Gamma-ray bursts (
GRBs) are
flashes of
gamma rays associated with
extremely energetic explosions in distant
galaxies. They are the most
luminous electromagnetic events occurring in the
universe. Bursts can last from milliseconds
to nearly an hour, although a typical burst lasts a few seconds.
The initial burst is usually followed by a longer-lived "afterglow"
emitting at longer wavelengths (
X-ray,
ultraviolet,
optical,
infrared,
and
radio).
Most observed GRBs are believed to be a narrow beam of intense
radiation released during a
supernova
event, as a rapidly rotating, high-mass star collapses to form a
black hole. A subclass of GRBs (the
"short" bursts) appear to originate from a different process,
possibly the merger of
binary neutron
stars.
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 could cause a
mass extinction on Earth.
GRBs were first detected in 1967 by the
Vela satellites, a series of satellites
designed to detect covert nuclear weapons tests. Hundreds of
theoretical models were proposed to explain these bursts in the
years following their discovery, 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. 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 and
connecting long GRBs with the deaths of massive stars.
History
Gamma-ray bursts were discovered 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 USSR
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
Scientific 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 as an
Astrophysical Journal article
entitled "Observations of Gamma-Ray Bursts of Cosmic Origin".
Many theories were advanced to explain these bursts, most of which
posited nearby sources within the
Milky
Way Galaxy. Little progress was made until the 1991 launch of
the
Compton Gamma Ray
Observatory and its Burst and Transient Source Explorer
(
BATSE) instrument, an extremely
sensitive gamma-ray detector. This instrument provided crucial data
indicating that the distribution of GRBs is
isotropic—not biased towards any particular
direction in space, such as toward the
Galactic plane or the
Galactic center. Because of the flat
structure of the Milky Way Galaxy, nearly all sources within our
own galaxy are 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 were still consistent with an
isotropic distribution.
For decades after the discovery of GRBs, astronomers searched for a
counterpart: any astronomical object in positional coincidence with
a recently observed burst. Astronomers considered many distinct
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.
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. Early
searches for this "
afterglow" were unsuccessful,
largely due to the difficulties in observing 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. Ground-based telescopes later
identified a fading optical counterpart as well. 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 by a bright supernova (
SN
1998bw), 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 2009 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 a few seconds before the
initial burst itself has ended.
New developments over the past few years include the recognition of
short gamma-ray bursts as a separate class (likely due to 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 most
distant (
GRB 090423) objects in the
universe.
Classification
Gamma-ray burst light curves
While most astronomical transient sources have simple and
consistent time structures (typically a rapid brightening followed
by gradual fading, as in a
nova or
supernova), 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 on differences in the shapes of
light curves, but these differences may not actually be caused by a
variety in the types of progenitors. 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.
Long gamma-ray bursts
Most observed events 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 studied in much
greater detail than their short counterparts. Almost every
well-studied long gamma-ray burst has been associated with a
rapidly star-forming galaxy and in many cases a
core-collapse supernova as well,
unambiguously linking long GRBs with the deaths of massive
stars.
Short gamma-ray bursts
Events with a duration of less than about two seconds are
classified as short gamma-ray bursts. 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, including large elliptical galaxies and the intracluster
medium. This rules out an association with massive stars,
confirming that short events are physically distinct from long
events. The true nature of these objects (or even whether the
current classification scheme is accurate) remains unknown,
although the leading hypothesis is that they originate from the
mergers of binary neutron stars. A small fraction of short
gamma-ray bursts are probably associated with giant flares from
soft gamma repeaters in nearby
galaxies.
Energetics and beaming
Gamma-ray bursts are very bright as observed from Earth despite
typical immense distances. An average long GRB has comparable
bolometric flux to a bright
Galactic star despite a distance of billions of light years
(compared to a few tens of light years for most 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 requires 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.)
No known process in the Universe can produce this much energy in
such a short time. However, gamma-ray bursts are thought to be
highly focused explosions, with most of the explosion energy
collimated into a narrow
jet
traveling at speeds exceeding 99.995% of the
speed of light. The approximate angular width
of the jet (that is, the degree of beaming) can be estimated
directly by observing "jet breaks" in afterglow light curves: a
time after which the slowly-decaying afterglow abruptly begins to
fade rapidly as the jet slows down 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 beamed, 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 corrected for the use of
estimates of the beaming angle, typical gamma-ray bursts have a
true energy release of about 10
44 J, or about
1/2000 of a
Solar mass energy equivalent.
