Black holes are usually either relatively small (with 10 times the mass of our Sun) or giant (equivalent to the mass of 10 billion Suns); but what about medium size?
"Exactly how intermediate-sized black holes would form remains an open issue," Dominic Walton of the California Institute of Technology, Pasadena, said in a NASA's Jet Propulsion Laboratory news release."Some theories suggest they could form in rich, dense clusters of stars through repeated mergers, but there are a lot of questions left to be answered."
Supermassive black holes are normally found at the center of a galaxy; these objects pull material inward and emit powerful X-rays.
Evidence of medium black holes may come from the detection of ultraluminous X-ray source (UXLs). These objects come in pairs, and feed off normal stars. Researcher have found the feeding process is not as "big and messy" as it is in supermassive black holes.
The glow from the UXLs is also too bright to come from a typical small black hole. The team suggested the objects emitting the X-rays were between 100 to 10,000 times the mass of our Sun.
Irregular feeding by a larger black hole could also cause the phenomenon.
Matteo Bachetti of the Institut de Recherche en Astrophysique et Planétologie also looked at ULXs in what is known as the "topsy-turvy galaxy." These UXLs proved to be the clearest and most informative. Using the NuSTAR telescope the team found the UXLs did not fit into the "model" of a medium-sized back hole. The team suggested instead they could be "small, stellar-mass black holes." One of the objects was "70 to 100 solar masses," which is too big for its size category.
"It's possible that these objects are ultraluminous because they are accreting material at a high rate and not because of their size," Bachetti said. "If intermediate-mass black holes are out there, they are doing a good job of hiding from us."
