Super portrait: x-ray telescope eyes supernova remnant

Science News, August 28, 2004 by R. Cowen

When light from a massive star that exploded in the constellation Cassiopeia reached Earth some 340 years ago, few if any sky watchers recorded the event. But over the past several decades, the glowing remains of that explosion--a vast bubble of hot gas and dust called Cassiopeia A--has become one of the most studied supernova remnants in the heavens.

Trained on Cassiopeia A for viewing sessions totaling 11.5 days, NASA's Chandra X-ray Observatory has now taken the most detailed portrait ever recorded of any supernova remnant. The image provides new evidence linking supernova explosions to gamma-ray bursts, the most energetic flashes of radiation in the cosmos, says Chandra researcher J. Martin Laming of the Naval Research Laboratory in Washington, D.C.

Laming, Una Hwang of NASA's Goddard Space Flight Center in Greenbelt, Md., and their colleagues analyzed the portrait of Cassiopeia A. At 10,000 light-years, it's the closest supernova remnant to Earth. NASA released the image of the remnant this week. The portrait contains about 200 times as much detail as a shorter exposure of the remnant taken by Chandra soon after its 1999 launch (SN: 10/21/00, p. 266).

The new image shows two oppositely directed jets, each about 10 light-years long, shooting out from the remnant's center. Previous images had shown only one jet. X-ray spectra reveal that the jets are rich in silicon ions and poor in iron. The iron deficiency suggests that the jets didn't trigger the explosion, as they would have carried large amounts of iron from the massive star's central region. Instead, the supernova explosion is likely to have produced the jets, Laming says.

His team conjectures that the jets are low-energy versions of those created by hypernovas--relatively rare and extremely powerful supernovas. According to a leading theory, a hypernova jet generates gamma-ray bursts when particles within it collide (SN: 7/10/99, p. 28).

The jets found in Cassiopeia A aren't strong enough to have generated a gamma-ray burst, suggesting that "jets [in super-novas] may be a more common phenomenon than had been previously suspected," says Laming. Establishing that run-of-the-mill supernova explosions produce jets "would be an important step forward" in understanding what it takes to generate a gamma-ray burst, he adds.

The new Chandra image also confirms that the collapsed core of the exploded star doesn't lie exactly at the center of the remnant. Laming mad other astronomers conjecture that the core was kicked off center because the explosion wasn't equally strong in all directions.

"The new results are breathtaking," says Start Woosley of the University of California, Santa Cruz. "We have seen evidence for jetlike outflows in the Cassiopeia A remnant before, but these are much clearer and the compositional information will aid in diagnosing what happened," he adds. At the same time, Woosley cautions that Cassiopeia A might not represent the typical supernova.