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In gamma-ray astronomy, gamma-ray bursts ( GRBs) are extremely energetic events occurring in distant Galaxy which represent the brightest and most powerful class of explosion in the universe. These extreme electromagnetic emissions are second only to the Big Bang as the most energetic and luminous phenomenon ever known. Gamma-ray bursts can last from a few milliseconds to several hours. After the initial flash of , a longer-lived afterglow is emitted, usually in the longer wavelengths of X-ray, ultraviolet, visible spectrum, infrared, microwave or radio waves frequencies.Vedrenne & Atteia 2009
The intense radiation of most observed GRBs is thought to be released during a supernova or superluminous supernova as a high-mass star implodes to form a neutron star or a black hole. Short-duration (sGRB) events are a subclass of GRB signals that are now known to originate from the cataclysmic merger of binary neutron stars.
The sources of most GRB are billions of away from Earth, implying that the explosions are both extremely energetic (a typical burst releases as much energy in a few seconds as the Sun will in its entire 10-billion-year lifetime) and extremely rare (a few per galaxy per million years).Podsiadlowski 2004 All GRBs in recorded history have originated from outside the Milky Way galaxy, although a related class of phenomena, soft gamma repeaters, are associated with within our galaxy. A gamma-ray burst in the Milky Way pointed directly at Earth would likely sterilize the planet or effect a mass extinction.Melott 2004 The Late Ordovician mass extinction has been hypothesised by some researchers to have occurred as a result of such a gamma-ray burst.
GRB signals were first detected in 1967 by the Vela satellites, which were designed to detect covert nuclear weapons tests; after an "exhaustive" period of analysis, this was published as academic research in 1973. Following their discovery, hundreds of theoretical models were proposed to explain these bursts, such as collisions between and .Hurley 2003 Little information was available to verify these models until the 1997 detection of the first X-ray and optical afterglows and direct measurement of their using optical spectroscopy, and thus their distances and energy outputs. These discoveries—and subsequent studies of the galaxies and associated with the bursts—clarified the distance and luminosity of GRBs, definitively placing them in distant galaxies.
Most early hypotheses of gamma-ray bursts posited nearby sources within the Milky Way Galaxy. From 1991, the Compton Gamma Ray Observatory (CGRO) and its Burst and Transient Source Explorer (BATSE) instrument, an extremely sensitive gamma-ray detector, provided data that showed the distribution of GRBs is Isotropy (that is, not biased towards any particular direction in space).Meegan 1992 If the sources were from within our own galaxy, they would be strongly concentrated in or near the galactic plane. The absence of any such pattern in the case of GRBs provided strong evidence that gamma-ray bursts must come from beyond the Milky Way.Vedrenne & Atteia 2009, pp. 16–40Schilling 2002, pp. 36–37Paczyński 1999, p. 6Piran 1992 However, some Milky Way models are still consistent with an isotropic distribution.Lamb 1995
Because of the very faint luminosity of this galaxy, its exact distance was not measured for several years. Well after then, another major breakthrough occurred with the next event registered by BeppoSAX, GRB 970508. This event was localized within four hours of its discovery, allowing research teams to begin making observations much sooner than any previous burst. The spectrum of the object revealed a redshift of z = 0.835, placing the burst at a distance of roughly 6 billion from Earth.Reichart 1995 This was the first accurate determination of the distance to a GRB, and together with the discovery of the host galaxy of 970228 proved that GRBs occur in extremely distant galaxies.Schilling 2002, pp. 118–123 Within a few months, the controversy about the distance scale ended: GRBs were extragalactic events originating within faint galaxies at enormous distances. The following year, GRB 980425 was followed within a day by a bright supernova (SN 1998bw), coincident in location, indicating a clear connection between GRBs and the deaths of very massive stars. This burst provided the first strong clue about the nature of the systems that produce GRBs.Galama 1998
The Space Variable Objects Monitor is a small X-ray telescope satellite for studying the explosions of massive stars by analysing the resulting gamma-ray bursts, developed by China National Space Administration (CNSA), Chinese Academy of Sciences (CAS) and the French Space Agency (CNES), launched on 22 June 2024 (07:00:00 UTC).
The Taiwan Space Agency is launching a CubeSat called The Gamma-ray Transients Monitor to track GRBs and other bright gamma-ray transients with energies ranging from 50 keV to 2 MeV in Q4 2026.
