# Gamma Ray Burst

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Image:Gammarayburst-GRB990123.jpeg
The image above shows the optical afterglow of gamma ray burst GRB-990123 taken on January 23, 1999. The burst is seen as a bright dot denoted by a square on the left, with an enlarged cutout on the right. The object above it with the finger-like filaments is the originating galaxy. This galaxy seems to be distorted by a collision with another galaxy.
Image:Gamma ray burst.jpg
Drawing of a massive star collapsing to form a black hole. Energy released as jets along the axis of rotation forms a gamma ray burst. Credit: Nicolle Rager Fuller/NSF

Gamma-ray bursts (GRBs) are the most luminous events known in the universe since the Big Bang. They are flashes of gamma rays coming from seemingly random places in deep space at random times. GRBs last from milliseconds to many minutes, and are often followed by "afterglow" emission at longer wavelengths (X-ray, UV, optical, IR, and radio). Gamma-ray bursts are detected by orbiting satellites about two to three times a week, as of 2007, though their actual rate of occurrence is much higher.

The majority of observed GRBs appear to be collimated emissions from the core-collapse of rapidly rotating, high-mass stars into black holes. A subclass of GRBs (the "short" bursts) appear to come from a different process, possibly the collision of neutron stars orbiting in a binary system. All known GRBs come from outside our own galaxy, though a related class of phenomena, SGR flares, are associated with Galactic magnetars). Most GRBs come from billions of light years away.

When a GRB occurs, the energy output exceeds that of the entire known universe for the duration of the burst. For an idea of just how much energy this is, consider this: the total energy produced in all the years mankind has been utilizing energy (via fire, electricity, etc.) does not even add up to 1/1,000,000th of 1% of the energy put out by our own sun in one second.

## Discovery and history

### Vela and the discovery of GRBs

Cosmic gamma-ray bursts were discovered in the late 1960s by the US Vela nuclear test detection satellites. The Velas were built to detect the gamma-radiation pulses emitted by nuclear weapons tests in space. The United States suspected that the USSR might attempt to conduct secret tests after signing the Nuclear Test Ban Treaty in 1963. The discovery of weapons tests was never publicly declared, however, details of the Vela Incident remain classified. In a classic example of scientific serendipity, the satellites did detect flashes of radiation that looked nothing like a nuclear weapons signature, coming from seemingly random directions in deep space. These results were published in 1973,[1] launching the modern scientific study of GRBs.

### BATSE

The discovery of GRBs was confirmed by many later space missions, including Apollo and the Soviet Venera probes. Many speculative theories about these events were presented, most of which involved nearby Galactic sources. There were no major advances until the 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 that gamma-ray bursts are isotropic[2], not biased towards any particular direction in space, such as the galactic plane or the Galactic center, ruling out nearly all galactic origins. BATSE data also showed that GRBs fall into two apparently distinct categories: short-duration, hard-spectrum bursts ("short bursts"), and long-duration, soft-spectrum bursts ("long bursts").[3] 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 suggested two different classes of progenitors.

### BeppoSAX and the afterglow era

Because of the poor resolution of gamma-ray detectors, no GRB was associated with a known counterpart or host, such as a star or galaxy, for decades after the initial discovery. The best hope for changing this situation seemed to be in finding fainter, fading emission at longer wavelengths following the burst itself, the "afterglow" of a GRB, as predicted by most models.[4] However, despite intensive searches, no such emission had been found.

This changed in 1997 when the Dutch/Italian satellite BeppoSAX detected a gamma-ray burst (GRB 970228[5]), pointed its X-ray camera at the direction from which the burst had originated, and detected a fading X-ray emission. Additional study from ground-based telescopes identified a fading optical counterpart as well.[6] With the position of this event precisely known, once the GRB faded, deep imaging was able to identify a faint, very distant host galaxy at the GRB location, the first of many to be localized.[7] Within only a few weeks, the long controversy about the distance scale had ended: GRBs were extragalactic events, originating inside faint galaxies[8] at enormous distance. By finally establishing the distance scale, characterizing the environments in which GRBs occur, and providing a new window on GRBs both observationally and theoretically, this discovery revolutionized the study of GRBs.[9]

### Swift and GRBs today

A similar revolution in GRB astronomy is in progress as of 2007, largely as a result of the successful launch of NASA's Swift satellite in November 2004, which combines a sensitive gamma-ray detector with the ability to slew on-board X-ray and optical telescopes to the direction of a new burst in less than one minute.[10] Swift's discoveries include the first observations of short burst afterglows and vast amounts of data on the behavior of GRB afterglows at early times in their evolution, even before the GRB's gamma-ray emission has stopped. The mission has also discovered huge X-ray flares appearing from minutes to days after the end of the GRB.

