Gamma Ray Burst

A Gamma Ray Burst is an outburst that radiates tremendous amounts of energy, equal to or greater than a supernova, in the form of gamma rays and X-rays. These bursts take place in less than a few minutes. The gamma-ray burster, as the intriguing phenomenon is sometimes called, represents one of the greatest astronomical mysteries.

Starting in the late 1960s, astronomers observing the heavens at very short wavelengths began detecting incredibly brief and intense bursts of gamma rays from seemingly random locations in the sky. They observed that a few times a day, the sky lights up with an incredible flash, or burst, of gamma rays. Often the burst outshines all the other sources of cosmic radiation added together. The source of the burst then disappears completely. Even more puzzling, no one was able to predict when the next burst would occur or from what direction in the sky it would come.

Following the discovery of the gamma-ray burst in the late 1960s, astronomers used the best available space-based gamma-ray detection instruments to construct a catalog of GRBs. As the number of observed GRBs grew, many theories emerged within the international astronomical community concerning their origin. Scientists also argued among themselves as to whether the GRBs were taking place in the Milky Way galaxy or in other galaxies. Unfortunately, the addition of each newly observed GRB tended to reveal little more than the fact that GRBs never repeated from the same cosmic source or location.

Then NASA’s Compton Gamma-Ray Observatory (CGRO), launched in 1991, provided a wealth of new GRB observations. The spacecraft’s burst and transient source experiment (BATSE) was capable of monitoring the sky with unprecedented sensitivity. One thing soon became clear as the catalog of GRBs observed by BATSE grew - the GRBs were in no way correlated with sources in the Milky Way galaxy. In 1997 the Italian-Dutch BeppoSAX satellite made a breakthrough in our basic understanding of gamma-ray bursts. Using a highly effective combination of gamma-ray and X-ray telescopes, BeppoSAX detected afterglows from a few GRBs and precisely located the sources in a way that allowed other telescopes (sensitive to different portions of the electromagnetic spectrum) to promptly study the same location in the sky. This effort showed astronomers that GRBs are produced in very distant galaxies, requiring that the explosions producing the gamma-ray bursts must be incredibly powerful.

The next important breakthrough in GRB understanding occurred on January 23, 1999, when an enormously powerful event (called GRB990123) was detected. Alerted by sophisticated space-based detectors, astronomers were able to quickly observe this event at an unprecedented range of wavelengths and timing sensitivities. The event was very far away. If astronomers assumed isotropic emission of the gamma rays, this powerful GRB would have involved the release of the energy equivalent of two times the rest mass energy of a neutron star. If, on the other hand, astronomers assumed that the emitted energy was being beamed out of this distant GRB in a preferred direction, then the required energies became more reasonable and easier to explain. Today astronomers tenuously suggest that gamma-ray bursts are produced by materials shooting toward Earth at nearly the speed of light. The material in question is ejected during the collision of two neutron stars or black holes. An alternate speculation is that the GRBs arise from a hypernova - the huge explosion hypothesized to occur when a supermassive star ends its life and collapses into a black hole. However, astronomers also recognize that many more multi-wavelength, prompt observations of future GRBs will be required before they can confidently model the central engine that produces these powerful and mysterious gamma-ray bursts.

Questions to Ponder

• What could be the mass of the Super massive stars that cause gamma ray bursts?
• Exactly how energetic are Gamma-ray Bursts?
• What is "hypernova"?
• What is the danger of a close GRB to Earth?