In October 2018, astronomers reported that (detected in 2015) and GW170817, a gravitational wave event detected in 2017 (which has been associated with , a burst detected 1.7 seconds later), may have been produced by the same mechanism—the merger of two . The similarities between the two events, in terms of gamma ray, optical, and x-ray emissions, as well as to the nature of the associated host Galaxy, were considered "striking", suggesting the two separate events may both be the result of the merger of neutron stars, and both may be a kilonova, which may be more common in the universe than previously understood, according to the researchers.
The highest energy light observed from a gamma-ray burst was one Electronvolt, from in 2019. Although enormous for such a distant event, this energy is around 3 orders of magnitude lower than the highest energy light observed from closer gamma ray sources within our Milky Way galaxy, for example a 2021 event of 1.4 petaelectronvolts.
Although some light curves can be roughly reproduced using certain simplified models,Simić 2005 little progress has been made in understanding the full diversity observed. Many classification schemes have been proposed, but these are often based solely on differences in the appearance of light curves and may not always reflect a true physical difference in the progenitors of the explosions. However, plots of the distribution of the observed durationThe duration of a burst is typically measured by T90, the duration of the period which 90 percent of the burst's energy is emitted. Recently some otherwise "short" GRBs have been shown to be followed by a second, much longer emission episode that when included in the burst light curve results in T90 durations of up to several minutes: these events are only short in the literal sense when this component is excluded. for a large number of gamma-ray bursts show a clear bimodality, suggesting the existence of two separate populations: a "short" population with an average duration of about 0.3 seconds and a "long" population with an average duration of about 30 seconds.Kouveliotou 1994 Both distributions are very broad with a significant overlap region in which the identity of a given event is not clear from duration alone. Additional classes beyond this two-tiered system have been proposed on both observational and theoretical grounds.Horvath 1998Hakkila 2003Chattopadhyay 2007Virgili 2009
The true nature of these objects was thus initially unknown, but the leading hypothesis was that they originated from the mergers of binary neutron stars or a neutron star with a black hole. Such mergers were hypothesized to produce , and evidence for a kilonova associated with short GRB 130603B was reported in 2013. The mean duration of sGRB events of around 200 milliseconds implied (due to causality) that the sources must be of very small physical diameter in stellar terms: less than 0.2 light-seconds (60,000 km or 37,000 miles)—about four times the Earth's diameter. The observation of minutes to hours of X-ray flashes after an sGRB was seen as consistent with small particles of a precursor object like a neutron star initially being swallowed by a black hole in less than two seconds, followed by some hours of lower-energy events as remaining fragments of tidally disrupted neutron star material would remain in orbit, spiraling into the black hole over a longer period of time.
The origin of short gamma-ray bursts in kilonovae was finally conclusively established in 2017, when short GRB 170817A co-occurred with the detection of gravitational wave GW170817, a signal from the merger of two neutron stars.
Unrelated to these cataclysmic origins, short-duration gamma-ray signals are also produced by giant flares from soft gamma repeaters in our own—or nearby—galaxies.Frederiks 2008Hurley 2005
In December 2022, astronomers reported the observation of GRB 211211A for 51 seconds, the first evidence of a long GRB likely associated with mergers of "compact binary objects" such as neutron stars or . Following this, GRB 191019A (2019, 64s) and GRB 230307A (2023, 35s) have been argued to signify an emerging class of long GRB which may originate from these types of progenitor events.
Gamma-ray bursts are thought to be highly focused explosions, with most of the explosion energy collimated light into a narrow relativistic jet.Rykoff 2009Abdo 2009 The jets of gamma-ray bursts are ultrarelativistic, and are the most relativistic jets in the universe. The matter in gamma-ray burst jets may also become superluminal, or faster than the speed of light in the jet medium, with there also being effects of time reversibility. The approximate angular width of the jet (that is, the degree of spread of the beam) can be estimated directly by observing the achromatic "jet breaks" in afterglow light curves: a time after which the slowly decaying afterglow begins to fade rapidly as the jet slows and can no longer beam its radiation as effectively.Sari 1999Burrows 2006 Observations suggest significant variation in the jet angle from between 2 and 20 degrees.Frail 2001
Because their energy is strongly focused, the gamma rays emitted by most bursts are expected to miss the Earth and never be detected. When a gamma-ray burst is pointed towards Earth, the focusing of its energy along a relatively narrow beam causes the burst to appear much brighter than it would have been were its energy emitted spherically. The total energy of typical gamma-ray bursts has been estimated at 3 × 1044 J,which is larger than the total energy (1044 J) of ordinary (type Ia, Ibc, II), with gamma-ray bursts also being more powerful than the typical supernova. Very bright supernovae have been observed to accompany several of the nearest GRBs. Further support for focusing of the output of GRBs comes from observations of strong asymmetries in the spectra of nearby type Ic supernovaeMazzali 2005 and from radio observations taken long after bursts when their jets are no longer relativistic.Frail 2000
The discovery of GRB 190114C suggests that previous observations may have underestimated the total energy output of GRBs. Measurements indicate that the energy released in very-high-energy gamma rays may be comparable to the combined energy emitted at all lower wavelengths.