## Distance scale and energetics

### Galactic vs. extragalactic models

Before the launch of BATSE, the distance scale to GRBs was completely unknown. Theories for the location of these events ranged from the outer regions of our own solar system to the edges of the known universe. The discovery that bursts were isotropic narrowed down these possibilities greatly, and by the mid 1990s only two theories were considered generally viable: that GRBs originate from a very large, diffuse halo (or "corona") around our own Galaxy, or that they originate from distant galaxies far beyond our local group. Supporters of the Galactic model pointed to the class of well-known objects known as soft gamma repeaters (SGRs), highly magnetized Galactic neutron stars known to periodically erupt in bright flares at gamma-ray and other wavelengths, and stated that there may very well be an unobserved population of similar objects at greater distances producing GRBs.[11] Furthermore, the sheer brightness of a typical gamma-ray burst would impose enormous requirements on the energy released in such an event if it really occurred in a distant galaxy. Supporters of the extragalactic model claimed that the Galactic neutron-star hypothesis involved too many ad-hoc assumptions in order to reproduce the degree of isotropy reported by BATSE and that an extragalactic model was far more natural regardless of its problems.[12]

### Extragalactic nature of GRBs

The discovery of afterglow emission associated with faraway galaxies definitively supported the extragalactic hypothesis. Not only are GRBs extragalactic events, but they are also observable practically to the limits of the visible universe: a typical GRB has a redshift of at least 1.0 (corresponding to a distance of 8 billion light-years), while the most distant event known (Gamma Ray Burst 050904) has a redshift of 6.3 (12.3 billion light years).[13] Observers are only able to acquire spectra of a small fraction of bursts, generally the brightest ones, so many GRBs could actually have come from even higher redshifts.

The confirmed immense distance scale of GRBs imposed equally immense demands on the energetics of a GRB explosion. Under the assumption that a given burst emits energy uniformly in all directions, some of the brightest bursts correspond to a total energy release of 1047 joules, nearly a solar mass converted into gamma-radiation (see mass-energy equivalence) in the matter of a few seconds. No known process in the universe is able to liberate that much energy so quickly. The energy requirements are eased somewhat if the burst is not symmetric. If, for example, the energy is funneled out along a narrow jet with an angle of a few degrees, the actual energy release for a typical GRB is comparable to that of a very luminous supernova.

### GRB Jets: collimated emission

Narrow jet emissions are widely believed to be the case, as of 2007. Many GRBs have been observed to undergo a "jet break" in their light curve, in which the optical afterglow quickly changes from slowly fading to rapidly fading as the jet slows down.[14] At least one supernova of a similar nature to the few supernova that have been seen to accompany GRBs has been shown to have features suggestive of significant asymmetry in its explosion (see "Progenitors"). The jet opening angle (degree of beaming), however, appears to vary greatly, from 2 degrees up to more than 20 degrees. There is some evidence that the jet angles and apparent energy released are correlated in such a way that the true energy release of a (long) GRB is approximately constant—about 1044 J, around 1/2000 of a solar mass.[15] This is comparable to the energy released in a bright type Ib/c supernova (sometimes termed a "hypernova"). Bright hypernovae do in fact appear to accompany some GRBs.[16]

The fact that GRBs are jetted also suggests that there are far more events occurring in the Universe than actually seen, even when factoring in the limited sensitivity of available detectors. Most jetted GRBs will "miss" the Earth and never be seen; only a small fraction happen to be pointed the right way to allow detection. Still, even with these considerations, the rate of GRBs is very small—about once per galaxy per 100,000 years.[17] The fact that GRBs are so incredibly luminous allows detection on a regular basis despite their rarity. For the brightest GRBs, if the burst's jet is directed at Earth, it is possible to detect it no matter how far away it may be—fueling speculation that some bursts may originate from redshifts of 7 or higher (distances of over 13 billion light-years) in the earliest Universe. Even the faintest GRBs are visible out to a distance over a billion light-years.

