The press release from the Stanford Linear Accelerator Center was giddy with excitement: the gamma-ray burst detected by the Fermi Gamma-ray Space Telescope was “one for the record books.”
Seen in the constellation Carina on Sept. 15, 2008, but announced last Thursday, the burst took place 12.5 billion light-years away. The accelerator center, in Menlo Park, Calif., insisted it had “the greatest total energy, the fastest motions, and the highest-energy initial emissions ever before seen.” Presumably, the scrambled syntax means “ever seen.”
(View of September 2008 supernova in Carina, NASA photo)
The orbiter’s instruments “provide a view of the blast’s gamma-ray emission from energies ranging from 3,000 to more than 5 billion times that of visible light …. [T]eam members showed that the blast exceeded the power of nearly 9,000 ordinary supernovae and that the gas bullets emitting the initial gamma rays must have moved at no less than 99.9999 percent the speed of light,” the release added.
Wait a minute. How could a supernova be 9,000 times as powerful as an “ordinary” supernova?
A supernova marks the death of a star more massive than our sun. Depending on its size, the star collapses into an extremely dense neutron star or a black hole. But if an ordinary supernova involves a star eight or ten times as large as the sun, what kind of star would be required to release as much energy as 9,000 of these? According to NASA, the biggest star known, the “pistol star,” is 100 times as massive as our Sol and 10 million times as bright.
Tom Matheson, who grew up in the Utah governor’s mansion and is a researcher in the field of cosmic rays, points out that the September blast may not have been much more powerful than an ordinary supernova.
Nightly News talked with Matheson — son of the late Gov. Scott M. Matheson and brother to Rep. Jim Matheson — about the claims. Matheson works in Tucson, Ariz., at the National Optical Astronomy Observatory, spending half of his time helping other scientists use the Kit Peak and Gemini telescopes, the other half researching supernovae.
In 2003, he was doing postdoctoral work at the Harvard-Smithsonian Center for Astrophysics, Cambridge, Mass., when a NASA satellite recorded a blast of gamma rays from a supernova 2 billion light-years away. He was part of the team that proved gamma-ray bursts are related to supernovae.
Speaking in a telephone interview about the September 2008 burst, he said, “This was a really fantastic burst in the sense that it was so much brighter than anything that had come before.”
It was bright. But not 9,000 times a powerful as other supernovae.
Two differing causes can be cited for its brightness. One possibility is that the blast was simply “intrinsically more energetic” than others that have been seen so far. That is the approach adopted by the accelerator center’s press release.
The other possibility is expressed in a more detailed statement from NASA’s Goddard Space Flight Center: knowing its distance, “Fermi team members showed that the blast exceeded the power of approximately 9,000 ordinary supernovae, if the energy was emitted equally in all directions. This is a standard way for astronomers to compare events even though gamma-ray bursts emit most of their energy in tight jets.”
Matheson explained it was the equivalent of 9,000 ordinary supernovae “only if you take the energy [recorded by the instruments] and average it over the whole sphere of the star.”
But theoretical models and some direct evidence show the blasts are not spread out evenly from the exploding stars. “The main idea for how these things work is, the gamma ray is produced by a jet coming out along the rotation axis of the black hole being formed,” he said. A great deal of energy is blasting out, but only in a “small cone.”
With most of the gamma ray bursters seen, the explosions are “pretty much comparable to a regular supernova.”
Staring down the barrel of one of these cosmic flamethrowers, we perceive an enormous blast. But we can’t take say the same amount of energy is coming out from every side of the flamethrower.
Changes in the light from gamma ray bursts allow scientists to estimate the angular size of the jet of material that’s rushing out. A vast quantity blasts into space, and if the jet is a small cone, “it would be incredibly bright like this one.”
To see it from Earth, “it has to be pointed right at you,” Matheson said.
“If this thing had been pointing just a very few degrees away, we never would have seen it, because it’s beamed very tightly.” Instead of seeming as powerful as 9,000 ordinary supernovae, it would have been undetectable.
It all depends on perspective.