Detectable for only a few seconds but possessing enormous energy, gamma-ray bursts are notoriously difficult to capture as their energy does not penetrate the Earth’s atmosphere.
Fortunately, the recently launched Fermi Gamma-Ray Space Telescope is helping astrophysicists fill in the unknowns surrounding such bursts.
“[The orbiting] Fermi is lucky to measure the highest energy portion of the gamma-ray burst emission, which last for hundreds to thousands of seconds – maybe 20 minutes,” explained Péter Mészáros, Eberly Chair Professor of Astronomy and Astrophysics and Physics, Penn State.
As Mészáros notes, most gamma-ray bursts occur when stars that are more than 25 times larger than our sun come to the end of their lives. When the internal nuclear reaction in these stars ends, the star collapses in on itself and forms a black hole. The outer envelope of the star is subsequently ejected, forming a supernova.
“The black hole is rotating rapidly and as it is swallowing the matter from the star, the rotation ejects a jet of material through the supernova envelope,” said Mészáros.
Essentially, this jet causes the gamma-ray burst, which briefly becomes the brightest thing in the sky. Unlike supernovas that radiate in all directions, gamma-ray bursts radiate in a very narrow area, and Fermi sees only jets ejecting in its direction. This, however, is the direction in which they send their highest energy photons. Any gamma-ray bursts on the other side of the black hole or even off at an angle are invisible to the telescope.
“We actually miss about 500 gamma-ray bursts for every one we detect,” Mészáros confirmed.
Nevertheless, the gamma-ray bursts Fermi observed are already helping astrophysicists clarify previous theories about the phenomena.
“We have been able to rule out the simplest version of theories which combine quantum mechanics with gravity, although others remain to be tested,” said Mészáros.
Mészáros also pointed out that Fermi and other programs like the SWIFT telescope have demonstrated gamma-ray bursts last longer than originally believed, and consist of both long and short gamma-ray bursts.
“Fermi has done much better in measuring how close to the speed of light the jet gets,” he acknowledged. “[Yet], we still don’t know if it is 99.9995 percent the speed of light or 99.99995 percent the speed of light.”
Gamma-ray bursts occur in many places in the universe, but because they are a product of aging stars they may be able to help astronomers understand the origins of the universe. The bursts are visible at the longest distance from earth and therefore at the earliest time in the universe.
Wherever a gamma-ray burst exists, any planets in the vicinity suffer. Further away, the radiation from a gamma-ray burst would destroy the protective ozone in the upper atmosphere, allowing ultraviolet radiation to kill terrestrial plant life and animals would starve. Only sea life would remain unharmed. However, it is estimated that such nearby bursts can be expected only every 300 million years.
Because scientists believe that gamma-ray bursts also emit cosmic rays and neutrinos, additional observatories are also observing these phenomena. For example, Ice Cube Neutrino Observatory at the South Pole is attempting to capture neutrinos, while the Pierre Auger Cosmic Ray Observatory in Argentina captures cosmic rays from these objects.
