Gamma-Ray Burst - Introduction
Gamma-Ray Bursts
What causes gamma-ray bursts? The first burst was detected nearly 50 years ago and the mystery that surrounds their origin continues to exist. We do know that gamma-ray bursts are the most energetic events to occur in the Universe!
In order to understand what a gamma-ray burst (or GRB) is, you must first realize that gamma-rays are a type of light. In fact, gamma-rays are the most energetic form of light known. Light is a form of energy called electromagnetic radiation. Electromagnetic radiation comes in tiny packets of energy called photons. Photons come in a wide range of energies. Electromagnetic radiation can be placed in an arrangement according to the energy amount of the photons. This orderly arrangement is known as the electromagnetic spectrum.
At the low-energy end of the spectrum we find radio waves. They have a very long wavelength. At the high-energy end of the spectrum we find gamma-rays. They possess a very short wavelength. For electromagnetic waves, the relationship between wavelength and energy is an inverse relationship. The shorter the wavelength, the greater the energy; the longer the wavelength, the less the energy. Humans cannot see the light forms at the low and high-energy ends of the spectrum. We can only see light that falls in the visible range of the spectrum. Visible light is in the middle of the spectrum and accounts for a very small percentage of the energy range on the whole spectrum.
If an astronomer were to study the Universe only in the visible range of the spectrum, the large majority of events would go unobserved. Cosmological events such as star birth and star death emit photons that occur across the entire electromagnetic spectrum. Thanks to considerable technological advances, astronomers now have the ability to view the Universe in radio waves, gamma-rays, and all energies in between. Distant quasars were first discovered by the radio waves they emit. Galactic dust can be observed in the infrared range while light from ordinary stars such as the Sun can be observed in the visible and ultraviolet range. Extremely hot gas can be observed by the X-rays that it emits. Observations in the gamma-ray range of the spectrum reveal a very energetic Universe. Such energetic phenomena as a blazar (which consist of a supermassive black hole with jets of particles blasting away from near the event horizon), solar flares, and the radioactive decay of atomic nuclei created in supernova explosions all produce gamma-rays.
So what exactly 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 (remember: gamma-rays are not in the visible range of the electromagnetic spectrum so we consequently are not aware of the phenomena). The brightness of this flash of gamma-rays can temporarily overwhelm all other gamma-ray sources in the Universe. Gamma-ray bursts can release more energy in 10 seconds than the Sun will emit in its entire 10 billion-year lifetime. 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.
When this booklet was first published in 2000, the exact cause of these flashes was unknown. At the time, astronomers had determined that the observed bursts came from outside the Milky Way Galaxy, and they believed that a gamma-ray burst would occur about once every few million years here in the Milky Way.
The first gamma-ray bursts were detected while scientists were looking for violations of the Nuclear Test Ban Treaty during the Cold War Era of the 1960s. Several satellites employed to monitor treaty compliance detected a large increase in the number of gamma-rays they counted each second. It was determined that the gamma-rays were coming from outer space and not from a nuclear bomb exploding in the Earth’s atmosphere. Although Ray Klebesadel and his colleagues at the Los Alamos National Laboratory in New Mexico found these bursts in data going back to 1967, their discovery was not reported to the world until 1973.
Satellites, such as NASA’s Compton Gamma-Ray Observatory and Hubble Space Telescope, and ESA’s BeppoSAX, gave us valuable data in our quest to solve the mystery of GRBs. Those satellites had limitations, however. One limitation was that once a burst was detected, it took too long to reposition the satellite in order to face the burst and collect data. The satellites were also limited as to the range of the electromagnetic spectrum in which they can make observations. In 1999, scientists were able to observe an optical counterpart to a burst as the burst was occurring, which occurred only through 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 Compton Gamma-Ray Observatory, BeppoSAX, the Internet, and a special robotic ground-based telescope, scientists were able to monitor the burst from start to finish at multiple wavelengths. It had the optical brightness of 10 million billion Suns, which was only one-thousandth of its gamma-ray brightness!
A few leading theories were developed that addressed the possible causes of gamma-ray bursts. One explanation proposed that they are the result of colliding neutron stars. Neutron stars are the corpses of massive stars (5 to 10 times the mass of our Sun) that have come to the ends of their life cycles. They are extremely dense. Although their diameter may only be 20 kilometers, their mass is about 1.4 times that of the Sun. A second theory proposed 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 third theory stated 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.
Swift, a satellite with the capacity to study the Universe in a multitude of wavelengths, was launched in 2004. The satellite is aptly named because once a burst is detected, it can be repositioned to face the gamma ray source within 50 seconds. Through simultaneous observations of the burst in the optical, ultraviolet, X-ray, and gamma-ray ranges of the electromagnetic spectrum, scientists began to answer the many questions surrounding gamma-ray bursts. In 2008, the Fermi Gamma-Ray Space Telescope was launched and provided scientists with additional insight into the gamma-ray burst mystery.
Until recently, gamma-ray bursts could arguably have been called the biggest mystery in high-energy astronomy. Today, however, evidence from recent satellites like Swift and Fermi indicate that the energy behind a gamma-ray burst comes from the collapse of matter into a black hole. However, the type of collapse depends on the type of gamma-ray burst.
When astronomers looked at the number of bursts versus how long they lasted, they found two different classes of bursts: long-duration and short-duration. These two classes are likely created by different processes, but the end result in both cases is a brand new black hole.
Graph of the time versus number of bursts for the gamma-ray bursts observed by the BATSE instrument on the Compton Gamma-ray Telescope.
Long-duration bursts last anywhere from 2 seconds to a few hundreds of seconds (several minutes), with an average time of about 30 seconds. They are associated with the deaths of massive stars in supernovas; though not every supernova produces a gamma-ray burst.
Short duration bursts are those that last less then 2 seconds; lasting anywhere from a few milliseconds to 2 seconds with an average duration of about 0.3 seconds (or 300 milliseconds). These bursts appear to be associated with the merger of two neutron stars into a new black hole or a neutron star with a black hole to form a larger black hole.
This modern science mystery that plagued scientist for the past 50 years is now nearly solved.