### Short GRBs

The above arguments apply only to long GRBs. Short GRBs, while also extragalactic, appear to come from a lower-redshift population and are less luminous than long GRBs.[18] They appear to be generally less beamed[19] - or possibly not beamed at all in some cases[20], intrinsically less energetic than their longer counterparts, and probably more frequent in the universe despite being rarer observationally.

## Progenitors: what makes GRBs explode?

For decades, almost nothing was known about gamma-ray bursts. Their distribution, distances and sources were all unknown. GRBs themselves showed an extraordinary degree of diversity: they could be anywhere from a fraction of a second to many minutes in duration; bursts could have a single profile or oscillate wildly up and down in intensity; their spectra were highly variable and like nothing ever seen. Unsurprisingly, the almost complete lack of observational constraint led to a profusion of theories: evaporating black holes, magnetic flares on white dwarfs, accretion of matter onto neutron stars, antimatter accretion, exotic types of supernovae, and rapid extraction of rotational energy from supermassive black holes (to provide only a small sample).[21]

The situation has cleared up greatly since then. It is almost certain that there are at least two different types of progenitors (sources) of GRBs: one responsible for the long-duration, soft-spectrum bursts and one (or possibly more) responsible for short-duration, hard-spectrum bursts. The progenitors of long GRBs are believed to be massive, low-metallicity stars exploding due to the collapse of their cores. The progenitors of short GRBs are still unknown but mergers of neutron stars is probably the most popular model as of 2007.

### Long GRBs: massive stars

#### Collapsar model

As of 2007, there is almost universal agreement in the astrophysics community that the long-duration bursts are associated with the deaths of massive stars in a specific kind of supernova-like event commonly referred to as a collapsar.[22] Very massive stars are able to fuse material in their centers all the way to iron, at which point a star cannot continue to generate energy by fusion and collapses, in this case, immediately forming a black hole. Matter from the star around the core rains down towards the center and (for rapidly rotating stars) swirls into a high-density accretion disk. The infall of this material into the black hole drives a pair of jets out along the rotational axis, where the matter density is much lower than in the accretion disk, towards the poles of the star at velocities approaching the speed of light, creating a relativistic shock wave[23] at the front. If the star is not surrounded by a thick, diffuse hydrogen envelope, the jets' material can pummel all the way to the stellar surface. The leading shock actually accelerates as the density of the stellar matter it travels through decreases, and by the time it reaches the surface of the star it may be traveling with a Lorentz factor of 100 or higher (that is, a velocity of 0.9999 times the speed of light). Once it reaches the surface, the shock wave breaks out into space, with much of its energy released in the form of gamma-rays. (See "Emission Mechanisms" for more information.)

Three very special conditions are required for a star to evolve all the way to a gamma-ray burst under this theory: the star must be very massive (probably at least 40 Solar masses on the main sequence) to form a central black hole in the first place, the star must be rapidly rotating to develop an accretion torus capable of launching jets, and the star must have low metallicity in order to strip off its hydrogen envelope so the jets can reach the surface. As a result, gamma-ray bursts are far rarer than ordinary core-collapse supernovae, which only require that the star be massive enough to fuse all the way to iron.

#### Evidence for the collapsar view

This consensus is based largely on two lines of evidence. First, long gamma-ray bursts are found without exception in systems with abundant recent star formation, such as in irregular galaxies and in the arms of spiral galaxies.[24] This is strong evidence of a link to massive stars, which evolve and die within a few hundred million years and are never found in regions where star formation has long ceased. This does not necessarily prove the collapsar model (other models also predict an association with star formation) but does provide significant support.

Second, there are now several observed cases where a supernova has immediately followed a gamma-ray burst. While most GRBs occur too far away for current instruments to have any chance of detecting the relatively faint emission from a supernova at that distance, for lower-redshift systems there are several well-documented cases where a GRB was followed within a few days by the appearance of a supernova. All such supernovae that have been successfully classified have been of type Ib/c, a rare class of supernovae that are due to core collapse but lack hydrogen absorption lines, consistent with the theoretical prediction of association with stars that have lost their hydrogen envelope. The GRBs with the most obvious supernova signatures include GRB 060218 (SN 2006aj),[25] GRB 030329 (SN 2003dh),[26] and GRB 980425 (SN 1998bw),[27] and a handful of more distant GRBs show supernova "bumps" in their afterglow light curves at late times.

