New observations of the galaxy Messier 101 have yielded the discovery of an unusual black hole that can sustain a hugely voracious appetite while consuming material in an efficient and tidy manner – something previously thought impossible.
This image shows ULX-1 near a spiral arm of the galaxy Messier 101. Image credit: Chandra X-ray Observatory / Spitzer Satellite / Hubble Space Telescope / GALEX Satellite.
Messier 101 (M101), also known as the Pinwheel Galaxy or NGC 5457, is a spiral galaxy located in the constellation Ursa Major about 22 million light-years away from Earth.
In 2004, Chandra and XMM-Newton space observatories detected an ultraluminous X-ray source (ULX) in this galaxy.
X-ray sources give off high- and low-energy X-rays, which astronomers call hard and soft X-rays, respectively. In what might seem like a contradiction, larger black holes tend to produce more soft X-rays, while smaller black holes tend to produce relatively more hard X-rays.
The X-ray source in Messier 101, labeled ULX-1, is dominated by soft X-rays, so scientists expected to find a larger black hole as its energy source. In a surprising twist, however, the new observations indicate that ULX-1’s black hole is on the small side, and they don’t understand why.
The new data suggest that this black hole is unexpectedly lightweight (20-30 times the mass of our Sun), and, despite the generous amount of dust and gas being fed to it by a massive stellar companion, it swallows this material in a surprisingly orderly fashion.
“It has elegant manners. Such lightweights must devour matter at close to their theoretical limits of consumption to sustain the kind of energy output observed,” said Dr Stephen Justham from the National Astronomical Observatories of China, a co-author of the article published in Nature.
This is an artist’s impression of the environment around M101 ULX-1, showing a stellar-mass black hole, in foreground, with accretion disk; gas from the Wolf-Rayet star, in background, feeds the black hole’s voracious appetite. Image credit: Lynette Cook / Gemini Observatory / AURA.
“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. We expected them to display more complicated behavior when eating so quickly. Apparently we were wrong.”
In theoretical models of how matter falls into black holes and radiates energy, the soft X-rays come primarily from the accretion disk, while hard X-rays are typically generated by a high-energy corona around the disk. The models show that the corona’s emission strength should increase as the rate of accretion gets closer to the theoretical limit of consumption. Interactions between the disk and corona are also expected to become more complex.
Based on the size of M101 ULX-1, the region around it should, theoretically, be dominated by hard X-rays and appear structurally more complicated. However, that isn’t the case.
“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 study lead author Dr Jifeng Liu, also from the National Astronomical Observatories of China.
“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.”
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Bibliographic information: Ji-Feng Liu et al. 2013. Puzzling accretion onto a black hole in the ultraluminous X-ray source M 101 ULX-1. Nature 503, pp. 500–503; doi: 10.1038/nature12762
