In gamma-ray astronomy, gamma-ray bursts (GRBs)
are extremely energetic explosions that have
been observed in distant galaxies. They are
the brightest electromagnetic events known
to occur in the universe. Bursts can last
from ten milliseconds to several hours. After
an initial flash of gamma rays, a longer-lived
"afterglow" is usually emitted at longer wavelengths
(X-ray, ultraviolet, optical, infrared, microwave
and radio).The intense radiation of most observed
GRBs is thought to be released during a supernova
or superluminous supernova as a high-mass
star collapses to form a neutron star or a
black hole.
A subclass of GRBs (the "short" bursts) appear
to originate from a kilonova (the merger of
binary neutron stars). The cause of the precursor
burst observed in some of these short events
may be the development of a resonance between
the crust and core of such stars as a result
of the massive tidal forces experienced in
the seconds leading up to their collision,
causing the entire crust of the star to shatter.The
sources of most GRBs are billions of light
years 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). All observed GRBs have
originated from outside the Milky Way galaxy,
although a related class of phenomena, soft
gamma repeater flares, are associated with
magnetars within the Milky Way. It has been
hypothesized that a gamma-ray burst in the
Milky Way, pointing directly towards the Earth,
could cause a mass extinction event.GRBs were
first detected in 1967 by the Vela satellites,
which had been designed to detect covert nuclear
weapons tests; this was declassified and published
in 1973. Following their discovery, hundreds
of theoretical models were proposed to explain
these bursts, such as collisions between comets
and neutron stars. 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
redshifts using optical spectroscopy, and
thus their distances and energy outputs. These
discoveries, and subsequent studies of the
galaxies and supernovae associated with the
bursts, clarified the distance and luminosity
of GRBs, definitively placing them in distant
galaxies.
== History ==
Gamma-ray bursts were first observed in the
late 1960s by the U.S. Vela satellites, which
were built to detect gamma radiation pulses
emitted by nuclear weapons tested in space.
The United States suspected that the Soviet
Union might attempt to conduct secret nuclear
tests after signing the Nuclear Test Ban Treaty
in 1963. On July 2, 1967, at 14:19 UTC, the
Vela 4 and Vela 3 satellites detected a flash
of gamma radiation unlike any known nuclear
weapons signature. Uncertain what had happened
but not considering the matter particularly
urgent, the team at the Los Alamos National
Laboratory, led by Ray Klebesadel, filed the
data away for investigation. As additional
Vela satellites were launched with better
instruments, the Los Alamos team continued
to find inexplicable gamma-ray bursts in their
data. By analyzing the different arrival times
of the bursts as detected by different satellites,
the team was able to determine rough estimates
for the sky positions of sixteen bursts and
definitively rule out a terrestrial or solar
origin. The discovery was declassified and
published in 1973.Most early theories 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 isotropic—not
biased towards any particular direction in
space. 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. However, some
Milky Way models are still consistent with
an isotropic distribution.In October 2018,
astronomers reported that GRB 150101B, a gamma-ray
burst event detected in 2015, may be directly
related to the historic GW170817, a gravitational
wave event detected in 2017, and associated
with the merger of two neutron stars. 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 galaxies, are "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.
=== Counterpart objects as candidate sources
===
For decades after the discovery of GRBs, astronomers
searched for a counterpart at other wavelengths:
i.e., any astronomical object in positional
coincidence with a recently observed burst.
Astronomers considered many distinct classes
of objects, including white dwarfs, pulsars,
supernovae, globular clusters, quasars, Seyfert
galaxies, and BL Lac objects. All such searches
were unsuccessful, and in a few cases particularly
well-localized bursts (those whose positions
were determined with what was then a high
degree of accuracy) could be clearly shown
to have no bright objects of any nature consistent
with the position derived from the detecting
satellites. This suggested an origin of either
very faint stars or extremely distant galaxies.
Even the most accurate positions contained
numerous faint stars and galaxies, and it
was widely agreed that final resolution of
the origins of cosmic gamma-ray bursts would
require both new satellites and faster communication.