Possible exceptions to this theory were recently discovered [28] [29] when two nearby long gamma-ray bursts lacked a signature of any type of supernova: both GRB060614 and GRB 060505 defied predictions that a supernova would emerge despite intense scrutiny from ground-based telescopes. Both events were, however, associated with actively star-forming stellar populations. One possible implication is that it now appears that a supernova can fail utterly during the core collapse of a massive star, perhaps when the black hole swallows the entire star before the supernova blast can reach the surface.

### Short GRBs: degenerate binary systems?

Short gamma-ray bursts appear to be an exception. Until 2007, only a handful of these events have been localized to a definite galactic host. However, those that have been localized appear to show significant differences from the long-burst population. While at least one short burst has been found in the star-forming central region of a galaxy, several others have been associated with the outer regions and even the outer halo of large elliptical galaxies in which star formation has nearly ceased. All the hosts identified so far have also been at low redshift.[18] Furthermore, despite the relatively nearby distances and detailed follow-up study for these events, no supernova has been associated with any short GRB.

#### Neutron star and Neutron star/Black hole mergers

While the astrophysical community has yet to settle on a single, universally favored model for the progenitors of short GRBs, the generally preferred model is the merger of two compact objects as a result of gravitational inspiral: two neutron stars,[30] or a neutron star and a black hole.[31] While thought to be rare in the Universe, a small number of cases of close neutron star - neutron star binaries are known in our Galaxy, and neutron star - black hole binaries are believed to exist as well. According to Einstein's theory of general relativity, systems of this nature will slowly lose energy due to gravitational radiation and the two degenerate objects will spiral closer and closer together, until in the last few moments, tidal forces rip the neutron star (or stars) apart and an immense amount of energy is liberated before the matter plunges into a single black hole. The whole process is believed to occur extremely quickly and be completely over within a few seconds, accounting for the short nature of these bursts. Unlike long-duration bursts, there is no conventional star to explode and therefore no supernova.

This model has been well-supported so far by the distribution of short GRB host galaxies, which have been observed in old galaxies with no star formation (for example, GRB050509B, the first short burst to be localized to a probable host) as well as in galaxies with star formation still occurring (such as GRB050709, the second), as even younger-looking galaxies can have significant populations of old stars. However, the picture is clouded somewhat by the observation of X-ray flaring[32] in short GRBs out to very late times (up to many days), long after the merger should have been completed, and the failure to find nearby hosts of any sort for some short GRBs.

#### Magnetar giant flares

One final possible model that may describe a small subset of short GRBs are the so-called magnetar giant flares (also called megaflares or hyperflares). It has been well-known for several decades that members of a rare class of powerfully magnetized neutron stars known as "magnetars" (only five such objects are known in our Galaxy) are capable of producing brief but enormous outbursts of high-energy photons. Indeed, for a long time outbursts of this nature were a favorite model for producing all gamma-ray bursts. However, none of these events were observed to be luminous enough for bursts from similar events outside our Galaxy and its satellites to be detectable until 27 December 2004, when a blast of radiation from the magnetar SGR 1806-20 saturated the detectors of every gamma-ray satellite in orbit and significantly disrupted Earth's ionosphere.[33] Such an event would easily be detectable from beyond our Galaxy, and it has been speculated that a handful of known GRBs may be associated with these events. As of 2007, a definitive link with any specific GRB is lacking, though there is suggestive evidence of association in the case of GRB051103. Furthermore, only a small fraction of known GRBs have spectral properties with any resemblance to the properties of giant flares.

## Emission mechanisms

The issue of exactly how the energy from the gamma-ray burst progenitor (regardless of the actual nature of the progenitor) is turned into radiation is a major topic of research unto itself. Neither the light curves nor the early-time spectra of GRBs show resemblance to the radiation emitted by any familiar physical process.

