Gamma-ray bursts

Gamma-ray bursts for a long time were an unexplained phenomenon in high-energy astrophysics. A variety of spacecraft have detected and studied these random, brief, and intense bursts of gamma rays, which come from all parts of the sky. Most theories associate them with neutron stars in the Milky Way galaxy, but an extragalactic source cannot be excluded.

Overview

Gamma-ray bursts (GRBs) constitute a unique phenomenon in astronomy. During their brief appearance, they are brighter than all other objects in the sky, including the Sun. About a hundred strong GRBs occur every year. It is unknown how many weaker bursts occur. For many years after their discovery, their source represented one of the greatest mysteries in astrophysics. They occur at random times and appear to be randomly distributed over the sky. There is no particular clustering of GRBs in any region, and until the Swift spacecraft was launched in 2004, they had not been associated with any known objects. GRBs are extremely difficult to study since it is never known when or where a GRB will occur.

It has become recognized that distinct classes of GRBs with different properties exist that entirely different objects or emission mechanisms may cause. The situation is analogous to the recognition, long after the first telescopes came into use, that not all nonstellar objects should be classified simply as nebulae since they include objects as diverse as supernova remnants, galaxies, star clusters, and planetary nebulae. Similarly, the phenomena that scientists call gamma-ray bursts will be found to be caused by more than one process or object.

The discovery of GRBs in 1972 by the Vela spacecraft was a classic serendipitous discovery—a discovery made while looking for something else. The Vela spacecraft was designed and operated to detect nuclear explosions from space. This series of small spacecraft, built by TRW and launched in the mid-1960s, contained various sensors that looked in all directions. The spacecraft was launched into high, eccentric orbits so that they could even scan the area behind the Moon for clandestine nuclear explosions.

Gamma-ray detectors aboard the Vela spacecraft were designed and built at the Los Alamos National Laboratory. They consisted of small scintillation detectors, the output of which was continuously monitored for an increased rate above the background. After several years of operation, occasional triggers were detected. Still, they were dismissed since no other sensors on board the spacecraft recorded the events and because such glitches were common to detectors on other spacecraft. Not until Los Alamos scientists began studying these triggers in greater detail did their nature become known. In many cases, two or more spacecraft would record a trigger nearly simultaneously. It was first suspected that a source of gamma rays from Earth, the Sun, or another object or region within the solar system was causing the GRBs. When precise gamma-ray-burst timing analysis was performed, it became evident that triggers were caused by a plane wave of gamma rays striking the array of widely separated spacecraft. This type of wave could be caused only by a powerful point source of gamma rays far beyond the solar system.

Los Alamos scientists announced their discovery at a meeting of the American Astronomical Society in Columbus, Ohio, in 1973 and published their findings in an astrophysical journal. Almost immediately, there was a flurry of activity to try to explain GRBs and to obtain more experimental data. As experimenters began to look through old data and data from still-operating spacecraft, many confirmed GRBs were uncovered in addition to those detected by the Vela spacecraft. Among the earlier spacecraft that confirmed the existence of GRBs were the Orbiting Solar Observatories, the Orbiting Geophysical Observatories, the Small Astronomy Satellites, the Interplanetary Monitoring Platform, Kosmos 461, Apollo 16, and the German spacecraft TD 1. It should be noted that none of these spacecraft had detectors that were specifically designed to detect GRBs. It was only because of the intensity of the bursts and their coincidence with other observations that they were detectable by instruments designed for other purposes.

By the late 1970s, a network of small detectors on interplanetary spacecraft was established in an attempt to locate the source of the GRBs more precisely. Included in this network were instruments aboard the Pioneer Venus orbiter, International Sun-Earth Explorer 3 (ISEE 3), Venera 11, 12, 13, and 14, Prognoz 7, and Helios 2. For the first time, these spacecraft provided long interplanetary baseline distances required to locate the GRBs within one arc minute. Unfortunately, with one important exception, no unusual objects were detected near the burst sources. The exception was the GRB of March 5, 1979. It occurred in or near a supernova remnant in the Large Magellanic Cloud. This burst was unusual in other respects, however, so it may have been part of a separate class of GRBs.