=== Afterglow ===
Several models for the origin of gamma-ray
bursts postulated that the initial burst of
gamma rays should be followed by slowly fading
emission at longer wavelengths created by
collisions between the burst ejecta and interstellar
gas. This fading emission would be called
the "afterglow". Early searches for this afterglow
were unsuccessful, largely because it is difficult
to observe a burst's position at longer wavelengths
immediately after the initial burst. The breakthrough
came in February 1997 when the satellite BeppoSAX
detected a gamma-ray burst (GRB 970228) and
when the X-ray camera was pointed towards
the direction from which the burst had originated,
it detected fading X-ray emission. The William
Herschel Telescope identified a fading optical
counterpart 20 hours after the burst. Once
the GRB faded, deep imaging was able to identify
a faint, distant host galaxy at the location
of the GRB as pinpointed by the optical afterglow.Because
of the very faint luminosity of this galaxy,
its exact distance was not measured for several
years. Well before 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 light years from Earth. 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. 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.
BeppoSAX functioned until 2002 and CGRO (with
BATSE) was deorbited in 2000. However, the
revolution in the study of gamma-ray bursts
motivated the development of a number of additional
instruments designed specifically to explore
the nature of GRBs, especially in the earliest
moments following the explosion. The first
such mission, HETE-2, launched in 2000 and
functioned until 2006, providing most of the
major discoveries during this period. One
of the most successful space missions to date,
Swift, was launched in 2004 and as of 2016
is still operational. Swift is equipped with
a very sensitive gamma ray detector as well
as on-board X-ray and optical telescopes,
which can be rapidly and automatically slewed
to observe afterglow emission following a
burst. More recently, the Fermi mission was
launched carrying the Gamma-Ray Burst Monitor,
which detects bursts at a rate of several
hundred per year, some of which are bright
enough to be observed at extremely high energies
with Fermi's Large Area Telescope. Meanwhile,
on the ground, numerous optical telescopes
have been built or modified to incorporate
robotic control software that responds immediately
to signals sent through the Gamma-ray Burst
Coordinates Network. This allows the telescopes
to rapidly repoint towards a GRB, often within
seconds of receiving the signal and while
the gamma-ray emission itself is still ongoing.New
developments since the 2000s include the recognition
of short gamma-ray bursts as a separate class
(likely from merging neutron stars and not
associated with supernovae), the discovery
of extended, erratic flaring activity at X-ray
wavelengths lasting for many minutes after
most GRBs, and the discovery of the most luminous
(GRB 080319B) and the former most distant
(GRB 090423) objects in the universe. The
most distant known GRB, GRB 090429B, is now
the most distant known object in the universe.
== Classification ==
The light curves of gamma-ray bursts are extremely
diverse and complex. No two gamma-ray burst
light curves are identical, with large variation
observed in almost every property: the duration
of observable emission can vary from milliseconds
to tens of minutes, there can be a single
peak or several individual subpulses, and
individual peaks can be symmetric or with
fast brightening and very slow fading. Some
bursts are preceded by a "precursor" event,
a weak burst that is then followed (after
seconds to minutes of no emission at all)
by the much more intense "true" bursting episode.
The light curves of some events have extremely
chaotic and complicated profiles with almost
no discernible patterns.Although some light
curves can be roughly reproduced using certain
simplified models, 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
duration 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. 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.
=== Short gamma-ray bursts ===
Events with a duration of less than about
two seconds are classified as short gamma-ray
bursts. These account for about 30% of gamma-ray
bursts, but until 2005, no afterglow had been
successfully detected from any short event
and little was known about their origins.
Since then, several dozen short gamma-ray
burst afterglows have been detected and localized,
several of which are associated with regions
of little or no star formation, such as large
elliptical galaxies and the central regions
of large galaxy clusters. This rules out a
link to massive stars, confirming that short
events are physically distinct from long events.