### The compactness problem

It has been known for many years that ejection of matter at relativistic velocities (velocities very close to the speed of light) is a necessary requirement for producing the emission in a gamma-ray burst. GRBs vary on such short timescales (as short as milliseconds in some cases) that the size of the emitting region must be very small, or else the time delay due to the finite speed of light would "smear" the emission out in time, wiping out any short-timescale behavior. At the energies involved in a typical GRB, so much energy crammed into such a small space would make the system opaque to photon-photon pair production, making the burst far less luminous and also giving it a very different spectrum from what is observed. However, if the emitting system is moving towards Earth at relativistic velocities, the burst is compressed in time (as seen by an Earth observer, due to the relativistic Doppler effect) and the emitting region inferred from the finite speed of light becomes much smaller than the true size of the GRB (see relativistic beaming).

### GRBs and internal shocks

A related constraint is imposed by the relative timescales seen in some bursts between the short-timescale variability and the total length of the GRB. Often this variability timescale is far shorter than the total burst length. For example, in bursts as long as 100 seconds, the majority of the energy can be released in short episodes less than 1 second long. If the GRB were due to matter moving towards Earth (as the relativistic motion argument enforces), it is hard to understand why it would release its energy in such brief interludes. The generally accepted explanation for this is that these bursts involve the collision of multiple shells traveling at slightly different velocities; so-called "internal shocks" [34]. The collision of two thin shells flash-heats the matter, converting enormous amounts of kinetic energy into the random motion of particles, greatly amplifying the energy release due to all emission mechanisms. Which physical mechanisms are at play in producing the observed photons is still an area of debate, but the most likely candidates appear to be synchrotron radiation and inverse Compton scattering.

As of 2007 there is no theory that has successfully described the spectrum of all gamma-ray bursts (though some theories work for a subset). However, the so-called Band function has been fairly successful at fitting, empirically, the spectra of most gamma-ray bursts:

$N(E)= \begin{cases} {E^\alpha \exp \left( { - \frac{E}Template:E 0} \right)}, & \mbox{if }E \ge (\alpha - \beta) E_0\mbox{ } \\ {\left[ {\left( {\alpha - \beta } \right)E_0 } \right]^{\left( {\alpha - \beta } \right)} E^\beta \exp \left( {\beta - \alpha } \right)}, & \mbox{if }E < (\alpha - \beta) E_0\mbox{ } \end{cases}$

### Afterglows and external shocks

The GRB itself is very rapid, lasting from less than a second up to a few minutes at most. Once it disappears, it leaves behind a counterpart at longer wavelengths (X-ray, UV, optical, infrared, and radio) known as the afterglow[35] that generally remains detectable for days or longer.

In contrast to the GRB emission, the afterglow emission is not believed to be dominated by internal shocks. In general, all the ejected matter has by this time coalesced into a single shell traveling outward into the interstellar medium (or possibly the stellar wind) around the star. At the front of this shell of matter is a shock wave referred to as the "external shock" [36] as the still relativistically-moving matter ploughs into the tenuous interstellar gas or the gas surrounding the star.

As the interstellar matter moves across the shock, it is immediately heated to extreme temperatures. (How this happens is still poorly understood as of 2007, since the particle density across the shock wave is too low to create a shock wave comparable to those familiar in dense terrestrial environments – the topic of "collisionless shocks" is still largely hypothesis but seems to accurately describe a number of astrophysical situations. Magnetic fields are probably critically involved.) These particles, now relativistically moving, encounter a strong local magnetic field and are accelerated perpendicular to the magnetic field, causing them to radiate their energy via synchrotron radiation.

Synchrotron radiation is well-understood and the afterglow spectrum has been modeled fairly successfully using this template.[37] It is generally dominated by electrons (which move and therefore radiate much faster than protons and other particles) so radiation from other particles is generally ignored.

