gamma_supernova_001108 New research presented this week sheds slivers of light on the most powerful explosions in the universe,
mysterious gamma-ray bursts.In the mere seconds that they exist, gamma-ray bursts -- known as GRBs -- emit more high-energy gamma rays than most of the rest of the universe combined. But what causes them is a mystery. Further, researchers don't even know if they are created by a single phenomenon or if multiple sources are responsible.
But one new study indicates there might be two sources, one producing long bursts and the other producing short bursts of energy. A separate pair of studies suggests that some GRBs may be the result of colossal stellar collapses known as
hypernovae. The long and short of GRBs
In one of three studies presented November 7 at a meeting in Honolulu of the High Energy Astrophysics Division of the American Astronomical Society, researchers found evidence for two types of gamma-ray bursts.
GRBs lasting less than two seconds have different characteristics than longer bursts, the researchers said, suggesting that two very different sources may be at work.
"The source of gamma-ray bursts remains one of the great mysteries in modern astronomy," said Jay Norris of NASA Goddard Space Flight Center, who led the study. "All gamma-ray bursters need not be the same type of object. The evidence is growing that shorter bursts are very different."
Norris and his colleagues examined the histories of GRBs recorded by Earth-orbiting satellites. These time histories show that each burst is made up of a series of pulses, groups of photons arriving just milliseconds apart. Short bursts were found to have fewer pulses, and a shorter lag time between pulses, than the longer bursts. The short bursts last two seconds or less.
Jerry Bonnell, also of Goddard, speculated that these shorter bursts might be caused by the merger of two neutron stars. This scenario was once thought to power all gamma-ray bursts, but two other new studies dispute that claim.
Hypernovae and GRBs
With the merger of two dense objects losing favor as a way of explaining GRBs, some other scientists suspect that gamma-ray bursts might be tied in some way to exploding stars called
supernovae, which might explode and collapse in an even larger event called a hypernova.The other pair of studies presented this week provides some of the first evidence of a direct connection.
When some stars near the end of their lives and their core of nuclear fuel burns out, they explode. The colossal supernova event can be more luminous than anything in the universe, as an expanding bubble of matter and energy radiates outward.
A supernova is also thought, in some cases, to then trigger a collapse of the remaining material into a
black hole. Some scientists think this black hole later generates a gamma-ray burst. But there has been no firm evidence of this. The scenario is referred to as a hypernova.The pair of studies found evidence that the energy from a GRB raced outward and encountered a slower-moving cloud of gas enriched with iron. Iron is produced in supernova explosions, so the researchers suspect that the gamma-ray burst is running into the expanding bubble created by the previous supernova.
"Our observations tell us that the material is moving with a velocity of 30,000 kilometers per second (67 million miles per hour), which is 10 percent of the speed of light, and that the iron-rich cloud is extremely dense," said the lead author of one of the studies, Luigi Piro of the Istituto de Astrofisica Spaziale in Italy. "The large mass of ejecta tells us that the progenitor was a very massive star."
Piro and colleagues who worked on both studies said the simplest explanation is that a supernova ejected the cloud about 10 years before the gamma-ray burst.
"The most straightforward scenario that emerges from all of the evidence we have gathered is that a massive progenitor -- like a hypernova ejects matter, shortly before the GRB," said Piro. "In other words, our data helps rule out the scenario where two neutron stars or black holes collide. We think GRBs result from something similar to a supernova explosion, but much more powerful."
Other models holds that the gamma-ray burst is fed by energy from a merger between two dense objects that have been spiraling inwards toward each other.
"We cannot rule out other scenarios yet, but this one is the simplest, and the most consistent with our results" said Italian researcher Filippo Frontera, who led the other study.
Piro and Frontera studied the X-rays emitted by gamma-ray bursts. The data were provided by the Chandra X-ray Observatory and the BeppoSAX satellite and reported in the November 3 issue of the journal Science.
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