In addition, there has been no association
with supernovae.The true nature of these objects
was initially unknown, and 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 theorized
to produce kilonovae, and evidence for a kilonova
associated with GRB 130603B was seen. The
mean duration of these events of 0.2 seconds
suggests (because of causality) a source of
very small physical diameter in stellar terms;
less than 0.2 light-seconds (about 60,000
km or 37,000 miles—four times the Earth's
diameter). The observation of minutes to hours
of X-ray flashes after a short gamma-ray burst
is consistent with small particles of a primary
object like a neutron star initially swallowed
by a black hole in less than two seconds,
followed by some hours of lesser energy events,
as remaining fragments of tidally disrupted
neutron star material (no longer neutronium)
remain in orbit to spiral into the black hole,
over a longer period of time. A small fraction
of short gamma-ray bursts are probably produced
by giant flares from soft gamma repeaters
in nearby galaxies.The origin of short GRBs
in kilonovae was confirmed when short GRB
170817A was detected only 1.7 s after the
detection of gravitational wave GW170817,
which was a signal from the merger of two
neutron stars.
=== Long gamma-ray bursts ===
Most observed events (70%) have a duration
of greater than two seconds and are classified
as long gamma-ray bursts. Because these events
constitute the majority of the population
and because they tend to have the brightest
afterglows, they have been observed in much
greater detail than their short counterparts.
Almost every well-studied long gamma-ray burst
has been linked to a galaxy with rapid star
formation, and in many cases to a core-collapse
supernova as well, unambiguously associating
long GRBs with the deaths of massive stars.
Long GRB afterglow observations, at high redshift,
are also consistent with the GRB having originated
in star-forming regions.
=== Ultra-long gamma-ray bursts ===
These events are at the tail end of the long
GRB duration distribution, lasting more than
10,000 seconds. They have been proposed to
form a separate class, caused by the collapse
of a blue supergiant star, a tidal disruption
event or a new-born magnetar. Only a small
number have been identified to date, their
primary characteristic being their gamma ray
emission duration. The most studied ultra-long
events include GRB 101225A and GRB 111209A.
The low detection rate may be a result of
low sensitivity of current detectors to long-duration
events, rather than a reflection of their
true frequency. A 2013 study, on the other
hand, shows that the existing evidence for
a separate ultra-long GRB population with
a new type of progenitor is inconclusive,
and further multi-wavelength observations
are needed to draw a firmer conclusion.
== Energetics and beaming ==
Gamma-ray bursts are very bright as observed
from Earth despite their typically immense
distances. An average long GRB has a bolometric
flux comparable to a bright star of our galaxy
despite a distance of billions of light years
(compared to a few tens of light years for
most visible stars). Most of this energy is
released in gamma rays, although some GRBs
have extremely luminous optical counterparts
as well. GRB 080319B, for example, was accompanied
by an optical counterpart that peaked at a
visible magnitude of 5.8, comparable to that
of the dimmest naked-eye stars despite the
burst's distance of 7.5 billion light years.
This combination of brightness and distance
implies an extremely energetic source. Assuming
the gamma-ray explosion to be spherical, the
energy output of GRB 080319B would be within
a factor of two of the rest-mass energy of
the Sun (the energy which would be released
were the Sun to be converted entirely into
radiation).Gamma-ray bursts are thought to
be highly focused explosions, with most of
the explosion energy collimated into a narrow
jet. 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. Observations suggest significant
variation in the jet angle from between 2
and 20 degrees.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.
When this effect is taken into account, typical
gamma-ray bursts are observed to have a true
energy release of about 1044 J, or about 1/2000
of a Solar mass (M☉) energy equivalent—which
is still many times the mass-energy equivalent
of the Earth (about 5.5 × 1041 J). This is
comparable to the energy released in a bright
type Ib/c supernova and within the range of
theoretical models. Very bright supernovae
have been observed to accompany several of
the nearest GRBs. Additional support for focusing
of the output of GRBs has come from observations
of strong asymmetries in the spectra of nearby
type Ic supernova and from radio observations
taken long after bursts when their jets are
no longer relativistic.Short (time duration)
GRBs appear to come from a lower-redshift
(i.e. less distant) population and are less
luminous than long GRBs. The degree of beaming
in short bursts has not been accurately measured,
but as a population they are likely less collimated
than long GRBs or possibly not collimated
at all in some cases.