In general, the GRB assumes the form of a power-law with three break points (and therefore four different power-law segments.) The lowest break point, $\nu_a$, corresponds to the frequency below which the GRB is opaque to radiation and so the spectrum attains the form Raleigh-Jeans tail of blackbody radiation. The two other break points, $\nu_m$ and $\nu_c$, are related to the minimum energy acquired by an electron after it crosses the shock wave and the time it takes an electron to radiate most of its energy, respectively. Depending on which of these two frequencies is higher, two different regimes are possible:

• Fast cooling ($\nu_m > \nu_c$) - Shortly after the GRB, the shock wave imparts immense energy to the electrons and the minimum electron Lorentz factor is very high. In this case, the spectrum looks like:

$F_\nu \propto \begin{cases} {\nu^{2}}, & \nu<\nu_a \\  {\nu^{1/3}}, & \nu_a<\nu<\nu_c \\ {\nu^{-1/2}}, & \nu_c<\nu<\nu_m \\ {\nu^{-p/2}}, & \nu_m<\nu  \end{cases}$

• Slow cooling ($\nu_m < \nu_c$) – Later after the GRB, the shock wave has slowed down and the minimum electron Lorentz factor is much lower.:

$F_\nu \propto \begin{cases} {\nu^{2}}, & \nu<\nu_a \\  {\nu^{1/3}}, & \nu_a<\nu<\nu_m \\ {\nu^{-(p-1)/2}}, & \nu_m<\nu<\nu_c \\ {\nu^{-p/2}}, & \nu_c<\nu  \end{cases}$

The afterglow changes with time. It must fade, obviously, but the spectrum changes as well. For the simplest case of adiabatic expansion into a uniform-density medium, the critical parameters evolve as:

$\nu_c \propto t^{1/2}$

$\nu_m \propto t^{-3/2}$

$F_{\nu,max} = const$

Here $F_{\nu,max}$ is the flux at the current peak frequency of the GRB spectrum. (During fast-cooling this is at $\nu_c$; during slow-cooling it is at $\nu_m$.) Note that because $\nu_m$ drops faster than $\nu_c$, the system eventually switches from fast-cooling to slow-cooling.

Different scalings are derived for radiative evolution and for a non-constant-density environment (such as a stellar wind), but share the general power-law behavior observed in this case.

Several other known effects can modify the evolution of the afterglow:

### Reverse shocks and the optical flash

There can be "reverse shocks", which propagate back into the shocked matter once it begins to encounter the interstellar medium.[38] [39] The twice-shocked material can produce a bright optical/UV flash, which has been seen in a few GRBs,[40] though it appears not to be a common phenomenon.

### Refreshed shocks and late-time flares

There can be "refreshed" shocks if the central engine continues to release fast-moving matter in small amounts even out to late times, these new shocks will catch up with the external shock to produce something like a late-time internal shock. This explanation has been invoked to explain the frequent flares seen in X-rays and at other wavelengths in many bursts, though some theorists are uncomfortable with the apparent demand that the progenitor (which one would think would be destroyed by the GRB) continues to remain active for very long.

### Jet effects

Gamma-ray burst emission is believed to be released in jets, not spherical shells.[41] Initially the two scenarios are equivalent: the center of the jet is not "aware" of the jet edge, and due to relativistic beaming we only see a small fraction of the jet. However, as the jet slows down, two things eventually occur (each at about the same time): First, information from the edge of the jet that there is no pressure to the side propagates to its center, and the jet matter can spread laterally. Second, relativistic beaming effects subside, and once Earth observers see the entire jet the widening of the relativistic beam is no longer compensated by the fact that we see a larger emitting region. Once these effects appear the jet fades very rapidly, an effect that is visible as a power-law "break" in the afterglow light curve. This is the so-called "jet break" that has been seen in some events and is often cited as evidence for the consensus view of GRBs as jets. Many GRB afterglows do not display jet breaks, especially in the X-ray, but they are more common in the optical light curves. Though as jet breaks generally occur at very late times (~1 day or more) when the afterglow is quite faint, and often undetectable, this is not necessarily surprising.

### Dust extinction and hydrogen absorption

There may be dust along the line of sight from the GRB to Earth, both in the host galaxy and in the Milky Way. If so, the light will be attenuated and reddened and an afterglow spectrum may look very different from that modeled.

At very high frequencies (far-ultraviolet and X-ray) interstellar hydrogen gas becomes a significant absorber. In particular, a photon with a wavelength of less than 912 Angstroms (91 nanometers) is energetic enough to completely ionize neutral hydrogen and is absorbed with almost 100% probability even through relatively thin gas clouds. (At much shorter wavelengths the probability of absorption begins to drop again, which is why X-ray afterglows are still detectable.) As a result, observed spectra of very high-redshift GRBs often drop to zero at wavelengths less than that of where this hydrogen ionization threshold (known as the Lyman break) would be in the GRB host's reference frame. Other, less dramatic hydrogen absorption features are also commonly seen in high-z GRBs, such as the Lyman alpha forest.

