Swift is a space-based gamma ray telescope designed to react quickly to locate and observe gamma ray bursts. On March 28, 2011, Swift detected a burst in the constellation Draco. Designated Sw J1644+57, the astronomers using Swift quickly identified the source as the black hole at the center of a galaxy about 3.9 billion light years from Earth. But, rather than fading quickly the way a gamma-ray burst normally behaves, Sw J1644+57 continued to glow for a few months. The source had to be something more unusual.

When a small object like a star gets too close to a black hole, the pull of gravity is strong enough to strip the outer layers off the star (see the animation, below.) However, rather than falling straight in to the black hole, the gas begins to orbit, spiraling down toward the center and forming a disk called an accretion disk. The closer it gets to the black hole, the faster it orbits and the hotter it gets. Some of the superheated gas is ejected in jets, perpendicular to the accretion disk. The gas that remains in the disk gets hot enough to emit x-rays. Astronomers look for these tell-tale emissions from the gas, just before it crosses the event horizon and vanishes. "You can think of it as hearing the star scream as it gets devoured, if you like" said Prof. Jon Miller, co-author of the paper that appeared last week in ScienceExpress.

Knowing that the accretion disk should glow brightly in x-rays, a team of astronomers, mostly from the University of Michigan, used two x-ray telescopes to observe Sw J1644+57. Nine days after Swift detected the event, they pointed the Suzaku x-ray observatory, operated by the Japan Space Agency (JAXA) in collaboration with NASA, at Sw J1644+57. 10 days later, they were able to begin an observation campaign with XMM-Newton, a European Space Agency x-ray satellite.

These x-ray observations revealed a 200 second variation in the x-ray emission. This variation, called a quasi-periodic oscillation or QPO, is a distinctive signature associated with matter on the inner edge of an accretion disk, just before it crosses the event horizon. QPOs are common with stellar mass black holes (black holes with a mass similar to that of a star) and have been studied extensively for nearby objects. However, only one supermassive black hole has ever exhibited a QPO before.


According to lead author Rubens Reis, "This is telling us that the same physical phenomenon we observe in stellar mass black holes is also happening in black holes a million times the mass of the Sun, and in black holes that were previously asleep."


QPOs originate in an area where General Relativity is important, and where it is usually very hard to get good information from distant objects. "Our discovery opens the possibility of studying orbits close to black holes that are very distant," Miller said. Acording to Reis, "[It] 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."

The paper is " A 200-Second Quasi-Periodicity After the Tidal Disruption of a Star by a Dormant Black Hole" by Reis, Miller, Reynolds, Gültekin, Maitra, King, and Strohmayer and appeared in the August 2, 2012 edition of Science. It is available online athttp://www.sciencemag.org/content/early/2012/08/01/science.1223940

Images and animation in this article came from: http://svs.gsfc.nasa.gov/vis/a010000/a010800/a010807/index.html

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