== Progenitors ==
Because of the immense distances of most gamma-ray
burst sources from Earth, identification of
the progenitors, the systems that produce
these explosions, is challenging. The association
of some long GRBs with supernovae and the
fact that their host galaxies are rapidly
star-forming offer very strong evidence that
long gamma-ray bursts are associated with
massive stars. The most widely accepted mechanism
for the origin of long-duration GRBs is the
collapsar model, in which the core of an extremely
massive, low-metallicity, rapidly rotating
star collapses into a black hole in the final
stages of its evolution. Matter near the star's
core rains down towards the center and swirls
into a high-density accretion disk. The infall
of this material into a black hole drives
a pair of relativistic jets out along the
rotational axis, which pummel through the
stellar envelope and eventually break through
the stellar surface and radiate as gamma rays.
Some alternative models replace the black
hole with a newly formed magnetar, although
most other aspects of the model (the collapse
of the core of a massive star and the formation
of relativistic jets) are the same.
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 to radiation pressure.
Eta Carinae and WR 104 have been cited as
possible future gamma-ray burst progenitors.
It is unclear if any star in the Milky Way
has the appropriate characteristics to produce
a gamma-ray burst.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 galaxy halos. The
favored theory 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 energy 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.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.
=== Tidal disruption events ===
This new class of GRB-like events was first
discovered through the detection of GRB 110328A
by the Swift Gamma-Ray Burst Mission on 28
March 2011. This event had a gamma-ray duration
of about 2 days, much longer than even ultra-long
GRBs, and was detected in X-rays for many
months. It occurred at the center of a small
elliptical galaxy at redshift z = 0.3534.
There is an ongoing debate as to whether the
explosion was the result of stellar collapse
or a tidal disruption event accompanied by
a relativistic jet, although the latter explanation
has become widely favoured.
A tidal disruption event of this sort is when
a star interacts with a supermassive black
hole shredding the star, and in some cases
creating a relativistic jet which produces
bright emission of gamma ray radiation. The
event GRB 110328A (also denoted Swift J1644+57)
was initially argued to be produced by the
disruption of a main sequence star by a black
hole of several million times the mass of
the Sun, although it has subsequently been
argued that the disruption of a white dwarf
by a black hole of mass about 10 thousand
times the Sun may be more likely.
== Emission mechanisms ==
The means by which gamma-ray bursts convert
energy into radiation remains poorly understood,
and as of 2010 there was still no generally
accepted model for how this process occurs.
Any successful model of GRB emission must
explain the physical process for generating
gamma-ray emission that matches the observed
diversity of light curves, spectra, and other
characteristics. Particularly challenging
is the need to explain the very high efficiencies
that are inferred from some explosions: some
gamma-ray bursts may convert as much as half
(or more) of the explosion energy into gamma-rays.
Early observations of the bright optical counterparts
to GRB 990123 and to GRB 080319B, whose optical
light curves were extrapolations of the gamma-ray
light spectra, have suggested that inverse
Compton may be the dominant process in some
events. In this model, pre-existing low-energy
photons are scattered by relativistic electrons
within the explosion, augmenting their energy
by a large factor and transforming them into
gamma-rays.The nature of the longer-wavelength
afterglow emission (ranging from X-ray through
radio) 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.
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.
== Rate of occurrence and potential effects
on life ==
Gamma ray bursts can have harmful or destructive
effects on life. Considering the universe
as a whole, the safest environments for life
similar to that on Earth are the lowest density
regions in the outskirts of large galaxies.
