Small Yet Voracious Black Hole Puzzles Astronomers

A small yet voracious black hole located some 22 million light years away in the Pinwheel Galaxy is forcing astronomers to reevaluate what they know about the already mysterious objects.

Based on the amount of gas and dust the black hole consumes, researchers expected to uncover a messy situation.

"We thought that when small black holes were pushed to these limits, they would not be able to maintain such refined ways of consuming matter," research team member Stephen Justham, of the National Astronomical Observatories of China, Chinese Academy of Sciences, said in a statement. "We expected them to display more complicated behavior when eating so quickly. Apparently we were wrong."

The black hole, Justham explained, boasts "elegant manners," despite devouring matter near the theoretical limits needed to sustain the output seen.

Using the Gemini Multi-Object Spectrograph at the Gemini North telescope on Mauna Kea, Hawaii, the researchers were able to measure the motion of the black hole's companion star and, in doing so, determine the mass of the black hole.

The results were baffling.

Large black holes usually produce soft, or low-energy, X-rays, and small black holes hard, or high-energy, X-rays. Given the amount of the former surrounding this X-ray source, called M101 ULX-1, researchers expected to find a sizable black hole as its energy source.

Published in the journal Nature, the findings indicate the black hole is quite small, though the study's authors are not sure how that is possible.

"Theories have been suggested which allow such low-mass black holes to eat this quickly and shine this brightly in X-rays. But those mechanisms leave signatures in the emitted X-ray spectrum, which this system does not display," said lead author Jifeng Liu, of the National Astronomical Observatories of China, Chinese Academy of Sciences. "Somehow this black hole, with a mass only 20-30 times the mass of our Sun, is able to eat at a rate near to its theoretical maximum while remaining relatively placid. It's amazing. Theory now needs to somehow explain what's going on."