Gamma-ray bursts

Gamma-ray bursts, or GRBs, are the brightest and most powerful explosions known, occurring in far-off galaxies. In terms of energy and luminosity, only the Big Bang outshines these extreme electromagnetic events. Their duration ranges from a few milliseconds to several hours. Following the initial gamma-ray flash, a longer-lasting afterglow appears, typically at longer wavelengths like X-ray, ultraviolet, optical, infrared, microwave, or radio.

Most observed GRBs are thought to release their intense radiation during a supernova or superluminous supernova, when a high-mass star collapses into a neutron star or black hole. A subtype, short-duration GRBs, come from the cataclysmic merger of binary neutron stars. Because their sources are billions of light-years away, these explosions are both incredibly energetic—a typical burst releases as much energy in seconds as the Sun will over its entire 10-billion-year lifetime—and extremely rare, occurring only a few times per galaxy per million years. Every GRB ever recorded originated outside the Milky Way, though a related phenomenon, soft gamma repeaters, is linked to magnetars within our galaxy. A GRB in the Milky Way aimed directly at Earth could sterilize the planet or cause a mass extinction; some researchers have proposed that the Late Ordovician mass extinction resulted from such an event.

GRB signals were first detected in 1967 by the Vela satellites, built to spot covert nuclear weapons tests. After an exhaustive analysis, the findings were published as academic research in 1973. Following their discovery, hundreds of theoretical models were proposed—including collisions between comets and neutron stars—but little data existed to verify them until 1997. That year, the first X-ray and optical afterglows were detected, and their redshifts were measured directly via optical spectroscopy, revealing their distances and energy outputs. These discoveries, along with subsequent studies of the associated galaxies and supernovae, confirmed that GRBs occur in distant galaxies.

The first observations came in the late 1960s from U.S. Vela satellites, designed to detect gamma radiation from space-based nuclear tests, as the U.S. suspected the Soviet Union might violate the 1963 Nuclear Test Ban Treaty. On July 2, 1967, Vela 4 and Vela 3 recorded a gamma-ray flash unlike any known nuclear weapon signature. The Los Alamos team, led by Ray Klebesadel, filed the data away without urgency. As more Vela satellites launched with better instruments, the team kept finding unexplained bursts. By comparing arrival times across satellites, they estimated sky positions for 16 bursts and ruled out a terrestrial or solar origin. The data was never classified, and after thorough analysis, the results were published in 1973. Most early theories placed GRB sources within the Milky Way. Starting in 1991, the Compton Gamma Ray Observatory and its BATSE instrument showed that GRB distribution is isotropic—not biased toward any direction. If sources were in our galaxy, they would cluster near the galactic plane; their absence there proved GRBs come from beyond the Milky Way, though some galactic models remain consistent with an isotropic distribution.

For decades after discovery, astronomers searched for counterparts at other wavelengths—any object coinciding with a burst’s position. Candidates included white dwarfs, pulsars, supernovae, globular clusters, quasars, Seyfert galaxies, and BL Lac objects. All searches failed; even well-localized bursts showed no bright objects matching the satellite-derived positions, suggesting origins in very faint stars or extremely distant galaxies. The most accurate positions still contained many faint stars and galaxies, and it was widely agreed that resolving the origins required new satellites and faster communication.

Several models predicted that the initial gamma-ray burst would be followed by an afterglow—a slowly fading emission at longer wavelengths from collisions between burst ejecta and interstellar gas. Early searches failed, mainly because observing a burst’s position at longer wavelengths right after the initial flash was difficult. The breakthrough came in February 1997 when the BeppoSAX satellite detected a gamma-ray burst (GRB 970228) and its X-ray afterglow.