new_chandra_010515 Using the unmatched X-ray vision of the orbiting Chandra observatory, researchers seem to have spotted the first regularly pulsing X-ray emissions ever observed from a black hole outside our immediate galactic neighborhood.
A study of the data suggests that the periodic change in emissions is created by the black hole's interaction with a giant companion star. While black holes are only theoretical, experts are nearly certain they exist, and the new study could provide another tool to study the exotic objects.
The pulsing black hole, which sits at the center of a galaxy in the constellation Circinus, is 50 times more massive than our Sun, astronomers estimate. The intensity of the emission changes every 7.5 hours, though further work is needed to confirm this cycle.
At odds with our galaxy
Observations of large galaxies regularly turn up handfuls of very luminous, pulsating X-ray sources that are likely to be black holes, said Franz Bauer, a Penn State University researcher who led the new study. This is not true of our own Milky Way Galaxy or the nearby Andromeda Galaxy, however.
"This begs the questions of why we see these bright X-ray sources only in other galaxies and what sort of object they are," Bauer said in an e-mail interview.
No one has been able to identify whether these objects constituted a new class of objects or were pairs of massive objects in fairly common arrangements known as binary systems.
In binary systems, two stars orbit around a common midpoint. In an X-ray binary system, a black hole or neutron star is thought to siphon matter off another large star. X-rays are created when hot gas and dust from the large star slaps into the neutron star or approaches the speed of light as it nears a black hole's event horizon, a point beyond which nothing can escape -- not matter, nor even light.
"The periodic variability in the Chandra data of Circinus provides us with a key signature that these objects are indeed X-ray binary systems," said Franz Bauer, a postdoctoral scholar at Penn State University and lead author of paper in the July issue of the Astronomical Journal.
Testbed for other black holes
One pulsating black hole has been found in the Milky Way, Bauer said, but its pulse is erratic and it is not clear whether it is part of a binary system or has some other engine that drives the pulsation.
"Further study of the periodic signal [in Circinus] will allow us to determine whether it is truly periodic or just quasi-periodic like the source in our own galaxy," Bauer said.
Bauer said the finding is important because black holes this massive are difficult to explain with current theories of star formation and destruction. The pulse matches what would be expected if a black hole and a massive star rotating around each other caused it, he said, and if this is the case then optical observations could measure the mass of each object. Theorists might then be able to use the new information to test the theories.
The researchers also detected a cone of hot gas extending out from the galaxy's core to a distance of 2,000 light-years.
"The large region of X-ray emitting gas extending out of the galaxy disk looks very similar to the gas seen in optical observations," said David Smith, a University of Maryland researcher who also worked on the study. "This may imply the X-ray gas is heated by material close to the galaxy's supermassive black hole."
Next page: Space doughnuts and more black holes
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Space doughnuts
The findings also support a theory in which a large doughnut-shaped ring of gas and dust is thought to obscure some black holes. Such a ring is inferred in the new study by the distribution of gas and the physical conditions near the center of the galaxy.
"The Chandra observations of Circinus show us how complex the gaseous environment of supermassive black holes can be," said another of the study's researchers, Rita Sambruna of George Mason University.
On astronomical scales, the galaxy in Circinus is considered close -- just 13 million light-years from Earth. But intervening gas and dust from the plane of our own galaxy obscure it. X-rays pierce all this. The proximity of Circinus makes it an important testbed for what might be happening in other, more distant galaxies with active supermassive black holes, Sambruna said.
Researchers have come to suspect in recent years that most, or perhaps all, galaxies have a supermassive black hole at their center.
Bevy of black holes found
The study also found a host of small black holes in the spiral arms of the Circinus Galaxy.
"There are at least seven X-ray sources in Circinus which would be considered black holes based on the intensity of the radiation we see from them," Bauer said.
The estimate is based on a current understanding of how much matter can fall into a black hole and how X-rays are created in the process. At a theoretical limit, a black hole emits as much matter as it takes in. At this point, no more accretion takes place, and so X-rays are no longer produced.
If the assumptions are not correct, then the Chandra data might show fewer black holes, or as many as 15.
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