Our knowledge of galaxy types and their distribution
suggests that life as we know it can only
exist in about 10% of all galaxies. Furthermore,
galaxies with a redshift, z, higher than 0.5
are unsuitable for life as we know it, because
of their higher rate of GRBs and their stellar
compactness.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
and its emission were beamed straight towards
Earth, the effects could be harmful and potentially
devastating for the ecosystems. Currently,
orbiting satellites detect on average approximately
one GRB per day. The closest observed GRB
as of March 2014 was GRB 980425, located 40
megaparsecs (130,000,000 ly) 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.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.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.
=== Effects on Earth ===
Earth's atmosphere is very effective at absorbing
high energy electromagnetic radiation such
as x-rays and gamma rays, so these types of
radiation would not reach any dangerous levels
at the surface during the burst event itself.
The immediate effect on life on Earth from
a GRB within a few parsecs would only be a
short increase in ultraviolet radiation at
ground level, lasting from less than a second
to tens of seconds. This ultraviolet radiation
could potentially reach dangerous levels depending
on the exact nature and distance of the burst,
but it seems unlikely to be able to cause
a global catastrophe for life on Earth.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, 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 levels in the atmosphere would wash
out and produce nitric 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 parsecs, 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.
==== Hypothetical effects on Earth in the
past ====
GRBs close enough to affect life in some way
might occur once every five million years
or so — around a thousand times since life
on Earth began.The major Ordovician–Silurian
extinction events 450 million years ago may
have been caused by a GRB. The late Ordovician
species of trilobites 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.
==== WR 104: A nearby GRB candidate ====
A Wolf–Rayet star in WR 104, about 8,000
light-years (2,500 pc) away, is considered
a nearby GRB candidate that could have destructive
effects on terrestrial life. It is expected
to explode in a core-collapse-supernova at
some point within the next 500,000 years and
it is possible that this explosion will create
a GRB. If that happens, there is a small chance
that Earth will be in the path of its gamma
ray jet.
== GRB candidates in the Milky Way ==
No gamma-ray burst from within our own galaxy,
the Milky Way, has 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.
== See also ==
List of gamma-ray bursts
GRB 020813
GRB 130427A
GRB 080916C
Soft gamma repeater
Gamma-ray Search for Extraterrestrial Intelligence
Stellar evolution
Terrestrial gamma-ray flashes
== Notes ==
== Citations ==
== References ==
== Further reading ==
Vedrenne, G.; Atteia, J.-L. (2009). Gamma-Ray
Bursts: The brightest explosions in the Universe.
Springer. ISBN 978-3-540-39085-5.
Chryssa Kouveliotou; Stanford E. Woosley;
Ralph A. M. J., eds. (2012). Gamma-ray bursts.
Cambridge: Cambridge University Press. ISBN
978-0-521-66209-3.
== External links ==
GRB mission sitesSwift Gamma-Ray Burst Mission:
Official NASA Swift Homepage
UK Swift Science Data Centre
Swift Mission Operations Center at Penn State
HETE-2: High Energy Transient Explorer (Wiki
entry)
INTEGRAL: INTErnational Gamma-Ray Astrophysics
Laboratory (Wiki entry)
BATSE: Burst and Transient Source Explorer
Fermi Gamma-ray Space Telescope (Wiki entry)
AGILE: Astro-rivelatore Gamma a Immagini Leggero
(Wiki entry)
EXIST: Energetic X-ray Survey Telescope
Gamma Ray Burst Catalog at NASAGRB follow-up
programsThe Gamma-ray bursts Coordinates Network
(GCN) (Wiki entry)
BOOTES: Burst Observer and Optical Transient
Exploring System (Wiki entry)
GROND: Gamma-Ray Burst Optical Near-infrared
Detector (Wiki entry)
KAIT: The Katzman Automatic Imaging Telescope
(Wiki entry)
MASTER: Mobile Astronomical System of the
Telescope-Robots
ROTSE: Robotic Optical Transient Search Experiment
(Wiki entry)
