StarChild Question of the Month for July 2002

Question:

Can you tell me about gamma-ray bursts?

Answer:

What is a gamma-ray burst?

At least once a day, the sky lights up with a spectacular flash of gamma-rays coming from deep space. The brightness of this flash of gamma-rays can temporarily overwhelm all other gamma-ray sources in the universe. The burst can last from a fraction of a second to over a thousand seconds. The time that the burst occurs and the direction from which it will come cannot be predicted. Gamma-ray bursts (GRBs) can release more energy in 10 seconds than the Sun will emit in its entire 10 billion-year lifetime!

In order to understand what a gamma-ray burst is, you must recognize gamma-rays as the most energetic form of light. Light, or electromagnetic radiation, can be thought of as coming in tiny packets of energy called photons. These photons come with a wide range of energies. At the low-energy end, we find radio waves. At the high-energy end, we find gamma-rays. The human eye is blind to nearly the entire electromagnetic spectrum, except for the very narrow range light that falls in what we call the "visible" (or "optical") range. If an astronomer were to study the universe only in the visible range of the spectrum, the large majority of cosmic events would go unobserved. Events such as star birth and star death emit photons that occur across the entire electromagnetic spectrum. Within the past 30 years, astronomers have developed the ability to view the universe in radio waves, gamma-rays, and all energies in between. This ability has allowed us to discover amazing events in our universe such as GRBs.

What causes gamma-ray bursts?

The first gamma-ray bursts were detected while scientists were using satellites to look for gamma-rays that would result from violations of the Nuclear Test Ban Treaty during the Cold War Era of the 1960s. Gamma-rays were found, but the gamma-rays were coming from outer space and not from a nuclear bomb exploding in the Earth's atmosphere.

There are several current theories about the possible causes of gamma-ray bursts. One explanation proposes that they are the result of colliding neutron stars -- corpses of massive stars (5 to 10 times the mass of our Sun) that have blown up as supernovae. A second theory proposes that gamma-ray bursts are the result of a merging between a neutron star and a black hole or between two black holes. Black holes result when supermassive (greater than 20 times the mass of our Sun) stars die. A theory that is attracting considerable attention states that gamma-ray bursts occur as the result of material shooting towards Earth at almost the speed of light as the result of a hypernova. A hypernova explosion can occur when the largest of the supermassive stars come to the end of their lives and collapse to form black holes. Hypernova explosions can be at least 100 times more powerful than supernova explosions.

One of the greatest difficulties in finding gamma ray bursts is that they are so short-lived. Once a burst is detected, it takes too long to rotate a satellite to face the burst and collect data. Recently, scientists were able to observe the visible light from a burst as the burst was occurring. This extraordinary event occurred as the result of a great deal of planning, cooperation, and luck. On January 23, 1999, a network of scientists was notified within 4 seconds of the start of a burst that a burst was in progress. Thanks to the coordinated efforts of scientists and observatories all around the world, data were collected on the burst from start to finish in several different energy ranges of the electromagnetic spectrum. It had the optical brightness of 10 million billion Suns, which was only one-thousandth of its gamma-ray brightness!

The future looks good for solving the mystery of GRBs. Swift, a NASA satellite with the capacity to study the universe in a multitude of wavelengths, is expected to launch 2003. The satellite is aptly named because, once a burst is detected, the satellite can be repositioned to face the gamma-ray source within 50 seconds. By observing the burst in the optical, ultraviolet, X-ray, and gamma-ray ranges of the electromagnetic spectrum, scientists hope to finally be able to answer the many questions surrounding gamma-ray bursts.

The StarChild site is a service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Alan Smale (Director), within the Astrophysics Science Division (ASD) at NASA/ GSFC.