Short (time duration) GRBs appear to come from a lower-redshift (i.e. less distant) population and are less luminous than long GRBs.Prochaska 2006 The degree of beaming in short bursts has not been accurately measured, but as a population they are likely less collimated than long GRBsWatson 2006 or possibly not collimated at all in some cases.Grupe 2006
The closest analogs within the Milky Way galaxy of the stars producing long gamma-ray bursts are likely the Wolf–Rayet stars, extremely hot and massive stars, which have shed most or all of their hydrogen envelope. Eta Carinae, Apep, and WR 104 have been cited as possible future gamma-ray burst progenitors.Plait 2008 It is unclear if any star in the Milky Way has the appropriate characteristics to produce a gamma-ray burst.Stanek 2006
The massive-star model probably does not explain all types of gamma-ray burst. There is strong evidence that some short-duration gamma-ray bursts occur in systems with no star formation and no massive stars, such as elliptical galaxies and . The favored hypothesis for the origin of most short gamma-ray bursts is the merger of a binary system consisting of two neutron stars. According to this model, the two stars in a binary slowly spiral towards each other because gravitational radiation releases energyAbbott 2007Kochanek 1993 until tidal forces suddenly rip the neutron stars apart and they collapse into a single black hole. The infall of matter into the new black hole produces an accretion disk and releases a burst of energy, analogous to the collapsar model. Numerous other models have also been proposed to explain short gamma-ray bursts, including the merger of a neutron star and a black hole, the accretion-induced collapse of a neutron star, or the evaporation of primordial black holes.Vietri 1998MacFadyen 2006Blinnikov 1984Cline 1996
An alternative explanation proposed by Friedwardt Winterberg is that in the course of a gravitational collapse and in reaching the event horizon of a black hole, all matter disintegrates into a burst of gamma radiation.Winterberg, Friedwardt (2001 Aug 29). "Gamma-Ray Bursters and Lorentzian Relativity". Z. Naturforsch 56a: 889–892.
Since 2011, only 4 jetted TDEs have been discovered, of which 3 were detected in gamma-rays (including Swift J1644+57). It is estimated that just 1% of all TDEs are jetted events.
The nature of the longer-wavelength afterglow emission (ranging from X-ray through radio waves) that follows gamma-ray bursts is better understood. Any energy released by the explosion not radiated away in the burst itself takes the form of matter or energy moving outward at nearly the speed of light. As this matter collides with the surrounding interstellar gas, it creates a relativistic shock wave that then propagates forward into interstellar space. A second shock wave, the reverse shock, may propagate back into the ejected matter. Extremely energetic electrons within the shock wave are accelerated by strong local magnetic fields and radiate as synchrotron emission across most of the electromagnetic spectrum.Meszaros 1997Sari 1998 This model has generally been successful in modeling the behavior of many observed afterglows at late times (generally, hours to days after the explosion), although there are difficulties explaining all features of the afterglow very shortly after the gamma-ray burst has occurred.Nousek 2006
All GRBs observed to date have occurred well outside the Milky Way galaxy and have been harmless to Earth. However, if a GRB were to occur within the Milky Way within 5,000 to 8,000 light-years and its emission were beamed straight towards Earth, the effects could be harmful and potentially devastating for its . Currently, orbiting satellites detect on average approximately one GRB per day. The closest observed GRB as of March 2014 was GRB 980425, located away (z=0.0085) in an SBc-type dwarf galaxy. GRB 980425 was far less energetic than the average GRB and was associated with the Type Ib supernova SN 1998bw.