## Mass extinction on Earth

One line of research has investigated the consequences of Earth being hit by a beam of gamma rays from a nearby (about 500 light years) gamma ray burst. This is motivated by the efforts to explain mass extinctions on Earth and estimate the probability of extraterrestrial life. The consensus seems to be that the damage that a gamma ray burst could do would be limited by its very short duration, but that a sufficiently close gamma ray burst could do serious damage to the atmosphere, perhaps wiping out the ozone layer and triggering a mass extinction. The damage from a gamma ray burst would probably be significantly greater than a supernova at the same distance.

The idea that a nearby gamma-ray burst could significantly affect the Earth's atmosphere and potentially cause severe damage to the biosphere was introduced in 1995 by physicist Stephen Thorsett, then at Princeton University. [42] Scientists at NASA and the University of Kansas in 2005 released a more detailed study that suggests that the Ordovician-Silurian extinction events of 450 million years ago could have been triggered by a gamma-ray burst. The scientists do not have direct evidence that such a burst activated the ancient extinction, rather the strength of their work is their atmospheric modeling, essentially a "what if" scenario. The scientists calculated that gamma-ray radiation from a relatively nearby star explosion, hitting the Earth for only ten seconds, could deplete up to half of the atmosphere's protective ozone layer. Recovery could take at least five years. With the ozone layer damaged, ultraviolet radiation from the Sun could kill much of the life on land and near the surface of oceans and lakes, disrupting the food chain. While gamma-ray bursts in our Milky Way galaxy are indeed rare, NASA scientists estimate that at least one nearby event probably hit the Earth in the past billion years. Life on Earth is thought to have appeared at least 3.5 billion years ago. Dr. Bruce Lieberman, a paleontologist at the University of Kansas, originated the idea that a gamma-ray burst specifically could have caused the great Ordovician extinction. "We don't know exactly when one came, but we're rather sure it did come - and left its mark. What's most surprising is that just a 10-second burst can cause years of devastating ozone damage." [43]

Comparative work in 2006 on galaxies in which GRBs have occurred suggests that metal-poor galaxies are the most likely candidates. The likelihood of the metal-rich Milky Way galaxy hosting a GRB was estimated at less than 0.15%, significantly reducing the likelihood that a burst has caused mass extinction events on Earth. [44]

## Notable GRBs

Many thousands of GRBs have been detected by numerous satellites. This list does not attempt anything close to a complete listing, including only those GRBs of significant historical or scientific importance.

• 670702 – The first GRB ever detected.
• 970228 – The first GRB with a successfully detected afterglow. The location of the afterglow was coincident with what was apparently a very faint galaxy, providing strong evidence that GRBs are extragalactic.
• 970508 – The first GRB with a measured redshift (distance). At z=0.835, it confirmed unambiguously that GRBs are extragalactic.
• 971214 – In 1997, this was believed by some to be the most energetic event in the universe. This claim has since been discredited.
• 980425 – The first GRB with an observed associated supernova (1998bw), providing strong evidence of the link between GRBs and supernovae. The GRB itself was very unusual for being extremely underluminous. Also the closest GRB to date.
• 990123 – This GRB had the optically brightest afterglow measured to date, momentarily reaching or exceeding a magnitude of 8.95, only slightly fainter than the planet Neptune despite its distance of 9.6 billion light years. This was also the first GRB for which optical emission was detected before the gamma-ray emission had ceased.
• 030329A – Extremely bright GRB with an unambiguous supernova association. Proved that GRBs and supernovae are linked.
• 050509B - The first short GRB with a host association. Provided evidence that (some) short GRBs, unlike long GRBs, occur in old galaxies and do not have accompanying supernovae.
• 050724 – The first short GRB with a secure elliptical galaxy association.
• 050904 – The most distant GRB observed to date, at z=6.29 (13 billion light-years).
• 060218 – The most recent low-redshift GRB with an accompanying supernova.
• 060505 - The first long GRB not accompanied by a bright supernova.