Astronomers have observed for the first time the entire life cycle of one of Nature's most powerful events as jets of energy emerge from a distant black hole, slow down while plowing through interstellar gas and then finally fade away.
The jets came from a so-called stellar black hole, a compact object in our own galaxy that resulted from the death of a massive star.
The four-year set of observations reveals new clues about how a stellar black hole converts matter into streams of energy, unleashed in a powerful flash, and how these jets interact with interstellar space. Studies stemming from the data suggest how the process might be similar to events a far grander scale, over longer time periods, around the colossal supermassive black holes that anchor some galaxies.
Eat, digest, spit
Black holes can't be seen, because even light cannot escape their gravitational clutches. But astronomers infer their existence by noting the motions of stars, gas and dust in the vicinity or by noting extreme outputs of energy.
When black holes attract matter, they don't swallow everything. In the process of accelerating material to nearly the speed of light, the black hole spits some of the incoming stuff out in two opposite directions along its axis of rotation.
These high-energy jets can be observed because of their strong emissions in wavelengths from radio to X-rays.
Partly because X-ray astronomy is still a young discipline, no black hole jet has ever been observed from cradle to grave. In fact, astronomers have wondered whether the jets ever ended, as theory says they should.
Then in 1998, NASA's Rossi X-ray Timing Explorer spotted jets emerging from a binary star system called XTE J1550-564, which is about 17,000 light years from Earth. This two-star system consists of a black hole that siphons matter from an orbiting companion, astronomers believe. Observations were picked up two years later with NASA's Chandra X-ray Observatory and a radio observatory called the Australia Telescope Compact Array.
Now researchers say they've got data covering the whole life cycle of the jets, something not possible with longer-lived jets emerging from supermassive black holes.
"Since the jets came from a stellar black hole in our galaxy, we watched in a few years developments that would have taken thousands of years to occur around a supermassive black hole in a distant galaxy," said Stephane Corbel of the University of Paris VII and the French Atomic Energy Commission. Corbel is the lead author of a paper that will detail the findings in the Oct. 4 issue of the journal Science.
Just bigger parts
Kim Weaver, an astrophysicist at NASA's Goddard Space Flight Center, said black holes can be thought of as engines that convert matter into energy. Baby black holes and the supermassive variety are similar.
"Their parts are just different sizes," said Weaver, who was not involved in the new findings. "That's why this result is so incredibly exciting."
Weaver said most of the energy extracted from a black hole is contained in the motion of the jets. With a supermassive black hole, the jet can be a billion times more powerful than with the stellar type, and it would take possibly millions of years to watch a larger jet over its lifetime.
The physics of jets like those from XTE J1550-564 -- if it can be understood -- could be applied to supermassive black holes, Weaver said. She added that the jets can also be used as probes for what's actually going on inside and near a black hole.
Like a bullet
Because XTE J1550-564 emits these detectable jets, it is termed a microquasar, its behavior mimicking the so-called quasar galaxies of the distant universe whose black holes can contain the masses of millions or even billions of stars.
Early on in the new study, the jets were seen moving at about half the speed of light. But like a bullet slowed by air, the jets interact with interstellar gas. They are now more than 3 light-years apart and slowing down. One of the jets has faded away.
These observations confirm what theorists had expected should happen.
But "this is the first time we've actually seen a jet slowing down," said Philip Kaaret of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. Kaaret said the jets probably generate shock waves akin to those of a supersonic airplane. The shocks take energy from the motion of the jets, converting matter to high-energy particles that make the phenomenon visible in X-rays.
Kaaret is lead author of another paper on the jets due out in a future issue of the Astrophysical Journal. He and other researchers discussed the results today at a NASA press conference.
The new data and imagery generate a mystery, however. One jet, moving east, appears to have traveled farther than the other, most likely because the eastern jet is streaming toward Earth, the scientists say, while the other is moving away. This should also mean the eastern jet would be brighter, but the opposite is the case.
"This poses a puzzle," Kaaret said. "Either the black hole may somehow be feeding more energy into the western jet, or that jet has run into a dense cloud."
Much remains to be learned from the new observations and more that will surely be attempted with other black holes. Kaarat and other black hole experts all admit on thing: They don't understand the mechanism that causes "blobs of matter to be catapulted" into space, as Neil DeGrasse Tyson, director of the Hayden Planetarium, put it.
Tyson said one thing seems certain, however: "We're pretty sure a catastrophic moment took place."
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