The history of gamma-ray burst research began with the serendipitous detection of a gamma-ray burst (GRB) on July 2, 1967, by the U.S. Vela satellites. After these satellites detected fifteen other GRBs, Ray Klebesadel of the Los Alamos National Laboratory published the first paper on the subject, Observations of Gamma-Ray Bursts of Cosmic Origin. Hundreds of models were developed in an attempt to explain GRB origins.
Discovery Gamma-ray bursts were discovered in the late 1960s by the U.S. Vela nuclear test detection satellites. The Velas were built to detect gamma radiation pulses emitted by nuclear weapon tests in space. The United States suspected that the USSR might attempt to conduct secret nuclear tests after signing the Nuclear Test Ban Treaty in 1963. While most satellites orbited at about 500 miles above Earth's surface, the Vela satellites orbited at an altitude of 65,000 miles. At this height, the satellites orbited above the Van Allen radiation belt, which reduced the noise in the sensors. The extra height also meant that the satellites could detect explosions behind the Moon, a location where the United States government suspected the Soviet Union would try to conceal nuclear weapon tests. The Vela system generally had four satellites operational at any given time such that a gamma-ray signal could be detected at multiple locations. This made it possible to localize the source of the signal to a relatively compact region of space. While these characteristics were incorporated into the Vela system to improve the detection of nuclear weapons, these same characteristics were what made the satellites capable of detecting gamma-ray bursts.Katz 2002, p. 4–5
On July 2, 1967, at 14:19 UTC, the Vela 4 and Vela 3 satellites detected a flash of gamma radiation that were unlike any known nuclear weapons signatures.Schilling 2002, p.12–16 Nuclear bombs produce a very brief, intense burst of gamma rays less than one millionth of a second. The radiation then steadily fades as the unstable nuclei decay. The signal detected by the Vela satellites had neither the intense initial flash nor the gradual fading, but instead there were two distinct peaks in the light curve. Solar flares and new supernovas were the two other possible explanations for the event, but neither had occurred on that day. Unclear on what had happened, but not considering the matter particularly urgent, the team at the Los Alamos Scientific Laboratory, led by Ray Klebesadel, filed the data away for later investigation.
Vela 5 was launched on May 23, 1969. Because the sensitivity and time resolution on these satellites were significantly more accurate than the instruments on Vela 4, the Los Alamos team expected these new satellites to detect more gamma-ray bursts. Despite an enormous amount of background signals picked up by the new detectors, the research team found twelve events which had not coincided with any solar flares or supernovas. Some of the new detections also showed the same double-peak pattern that had been observed by Vela 4.
Although their instrumentation offered no improvement over those on Vela 5, the Vela 6 satellites were launched on April 8, 1970, with the intention of determining the direction from which the gamma rays were arriving. The orbits for the Vela 6 satellites were chosen to be as far away from Vela 5 as possible, generally on the order of 10000 kilometers apart. This separation meant that, despite gamma rays traveling at the speed of light, a signal would be detected at slightly different times by different satellites. By analyzing the arrival times, Klebesadel and his team successfully traced sixteen gamma-ray bursts. The random distribution of bursts across the sky made it clear that the bursts were not coming from the Sun, Moon, or other planets in the Solar System.
In 1973, Ray Klebesadel, Roy Olson, and Ian Strong of the University of California Los Alamos Scientific Laboratory published Observations of Gamma-Ray Bursts of Cosmic Origin, identifying a cosmic source for the previously unexplained observations of gamma-rays. Shortly thereafter, Klebesadel presented his findings at the 140th meeting of the American Astronomical Society. Although he was interviewed only by The National Enquirer, news of the discovery quickly spread through the scientific community.Schilling 2002, p.16–17 Between 1973 and 2001 more than 5300 papers were published on GRBs.Hurley 2003
Early research missions Shortly after the discovery of gamma-ray bursts, a general consensus arose within the astronomical community that in order to determine what caused them, they would have to be identified with astronomical objects at other wavelengths, particularly visible light, as this approach had been successfully applied to the fields of radio X-ray astronomy. This method would require far more accurate positions of several gamma-ray bursts than the Vela system could provide.Katz 2002, p. 19 Greater accuracy required the detectors to be spaced farther apart. Instead of launching satellites only into Earth's orbit, it was deemed necessary to spread the detectors throughout the Solar System.
