Search for Intermediate Black Holes Returns Head-Scratching Results

December 25, 2013

Two black holes in the NGC 1313 galaxy a.k.a. Topsy Turvy galaxy, both ultraluminous X-ray sources.

Two black holes (two magenta spots in this image) in the NGC 1313 galaxy a.k.a. Topsy Turvy galaxy, both ultraluminous X-ray sources. Credit: NASA/JPL-Caltech/IRAP

A couple of teams of scientists hope to find hypothesized intermediate black holes by observing ultra-luminous X-ray sources.

For some reason, black holes come in only two sizes – stellar and supermassive. The former usually have between three and ten, sometimes up to 100 times the mass of the Sun. The latter have anywhere from a couple of hundreds of thousands to billions of solar masses. A few recent studies were hoping to find the evidence of an intermediate-sized black hole with predicted mass between 100 and one million solar masses.

Supermassive black holes can be found in centres of galaxies. The Milky Way has such black hole in its heart and we found many other galaxies to have one. Presumably all galaxies have a supermassive black hole at their core. The centre of a galaxy is a dense area with a lot of material for the black hole to attract. This material, gas mostly, gets heated up as it gets eaten by the black hole and emits powerful X-rays while rotating around it.

Something similar might happen with intermediate black holes. There are objects in space called ultraluminous X-ray sources (ULXs). ULX is a system made of black hole and a normal star, where the black hole feeds off normal star's material, releasing X-rays in the process. The brightness of these X-rays is too great for the black hole to be of stellar variety and astronomers hope to find bigger, intermediate black holes in these systems. It's either that or there's a mechanism of material accretion that we still don't fully understand.

"As if black holes weren't extreme enough, this is a really extreme one that is shining as brightly as it possibly can," said Joel Bregman of the University of Michigan, USA. "It's figured out a way to be more luminous than we thought possible."

In a paper from Dominic Walton of the California Institute of Technology, Pasadena, USA, he and his colleagues have studied the ULX in the Circinus spiral galaxy 13 million light-years away and the results indicate it has a black hole 100 times the mass of the Sun. 

In another study, Matteo Bacheti of the Institut de Recherche en Astrophysique et Planétologie, Toulouse, France, and his colleagues have observed two ULXs in NGC 1313, known as the Topsy Turvy galaxy, also 13 million light-years away. The black holes in these two systems turned out to be smaller than expected, classifying them as stellar black holes. "It's possible that these objects are ultraluminous because they are accreting material at a high rate and not because of their size," said Bachetti. "If intermediate-mass black holes are out there, they are doing a good job of hiding from us."

In yet another new study, a team led by Jifeng Liu of the Chinese Academy of Sciences, Beijing, China, studied a ULX in the Pinwheel Galaxy, 22 million light-years away. They too have expected to find an intermediate black hole, but it turns out the system has a black hole with the mass between 20 and 30 solar masses.

These results indicate that the reason the ULXs shine so bright isn't the yet to be found intermediate black hole, but something else. "Our findings may turn the trend of taking ultraluminous X-ray sources as promising intermediate black hole candidates," Liu said. "Our work shows, based on our conclusion of a stellar mass black hole, that our understanding of the black hole radiation mechanism is incomplete and needs revision."

Sources 1, Source 2


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