Lightweight Black Hole Has Elegant Manners
November 29, 2013

Stellar-Mass Black Hole Caught Consuming Matter In A Surprisingly Calm Manner

April Flowers for - Your Universe Online

Our understanding of how some black holes consume matter might be changed by new observations of a black hole powering an energetic X-ray source in a galaxy some 22 million light years away. The findings, published in a recent issue of Nature, indicate that this particular black hole, thought to be the engine behind the X-ray source’s high-energy light output, is unexpectedly lightweight. Additionally, in spite of 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,” says research team member Stephen Justham, of the National Astronomical Observatories of China, Chinese Academy of Sciences. Such lightweights, he explains, must devour matter at close to their theoretical limits of consumption to sustain the kind of energy output observed. "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," Justham explains. "We expected them to display more complicated behavior when eating so quickly. Apparently we were wrong."

X-ray sources emit high- and low-energy X-rays, called hard and soft X-rays. Counter-intuitively, larger black holes tend to emit more soft X-rays, while smaller black holes produce relatively harder X-rays. Researchers expected to find a larger black hole when investigating the new X-ray source, named M101 ULX-1, because it is dominated by soft X-rays.

They were surprised, therefore, when the Gemini Observatory data revealed that M101 ULX-1's black hole is on the small side. Astrophysicists are unable to explain why this is so.

According to theoretical models depicting how matter falls into black holes and radiates energy, soft X-rays come primarily from the accretion disk. Hard X-rays, meanwhile, are typically generated by a high-energy "corona" around the disk. The corona's emission strength should increase as the rate of accretion becomes closer to the theoretical limit of consumption, the models indicate. The interactions between the disk and corona are also expected to become more complex.

Based on the size of M101 ULX-1, the black hole should theoretically be dominated by hard X-rays and appear structurally more complicated. This is not the case, however.

“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,” says 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.”

For astronomers hoping to find conclusive evidence for an "intermediate-mass" black hole in M101 ULX-1, the new data delivers a blow. An intermediate-mass black hole would have a mass roughly between 100 and 1000 times the mass of the Sun. This would place them between normal stellar-mass black holes and the supermassive black holes found in the centers of galaxies. Such objects have been frustratingly elusive, however. So far, there are potential candidates but no broadly-accepted detection. One of the main proposed hiding places for intermediate-mass black holes has been ultra-luminous X-ray sources (ULXs), and M101 ULX-1 was one of the most promising contenders.

“Astronomers hoping to study these objects will now have to focus on other locations for which indirect evidence of this class of black holes has been suggested, either in the even brighter ‘hyper-luminous’ X-ray sources or inside some dense clusters of stars,” explains research team member Joel Bregman of the University of Michigan.

“Many scientists thought it was just a matter of time until we had evidence for an intermediate-mass black hole in M101 ULX-1,” says Liu. Some of the hope for solving that old puzzle is lost because of the new Gemini observations, while they add the fresh mystery of how this stellar-mass black hole can consume matter so calmly.

The research team used  the Gemini Multi-Object Spectrograph at the Gemini North telescope on Mauna Kea, Hawaii to determine the mass of the black hole by measuring the motion of its companion. M101 ULX-1's companion star is of the Wolf-Rayet variety, which emit strong stellar winds that can provide material for the black hole. The Gemini observations also reveal that the black hole captures more material from those stellar winds than astronomers had anticipated.

As an ultra-luminous black hole, M101 ULX-1 shines a million times more brightly than the Sun in both X-rays (from the black hole accretion disk) and in the ultraviolet (from the companion star) range. Paul Crowther from the University of Sheffield in the United Kingdom adds, "Although this isn't the first Wolf-Rayet black hole binary ever discovered, at some 22 million light-years away, it does set a new distance record for such a system. The Wolf-Rayet star will have died in a small fraction of the time it has taken for light to reach us, so this system is now likely a double black hole binary."

“Studying objects like M101 ULX-1 in distant galaxies gives us a vastly larger sampling of the diversity of objects in our universe,” says Bregman. “It’s absolutely amazing that we have the technology to observe a star orbiting a black hole in another galaxy this far away.”

Image 2 (below):  ULX-1 is located near a spiral arm of M101. The image for M101 is composed from X-ray (Chandra X-ray Observatory; Purple), Infrared (Spitzer Satellite; Red), Optical (Hubble Space Telescope; Yellow) and Ultraviolet (GALEX satellite; Blue).Credit: Chandra X-ray Observatory, Spitzer Satellite, Hubble Space Telescope, and GALEX Satellite