In collaboration with Bulgaria, Denmark, and France, the Russians launched the Granat observatory late in 1989. Outfitted with two instruments to investigate high-energy astrophysical objects from X-ray to gamma ray wavelengths, Granat lasted until 1998. However, five years into its orbital lifetime, the observatory ran out of attitude control gas, making directional surveys no longer possible. Among Granat’s most significant discoveries were the detection of electron-positron annihilation from a galactic microquasar, nineteen GRBs, and the identification of numerous objects that were candidates for black holes.

The Gamma Ray Observatory (GRO) was deployed into an independent orbit from the space shuttle Atlantis during the STS-37 mission in early April 1991. The second in the National Aeronautics and Space Administration’s (NASA’s) Great Observatory series, once in orbit, GRO was renamed the Compton Gamma Ray Observatory (CGRO or just Compton Observatory) after the Nobel Prize-winning physicist Arthur Compton, for whom an important effect involving an interaction of matter and electromagnetic radiation is also named. That interaction involves a shift in wavelength when a photon is scattered off a free electron. This effect is most pronounced in X-ray and gamma ray photons.

CGRO was left in a low, nearly circular Earth orbit (at an altitude of 450 kilometers) to keep it out of the Van Allen radiation belts. Ideally, it would have been deployed at an altitude well above those radiation zones, but it was far too heavy (17,000 kilograms) for the shuttle to put it up that high; use of an upper stage in concert with the shuttle carrying such a heavy payload was out of the realm of possibility as well.

Compton could not be repaired in orbit like the Hubble Space Telescope. CGRO’s systems began to degrade, especially threatening the loss of attitude control. It was, therefore, decided to drive the observatory into the atmosphere over a portion of the Pacific Ocean that was sparsely populated at best. Large debris that would survive reentry, such as a major portion of one of Compton’s detectors, would drop harmlessly into the ocean. CGRO was deorbited on June 4, 2000, reluctantly ending nine years of unprecedented gamma ray astrophysics research.

CGRO’s four instruments were the Burst and Transient Source Experiment (BATSE), the Oriented Scintillation Spectrometer Experiment (OSSE), the Imaging Compton Telescope (COMPTEL), and the Energetic Gamma-Ray Experiment Telescope (EGRET). Provided by NASA’s Marshall Space Flight Center, BATSE was designed to search for short GRBs ranging in energy from 20 to 600 keV and also generate full-sky surveys for long-duration gamma ray sources. Provided by the Naval Research Laboratory, OSSE was outfitted with four individually pointing detectors capable of picking up radiation from 0.05 to 10 mega-electron volts (MeV). A pair of these detectors would record emission from a source while the other two would record the background near those sources for contrast. COMPTEL was provided by a collaboration of the University of New Hampshire, the Netherlands Institute for Space Research, the Max Planck Institute, and the European Space Agency’s (ESA’s) Astrophysics Division. COMPTEL was designed with a wide field of view and sensors capable of identifying sources (in an energy range of 0.75 to 30 MeV) to within one degree. EGRET was provided by a collaboration of NASA’s Goddard Space Flight Center, Stanford University, and the Max Planck Institute. This was the highest energy-detecting portion of CGRO, capable of recording emissions in the range of 20 MeV to 30 GeV and identifying the location of incoming radiation to within a fraction of a degree. That level of resolution was useful in having other satellites precisely locate sources picked up by EGRET.

Compton could not be repaired in orbit like the Hubble Space Telescope. CGRO’s systems began to degrade, especially threatening the loss of attitude control. It was, therefore, decided to drive the observatory into the atmosphere over a portion of the Pacific Ocean that was sparsely populated at best. Large debris that would survive reentry, such as a major portion of one of Compton’s detectors, would drop harmlessly into the ocean. CGRO was deorbited on June 4, 2000, reluctantly ending nine years of unprecedented gamma ray astrophysics research.

The Swift Gamma-Ray Burst spacecraft was launched by a Delta II booster on November 20, 2004, and placed in an orbit 600 kilometers above the Earth’s surface. Swift was designed to provide the best all-sky survey of gamma rays yet, to provide alerts to transient astrophysical events, such as supernovae and GRBs of both short and long duration, to help identify the location of GRBs, and to assist in determining the distances to gamma ray bursters at cosmological distances representing a time early in the universe’s evolution. Swift was outfitted with just three instruments: the Burst Alert Telescope (BAT), X-ray Telescope (XRT), and UltraViolet/Optical Telescope (UVDT). The most important aspect of Swift’s capability was the rapidity with which it could respond to a gamma ray detector and precisely locate its source so that these three instruments and other assets available worldwide to the astronomy community could record and study the afterglow of a GRB. Within about fifteen seconds on average, Swift could identify the source of gamma rays to within approximately one arc minute of the sky. The Swift Mission Operation Center, located on the campus of Pennsylvania State University, serves as a clearinghouse alerting other astronomers around the world to GRB events.

