August 3, 2012
X-ray Signal Detected During Black Hole’s Destruction Of Passing Star
Lee Rannals for redOrbit.com — Your Universe Online
Astronomers have identified an X-ray signal that followed an observation made last year of a quiescent black hole in a distant galaxy.
The black hole discovered by the team last year was seen erupting after shredding and consuming a passing star. Astronomers then witnessed days following the observation a distinctive X-ray signal that comes from matter on the verge of falling into the black hole.
This quasi-periodic oscillation (QPO) is a characteristic of the accretion disks that often surround the most compact objects in the universe, including white dwarf stars, neutron stars and black holes.
QPOs have been seen in many stellar-mass black holes, and there is evidence for them in a few black holes that may have masses between 100 and 100,000 times the sun.
This event had previously just been detected around only one supermassive black hole, until the new finding.
"This discovery extends our reach to the innermost edge of a black hole located billions of light-years away, which is really amazing," Rubens Reis, an Einstein Postdoctoral Fellow at the University of Michigan in Ann Arbor, said in a press release. "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."
The X-ray source was discovered on March 28, 2011, and was originally thought to be a more common type of outburst known as a gamma-ray bursts. However, as it gradually faded-out, it matched nothing that they had seen before.
The team eventually went with the idea that what they were seeing was the aftermath of the awakening of a distant galaxy's dormant black hole, as it was shredding a passing star. The galaxy is so far away, that light from the event had to travel 3.9 billion years to reach Earth.
As the star inched closer to the black hole, some of its gas fell toward it and formed a disk around it. The innermost part of this disk was rapidly heated to temperatures of millions of degrees, making it hot enough to produce X-rays.
At the same time, oppositely directed jets perpendicular to the disk formed near the black hole. These jets blasted matter outward at velocities that were greater than 90 percent the speed of light, along the black hole's spin axis.
Nine days after the outburst, the astronomers observed the X-ray source using an X-ray satellite operating by the Japan Aerospace Exploration Agency known as Suzaku. Ten days later, they began using the European Space Agency's XMM-Newton observatory.
"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," Tod Strohmayer, an astrophysicist and co-author of the study published in the journal Science Express, said in a press release.
Hot gas located in the innermost disk eventually reaches a point astronomers call the innermost stable circular orbit (ISCO). Any closer to the black hole, and the gas would reach the point of no return. The inward spiraling gas tends to pile up around the ISCO, where it become heated and radiates X-rays.
The brightest of the X-rays varies in a pattern that repeats at nearly regular interval, which creates the QPO signal.
The astronomer's data shows a QPO cycled every 3.5 minutes, which places its source region between 2.2 and 5.8 million miles from the center of the black hole. The maximum distance is only about 6 times the diameter of the sun.
"QPOs send us information from the very brim of the black hole, which is where the effects of relativity become most extreme," Reis said in the release. "The ability to gain insight into these processes over such a vast distance is a truly beautiful result and holds great promise."