures, which are at most four million degrees Celsius. That's well below the temperatures expected for "hard" X-ray sources such as neutron stars or stellar black holes - which run between 10 million and 100 million degrees Celsius - and still hotter than "soft" X-ray sources like white dwarfs. But despite their low temperatures, these quasisoft objects emitted X-ray signals that are more powerful than expected from simple neutron stars or stellar black holes.

"We're finding objects in the temperature range for an accreting black hole that is more massive than a stellar black hole," Di Stefano told SPACE.com. "A big clue that we may be seeing intermediate black holes are the high luminosities."

Di Stefano cautioned that it's not certain that the objects her team found are definitely intermediate black holes. They could be neutron stars or stellar black holes that are behaving unlike anything ever seen before, but the method used to find them could eventually used to detect a definite black hole middleweight.

Though black holes are invisible, most astronomers are convinced of the existence of the so-called stellar black holes, which collapse out of single star and have masses up to 10 times that of the sun. Likewise, supermassive black holes, which can have the mass of billions of stars and sit in the center of galaxies, are also believed to exist.

A key part of that belief, Di Stefano said, are optical and radio telescope studies that support X-ray observations with additional data of the environment around black holes. Images of accretion disks, high temperatures and powerful radio emissions streaming from a massive point of empty space point to the existence of stellar and supermassive black holes, she added.

Such dynamical support is absent for middleweight black holes, which could have mass that range from 100 to 1,000 times that of the sun. In 2002, a team of astronomers announced the discovery of at least one intermediate black hole, but an error in their research pulled the bottom out of their find.

Albert Kong, an astrophysicist who also participated in Di Stefano's study, said that the low-temperature, luminous quasisoft objects found in the study fit in with theoretical models of intermediate black holes, white dwarfs, stellar black holes and neutron stars.

"Basically we can rule out white dwarfs," he said. "The temperature is just too high."

Di Stefano said more quasisoft X-ray sources need to be catalogued in order to differentiate which objects are ordinary black holes and which may be middleweights.