By the end of 1978, the first Inter-Planetary Network (IPN) had been completed. In addition to the Vela satellites, the IPN included 5 new space probes: the Soviet Prognoz 7, in orbit around the Earth, the German Helios 2, in elliptical orbit around the Sun, and NASA's Pioneer Venus Orbiter, Venera 11, and Venera 12, each of which orbited Venus. The research team at the Russian Institute for Space Research in Moscow, led by Kevin Hurley, was able to use the data collected by the IPN to accurately determine the position of gamma-ray bursts with an accuracy of a few minutes of arc. However, even when using the most powerful telescopes available, nothing of interest could be found within the determined regions.Schilling 2002, p. 19–20
To explain the existence of gamma-ray bursts, many speculative theories were advanced, most of which posited nearby galactic sources. Little progress was made, however, until the 1991 launch of the Compton Gamma Ray Observatory and its Burst and Transient Source Explorer (BATSE) instrument, an extremely sensitive gamma-ray detector. This instrument provided crucial data indicating that GRBs are isotropic (not biased towards any particular direction in space, such as toward the galactic plane or the Galactic Center).Meegan 1992 Because the Milky Way galaxy has a very flat structure, if gamma-ray bursts were to originate from within the Milky Way, they would not be distributed isotropically across the sky, but instead concentrated in the plane of the Milky Way. Although the luminosity of the bursts suggested that they had to be originating within the Milky Way, the distribution provided very strong evidence to the contrary.Schilling 2002, p.36–37Paczyński 1999, p. 6
BATSE data also showed that GRBs fall into two distinct categories: short-duration, hard-spectrum bursts ("short bursts"), and long-duration, soft-spectrum bursts ("long bursts").Kouveliotou 1993 Short bursts are typically less than two seconds in duration and are dominated by higher-energy photons; long bursts are typically more than two seconds in duration and dominated by lower-energy photons. The separation is not absolute and the populations overlap observationally, but the distinction suggests two different classes of progenitors. However, some believe there is a third type of GRBs.Mukherjee 1998Horvath 1998Hakkila 2003Horvath 2006 The three kinds of GRBs are hypothesized to reflect three different origins: mergers of neutron star systems, mergers between white dwarfs and neutron stars, and the collapse of massive stars.Chattopadhyay 2007
For decades after the discovery of GRBs, astronomers searched for a counterpart: any astronomical object in positional coincidence with a recently observed burst. Astronomers considered many distinct objects, including white dwarfs, pulsars, supernovae, globular clusters, quasars, Seyfert galaxies, and BL Lac objects.Liang 1986, p. 33 Researchers specifically looked for objects with unusual properties which might relate to gamma-ray bursts: high proper motion, polarization, orbital brightness modulation, fast time scale flickering, extreme colors, emission lines, or an unusual shape.Liang 1986, p. 39 From the discovery of GRBs through the 1980s, GRB 790305bGRBs are named after the date on which they are discovered: the first two digits being the year, followed by the two-digit month and two-digit day, then a letter corresponding to the order upon which it was detected (A for first of that day, B for second, and so on). Before 2010 this was only signified if two or more GRBs were detected on a given day. was the only event to have been identified with a candidate source object: nebula N49 in the Large Magellanic Cloud.Schilling 2002, p. 20 All other attempts failed due to poor resolution of the available detectors. The best hope seemed to lie in finding a fainter, fading, longer wavelength emission after the burst itself, the "afterglow" of a GRB.Fishman 1995
As early as 1980, a research group headed by Livio Scarsi at the University of Rome began working on Satellite per Astronomia X, an X-ray astronomy research satellite. The project developed into a collaboration between the Italian Space Agency and the Netherlands Agency for Aerospace Programmes. Though the satellite was originally intended to serve the sole purpose of studying X-rays, Enrico Costa of the Istituto di Astrofisica Spaziale suggested that the satellite's four protective shields could easily serve as gamma-ray burst detectors.Schilling 2002, p. 58–60 After 10 years of delays and a final cost of approximately $350 million,Schilling 2002, p. 63 the satellite, renamed BeppoSAX in honor of Giuseppe Occhialini,Schilling 2002, p. 65 was launched on April 30, 1996.Schilling 2002, p. 67
In 1983, a team composed of Stan Woosley, Don Lamb, Ed Fenimore, Kevin Hurley, and George Ricker began discussing plans for a new GRB research satellite, the High Energy Transient Explorer (HETE).Schilling 2002, p. 62–63 Although many satellites were already providing data on GRBs, HETE would be the first satellite devoted entirely to GRB research.Schilling 2002, p. 56 The goal was for HETE to be able to localize gamma-ray bursts with much greater accuracy than the BATSE detectors. The team submitted a proposal to NASA in 1986 under which the satellite would be equipped with four gamma ray detectors, an X-ray camera, and four electronic cameras for detecting visible and ultraviolet light. The project was to cost $14.5 million, and the launch was originally planned for the summer of 1994. The Pegasus XL rocket, which launched HETE on November 4, 1996, did not release its two satellites, so the HETE and SAC-B, an Argentinian research satellite also on board, missions were attached to the rocket and unable to direct their solar panels towards the sun, and within one day of the launch, all radio contact with the satellites was lost.Schilling 2002, p. 69–70 The eventual successor to the mission, HETE 2, was successfully launched on 9 October 2000. It observed its first GRB on 13 February 2001.Schilling 2002, p. 252–253
Observations and analysis