Black Holes May Come in Medium, Pair of Studies Suggest

Researchers have identified what may be the first examples of medium-sized black holes.

For years, scientists could only point to small and large black holes -- either they were the corpses of stars bigger than our Sun, or the supermassive hearts of galaxies, where they steadily reeled in all surrounding material. Now NASA's Nuclear Spectroscopic Telescope Array (NuSTAR) is hard at work examining a new class of black holes that may constitute the middle ground between the two.

"Exactly how intermediate-sized black holes would form remains an open issue," Dominic Walton of the California Institute of Technology, said in a statement. "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."

Possible evidence for these middle-of-the-road celestial bodies may come from ultraluminous X-ray sources (ULXs), or a black hole preying upon a normal star. ULXs are found throughout the galaxy, emanating X-rays too great to be the result of small black holes. This and other evidence suggests the objects may contain a mass between 100-10,000 times the mass of our Sun -- versus small black holes, which generally run around 10 times bigger, and supermassive black holes, estimated to be some 10 billion times bigger.

By joining with other telescopes, NuSTAR and is offering some of the first looks at these objects in focused, high-energy X-rays, allowing scientists to put together estimates of their masses and other features.

The results, published in Astrophysical Journal, describe multiple ULXs located roughly 13 million light years away.

Archival data from NASA's Chandra, Swift and Spitzer space telescopes, as well as NuSTAR, the European Space Agency's XMM-Newton satellite and Japan's Suzaku satellite, have all been used to piece together a clearer picture of one of these objects, located in the Circinus spiral galaxy.

"We went to town on this object, looking at a range of epochs and wavelengths," Walton said.

Their findings suggest the black hole in question is some 100 times the mass of the Sun.

Another study, led by Matteo Bachetti of the Institut de Recherche en Astrophysique et Planétologie, looked at two ULXs in a spiral galaxy known as the "Topsy Turvy galaxy," also about 13 million light years away. A single view with NuSTAR reveals the black holes found here don't fit with models of medium-sized black holes, which has researchers thinking both ULXs may be home to small, stellar-mass black holes. However, one object the researchers discovered is believed to be too big for this category, measuring at 70-100 solar masses.

Still, there's a possible catch, according to Bachetti.

"It's possible that these objects are ultraluminous because they are accreting material at a high rate and not because of their size," the researcher said. "If intermediate-mass black holes are out there, they are doing a good job of hiding from us."