A Delta II booster launched NASA’s Gamma-Ray Large Area Space Telescope (GLAST) on June 11, 2008. GLAST involved an international team of space agencies and research groups from the United States, France, Germany, Italy, Sweden, and Japan. GLAST was designed as a follow-on gamma ray astronomical observatory to the lost Compton GRO; however, it was not considered a member of NASA’s Great Observatory program. It was intended to investigate cosmological questions raised by Compton's observations and energies far above what can be produced in particle accelerators on Earth. Scientists expected to use GLAST to understand better black holes, neutron stars, and high-speed gas and how they produce gamma radiation.

Knowledge Gained

Since GRBs have not been identified with known objects, their distance is highly uncertain. This, in turn, makes it difficult to speculate on their origin. Since the distance to the burst sources is unknown, the source's intrinsic luminosity is even more uncertain. Many of the early theories of GRBs posited exotic phenomena or objects to explain them. In later years, most models have associated GRBs with explosive events near, or at the surfaces of, neutron stars within the Milky Way galaxy. These explosions could be caused by thermonuclear reactions resulting from the collision of interstellar material, comets, or asteroids with neutron stars or from the annihilation of strong magnetic fields near such stars. Another theory attributes GRBs to a sudden shift of the solid crust that is thought to be present in neutron stars. There are also models of GRBs that attribute them to enormous explosions occurring at cosmological distances or distances near the edge of the observable universe. At these distances, the luminosity of a GRB would be equivalent to that of a supernova, although all its energy would be emitted at gamma ray wavelengths and within the duration of a GRB.

The three observable properties of GRBs that are most often studied are their time histories, their energy spectra, and the statistical properties of their intensity and distribution over the sky. Attempts to locate a GRB and identify it with a known object have thus far been unsuccessful. Very sensitive optical, radio, and X-ray searches have been made of precisely located gamma-ray-burst error boxes (the region of uncertainty in the position of a celestial source). These searches have been either inconclusive or controversial. A search of old photographic plates from telescopes in the Southern Hemisphere, however, has shown two or three transient, starlike optical images at the locations of GRBs. The authenticity and significance of these observations are still being debated.

The time history of a GRB refers to the intensity variations of the burst as a function of time. Some GRBs show extremely rapid fluctuations over their entire duration, which may encompass a minute or two. Other bursts last a few seconds, and only smooth variations are seen. Still, others exhibit a single spike lasting only a fraction of a second. The rapid variations indicate that the source of a GRB is a tiny region or compact object, such as a neutron star or a black hole. The GRB of March 5, 1979, was unique in that it had a single, intense spike followed by a lower-level emission with eight seconds lasting more than two hundred seconds.

The spectra of GRBs indicate that the sources contain regions of extremely high temperatures—perhaps the highest in the universe. In many cases, the gamma ray energies extend up to 100 MeV. Extremely rapid variations are observed in the spectra of most GRBs. Additionally, gamma ray line features may be explained by the gamma rays from regions with extremely high magnetic fields, such as those expected near neutron stars. There is also a class of GRBs that have softer spectra or lower temperatures and are observed to be repetitive. Soviet researchers also reported gamma ray line features near 400 keV, which some interpreted as caused by the destruction and redshift of electron-positron plasma near the surface of a neutron star.

Granat began working in a survey mode in late 1994. Before the demise of this Russian observatory, data from Granat detailed spectral and temporal variability of potential black holes, discovered the specific radiation emerging from electron-positron annihilation from the X-ray nova Muscae and a galactic micro-quasar, and improved imaging of the Milky Way’s galactic center.

In February 1997, telescopes sighted the source of a burst, a diffuse, elongated object with a bright core. Astronomers thought the object might be a very distant galaxy, but they could not be certain without more data.

