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Cry of a Shredded Star Indicates a New Era

First Posted: Aug 04, 2012 07:31 AM EDT
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An inactive black hole was discovered last year by the astronomers in the distant galaxy that erupted after shredding and consuming a passing star. Days after the outburst the researchers have identified a distinctive X ray signal that originates from the matter on the verge of falling into the black hole.

Led by researchers at the University of Michigan, the team documented the event with the Suzaku and XMM-Newton orbiting X-ray telescopes. These instruments picked up semi-regular blips in the light from a numerically-named galaxy 3.9 billion light years away in the northern constellation Draco the dragon.

These blips are referred as quasi-periodic oscillation or QPO. This is a characteristic feature of the accretion disks that often surround the most compact objects in the universe like the white dwarf stars, neutron stars and black holes.  

Till date QPOs have been identified in several stellar-mass black holes, and there is alluring evidence for them in a few black holes that may have middleweight masses between 100 and 100,000 times the sun's. Till date QPOs was detected in Seyfert-type galaxy REJ 1034+396, which at a distance of 576 million light-years lies relatively nearby.

"This discovery extends our reach to the innermost edge of a black hole located billions of light-years away, which is really amazing. This gives us an opportunity to explore the nature of black holes and test Einstein's relativity at a time when the universe was very different than it is today," said Rubens Reis, an Einstein Postdoctoral Fellow at the University of Michigan in Ann Arbor.

Reis led the team that uncovered the QPO signal using data from the orbiting Suzaku and XMM-Newton X-ray telescopes, a finding described in a paper published August 2 in Science Express.

The black hole, identified as Swift J1644+57, lies 3.9 billion light-years away in the constellation Draco. That name arises from its discovery on March 28, 2011, by NASA's Swift satellite, which searches the universe for gamma-ray bursts. At first, astronomers thought the signal was a common gamma-ray burst, but the gradual fade-out of the signal matched nothing that had ever been seen before from such a source. Closer observation of the object with the orbiting Suzaku and XMM-Newton  X-ray  telescopes ultimately revealed a faint, periodic signal that, Miller said, corresponds in frequency to an ultra-low D-sharp.

The star experienced intense tides as it reached its closest point to the black hole and was torn apart. The gas that fell towards the black hole and formed a disk around it, the innermost part of the disk was rapidly heated to temperatures of millions of degrees that were capable of emitting X rays.

Simultaneously they noticed oppositely directional jets perpendicular to the disk being formed near the black hole that blasted matter outwards at velocity greater than 90 percent the speed of light along black hole's spin axis. One of these jets just happened to point straight at Earth.

Tod Strohmayer, an astrophysicist and co-author of the study at NASA's Goddard Space Flight Center in Greenbelt, Md. Said, "Because matter in the jet was moving so fast and was angled nearly into our line of sight, the effects of relativity boosted its X-ray signal enough that we could catch the QPO, which otherwise would be difficult to detect at so great a distance."

As hot gas in the innermost disk spirals toward a black hole, it reaches a point astronomers refer to as the innermost stable circular orbit (ISCO). 

This inwards spiraling gas piles up at the ISCO, where it becomes extremely heated and radiates a stream of X-rays. The brightness of these X-rays varies in a pattern that repeats at a nearly regular interval, creating the QPO signal.

"QPOs send us information from the very brim of the black hole, which is where the effects of relativity become most extreme," Reis said. "The ability to gain insight into these processes over such a vast distance is a truly beautiful result and holds great promise."

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