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Black hole bonanza

Two groups of European researchers are going ga-ga over gamma-ray blasts from different sources, both thought to be black holes acting up.

One gamma-ray source, known as IGR J717497-2821 (let's call it IGR for short), appears to be a newborn black hole. The other, LS 5039, is an unusual high-energy modulator that researchers call the first-ever "gamma-ray clock."


In this animated before-and-after
image from Integral, the arrow
indicates the gamma-ray source
known as IGR J717497-2821. The other variable sources are
well-known X-ray binaries.

IGR was spotted by the European Space Agency's Integral gamma-ray space observatory on Sept. 17, during a survey focusing on the center of our own Milky Way galaxy. "The galactic center is one of the most exciting regions for gamma-ray astronomy, because there are so many potential gamma-ray sources," Roland Walter, an astronomer at the Swiss-based Integral Science Data Center, explained in today's ESA advisory.

Once the Integral team spotted the blast, other astronomers around the world were alerted to watch the phenomenon as well. Ground-based telescopes as well as the ESA's XMM-Newton X-ray observatory, NASA's Chandra X-Ray Observatory and the Swift gamma-ray space telescope gathered more data about the flash, which became brighter for a few days, then faded away over a matter of weeks.

The rise and fall of gamma-ray emissions followed the pattern that astronomers associate with a double-star system - in which one of the stars is like our sun, and the other is actually a matter-sucking black hole. Such black holes result when a massive star collapses into a gravitational singularity so dense that not even light can escape from its grip.

In IGR's case, astronomers believe matter was pulled off the sunlike star to swirl into the black hole like water circling a bathtub drain. The swirling gas, known as an accretion disc, became unstable and collapsed into the black hole - sparking the outburst seen by Integral.

"Astronomers are still not sure why the accretion disc should collapse like this, but one thing is certain: When it does collapse, it releases thousands of times more energy than at other times," the ESA advisory observed.

Check out the Integral news release for still more information. A report on the nova is due to appear in the journal Astronomy and Astrophysics.

The same journal carries the report about LS 5039, from astronomers using the telescopes of the High-Energy Stereoscopic System in Namibia. LS 5039 was discovered last year by the HESS team, and found to be another type of double-star system.

One of the stars is known to be a hot blue star, 20 times as massive as our sun, and the other is presumed to be a black hole. The high-energy gamma-ray emissions from the system rise and fall in a complex cycle - apparently due to the way the stars orbit each other.

It takes four days to complete one orbit, and during that time the presumed black hole passes in front of the blue giant, then behind it. The gamma rays are strongest when the black hole is in front, and weakest when the blue star is in front.

Astronomers believe the ups and downs of the gamma-ray emissions have something to do with the interaction between the black hole and the blue giant's flux of subatomic particles. The weirder aspects of Albert Einstein's theories come into play, according to today's advisory from the HESS team:

"We know since Einstein derived his famous equation (E=mc2) that matter and energy are equivalent, and that pairs of particles and antiparticles can mutually annihilate to give light. Symmetrically, when very energetic gamma rays meet the light from a massive star, they can be converted into matter (an electron-positron pair in this case). So the light from the star resembles, for gamma rays, a fog which masks the source of the gamma rays when the compact object is behind the star, partially eclipsing the source."

The researchers say such interactions are what lead to the clocklike oscillations in the gamma rays, at cosmic energies 100,000 times higher than previously known. And the HESS team members say they'll keep their eyes on this clock for years to come.

"The way in which the gamma-ray signal varies makes LS 5039 a unique laboratory for studying particle acceleration near compact objects such as black holes," said the University of Durham's Paula Chadwick.