Compton provided a means to differentiate GRBs of short duration from longer-duration bursts. Its instruments picked up the first known gamma ray repeaters. Its instruments picked up the first known gamma ray repeaters. Over its lifetime, BATSE picked up at least one event per day, giving it a total of 2,700 gamma ray detections. OSSE was used to survey the Milky Way’s center and provided evidence suggesting that a cloud of antimatter existed exterior to the central region. COMPTEL observed gamma rays originating from the decay of the radioactive isotope aluminum 26 (²⁶Al) and generated a full-sky survey of that emission. EGRET’s full-sky survey of high-energy gamma ray emissions picked up 271 sources. However, only 100 of them were definitively identified. One surprise was the detection of terrestrial gamma rays originating from thunderstorms. CGRO instruments surveyed both pulsars and supernova remnants.

CGRO provided an enormous amount of data, and its instruments were still working at the time of its demise. Its data helped astrophysicists better characterize the nature of GRBs and provided the insight needed to design even better observations to follow, such as the Swift spacecraft and GLAST telescope.

The Swift Observatory represented a major advance in the study of GRBs. Outfitted with detectors capable of observing from the visible through gamma ray wavelengths, Swift was designed to scan the sky continuously for the signature of a GRBer. It could slew quickly and locate the source of the burst. At this point, the worldwide astronomical community would be alerted to use space-based and ground-based observatories to quickly measure and record the burster’s afterglow. Although there were some anomalies with Swift’s XRT instrument, the observatory was commissioned on February 1, 2005. It had detected an initial burster earlier on January 17, 2005, and was quickly able to identify the bright source in its field of view, thereby triggering an alert for further study by other observatories, such as the Chandra X-Ray Observatory. During its first year of operation alone, Swift found 90 GRBs. Swift was the first to detect the location of a short-duration GRB. GRB 050509b, observed by Swift on May 9, 2005, had a burst duration of merely fifty milliseconds; this demonstrated the rapid response time of Swift.

On September 4, 2005, Swift discovered the most distant known GRB, GRB 050904, located 12.6 billion light-years distant. In 2006, Swift identified the location of GRB 060614 to be 1.6 billion light-years distant. This burster lasted 102 seconds and had a signature indicating it most likely resulted from the formation of a black hole. Another great discovery came on January 9, 2008. While Swift was studying a supernova in NGC 2770, it detected an X-ray burst in the very same galaxy. This triggered coordinated studies with Chandra, the Very Large Array, the Keck I telescope, the 200- and 60-inch telescopes at Palomar, and the Hubble Space Telescope, making this perhaps the most intensely studied supernova at a very early point in its development, a study that was performed with instruments across the electromagnetic spectrum. On March 19, 2008, Swift detected four GRBs, a record for one day’s observations. Even more important, the second of the four discovered that day, GRB 080319B, turned out to be the brightest celestial object ever detected. Located 7.5 billion light-years from Earth, this GRB was 2.5 million times brighter than any other supernova.

Among the scientific objectives of GLAST research were the examination of high-energy astrophysical objects at energies greater than could be duplicated on Earth in laboratories; the search for sources of dark matter to illuminate the identity and physics of such exotic matter; investigation of gamma-ray bursters; investigation of the mechanisms whereby black holes produce jets and accelerate the material in them to speeds very close to that of light; and investigation of solar flares, cosmic rays, and pulsars at high energy.

In 2022, a gamma-ray burst occurred that was so large scientists called it the brightest of all time (BOAT). Later called GRB 221009A, the burst was said to likely only occur every 10,000 years.

Context

The field of high-energy astrophysics is a product of the space age. It is necessary to carry instruments and telescopes above Earth’s atmosphere to observe the universe at X-ray and gamma ray wavelengths. This relatively new branch of astronomy has taught scientists more about objects they already knew existed and revealed new types of objects and phenomena, including X-ray stars, black holes, and GRBs. These objects are among the most energetic and violent in the universe. Most of them are associated with the final stages of massive stars' life cycles.

GRBs once represented the most significant unsolved problem in high-energy astrophysics. Their distance and luminosity were unknown for a long time. They are challenging to study because of their random and transient nature. Although groups in the United States made the initial discovery and studies of GRBs, the Soviets, often with French collaborators, also obtained much of the gamma-ray-burst data. Establishing an international gamma-ray-burst observation network, combining data from as many as nine spacecraft, became a model for international collaboration in space exploration. It is expected that the continued study of GRBs will help astrophysicists understand these objects and enable scientists to study conditions of extreme temperature, pressure, and density that are unavailable anywhere else.

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