Estimating the exact rate at which GRBs occur is difficult; for a galaxy of approximately the same size as the Milky Way, estimates of the expected rate (for long-duration GRBs) can range from one burst every 10,000 years, to one burst every 1,000,000 years. Only a small percentage of these would be beamed towards Earth. Estimates of rate of occurrence of short-duration GRBs are even more uncertain because of the unknown degree of collimation, but are probably comparable.Guetta and Piran 2006
Since GRBs are thought to involve beamed emission along two jets in opposing directions, only planets in the path of these jets would be subjected to the high energy gamma radiation. A GRB could potentially vaporize anything in its beams' paths within a range of around 200 light-years.
Although nearby GRBs hitting Earth with a destructive shower of gamma rays are only hypothetical events, high energy processes across the galaxy have been observed to affect the Earth's atmosphere.
The long-term effects from a nearby burst are more dangerous. Gamma rays cause chemical reactions in the atmosphere involving oxygen and nitrogen molecules, creating first nitrogen oxide then nitrogen dioxide gas. The nitrogen oxides cause dangerous effects on three levels. First, they deplete ozone layer, with models showing a possible global reduction of 25–35%, with as much as 75% in certain locations, an effect that would last for years. This reduction is enough to cause a dangerously elevated UV index at the surface. Secondly, the nitrogen oxides cause photochemical smog, which darkens the sky and blocks out parts of the sunlight spectrum. This would affect photosynthesis, but models show only about a 1% reduction of the total sunlight spectrum, lasting a few years. However, the smog could potentially cause a cooling effect on Earth's climate, producing a "cosmic winter" (similar to an impact winter, but without an impact), but only if it occurs simultaneously with a global climate instability. Thirdly, the elevated nitrogen dioxide levels in the atmosphere would wash out and produce acid rain. Nitric acid is toxic to a variety of organisms, including amphibian life, but models predict that it would not reach levels that would cause a serious global effect. The nitrates might in fact be of benefit to some plants.
All in all, a GRB within a few kiloparsecs, with its energy directed towards Earth, will mostly damage life by raising the UV levels during the burst itself and for a few years thereafter. Models show that the destructive effects of this increase can cause up to 16 times the normal levels of DNA damage. It has proved difficult to assess a reliable evaluation of the consequences of this on the terrestrial ecosystem, because of the uncertainty in biological field and laboratory data.
The major Ordovician–Silurian extinction event 450 million years ago may have been caused by a GRB. Estimates suggest that approximately 20–60% of the total phytoplankton biomass in the Ordovician oceans would have perished in a GRB, because the oceans were mostly oligotrophic and clear. The late Ordovician species of that spent portions of their lives in the plankton layer near the ocean surface were much harder hit than deep-water dwellers, which tended to remain within quite restricted areas. This is in contrast to the usual pattern of extinction events, wherein species with more widely spread populations typically fare better. A possible explanation is that trilobites remaining in deep water would be more shielded from the increased UV radiation associated with a GRB. Also supportive of this hypothesis is the fact that during the late Ordovician, burrowing bivalve species were less likely to go extinct than bivalves that lived on the surface.
A case has been made that the 774–775 carbon-14 spike was the result of a short GRB, though a very strong solar flare is another possibility.
Hypothetical effects on Earth in the past
There is a very good chance (but no certainty) that at least one lethal GRB took place during the past 5 billion years close enough to Earth as to significantly damage life. There is a 50% chance that such a lethal GRB took place within two kiloparsecs of Earth during the last 500 million years, causing one of the major mass extinction events.
GRB candidates in the Milky Way
No gamma-ray bursts from within our own galaxy, the Milky Way, have been observed, and the question of whether one has ever occurred remains unresolved. In light of evolving understanding of gamma-ray bursts and their progenitors, the scientific literature records a growing number of local, past, and future GRB candidates. Long duration GRBs are related to superluminous supernovae, or hypernovae, and most luminous blue variables (LBVs) and rapidly spinning Wolf–Rayet stars are thought to end their life cycles in core-collapse supernovae with an associated long-duration GRB. Knowledge of GRBs, however, is from metal-poor galaxies of former epochs of the universe's evolution, and it is impossible to directly extrapolate to encompass more evolved galaxies and stellar environments with a higher metallicity, such as the Milky Way. (2025). 9780521791410, Cambridge University Press. ISBN 9780521791410
See also
Notes
Citations
Further reading
(2025). 9780521662093, Cambridge University Press. ISBN 9780521662093
(2025). 9781139226530, Cambridge University Press. ISBN 9781139226530
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