This is comparable to the energy released in a bright
type Ib/c supernova (sometimes termed a "
hypernova") and within the range of theoretical
models. Very bright supernovae have been observed to accompany
several of the nearest GRBs. Additional support for strong beaming
in 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 GRBs appear to come from a lower-redshift 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 beamed than long GRBs or possibly not beamed 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 particularly 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 Galactic analogs 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 due to
radiation pressure.
Eta Carinae and
WR 104
have been cited as possible 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 where
no massive stars are present, such as
galaxy
halos and
intergalactic
space. 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 due to the release of energy via
gravitational radiation
until the neutron stars suddenly rip each other apart due to
tidal forces and collapse into a single
black hole. The infall of matter into the new black hole in an
accretion disk then powers an explosion, similar 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.
Emission mechanisms
The means by which gamma-ray bursts convert energy into radiation
remains poorly understood, and as of 2007 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. Recent observations of the bright optical
counterpart of
GRB 080319B, whose light
curve was correlated with the gamma-ray light curve, has 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.
Rates and impacts on life
Currently orbiting satellites detect an average of about one
gamma-ray burst per day. Because gamma-ray bursts are visible to
distances encompassing most of the
observable universe, a volume
encompassing many billions of galaxies, this suggests that
gamma-ray bursts must be exceedingly rare events per galaxy.
Measuring the exact rate is difficult, but for a galaxy of
approximately the same size as the
Milky
Way, the expected rate (for long GRBs) is about one burst every
100,000 to 1,000,000 years. Only a few percent of these would be
beamed towards Earth. Estimates of rates of short GRBs are even
more uncertain because of the unknown beaming fraction, but are
probably comparable.
A gamma-ray burst in the Milky Way, if close enough to Earth and
beamed towards it, could have significant effects on the
biosphere. The absorption of radiation in the
atmosphere would cause
photodissociation of
nitrogen, generating
nitric
oxide that would act as a catalyst to destroy
ozone. According to a 2004 study, a GRB at a distance
of about a
kiloparsec could destroy up to
half of Earth's
ozone layer; the direct
UV irradiation from the burst combined with additional solar UV
radiation passing through the diminished ozone layer could then
have potentially significant impacts on the
food chain and potentially trigger a mass
extinction. The authors estimate that one such burst is expected
per billion years, and hypothesize that the
Ordovician-Silurian
extinction event could have been the result of such a
burst.
There are strong indications that long gamma-ray bursts
preferentially or exclusively occur in regions of low metallicity.
Because the Milky Way has been metal-rich since before the Earth
formed, this effect may diminish or even eliminate the possibility
that a long gamma-ray burst has occurred within the Milky Way
within the past billion years. No such metallicity biases are known
for short gamma-ray bursts. Thus, depending on their local rate and
beaming properties, the possibility for a nearby event to have had
a large impact on Earth at some point in geological time may still
be significant.
See also
Footnotes
Notes
- Podsiadlowski 2004
- Melott 2004
- Hurley
2003
- Schilling
2002, p.12–16
- Klebesadel
1973
- Meegan 1992
- Schilling
2002, p.36–37
- Paczyński 1999, p.
6
- Piran 1992
- Lamb 1995
- Hurley 1986,
p. 33
- A notable exception is the 5 March event of 1979, an extremely bright burst
that was successfully localized to supernova remnant N49 in the
Large Magellanic Cloud. This event is
now interpreted as a magnetar giant
flare, more related to SGR flares than "true" gamma-ray
bursts.
- Pedersen
1987
- Hurley 1992
- Paczynski 1993
- GRBs are named after the date on which they are discovered: the
first two digits being the year, followed by the two-digit month
and two-digit day. If two or more GRBs occur on a given day, the
letter 'A' is appended to the name for the first burst identified,
'B' for the second, and so on.
- van Paradijs
1997
- Schilling
2002, p. 102
- Reichart
1995
- Schilling
2002, p. 118–123
- Galama 1998
- Ricker 2003
- McCray 2008
- Gehrels
2004
- Akerlof 2003
- Akerlof
1999
- Bloom 2009
- Reddy 2009
- Katz 2002, p. 37
- Marani 1997
- Lazatti 2005
- Fishman & Meegan
1995
- Simić 2005
- The 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.
- Kouveliotou
1994
- Horvath
1998
- Hakkila 2003
- Chattopadhyay 2007
- Virgili 2009
- Woosley &
Bloom 2006
- Bloom 2006
- Hjorth 2005
- Berger 2007
- Nakar 2007
- Frederiks
2008
- Hurley 2005
- Racusin
2008
- Rykoff 2009
- Abdo 2009
- Sari 1999
- Burrows 2006
- Frail 2001
- Mazzali 2005
- Frail
2000
- Prochaska
2006
- Watson 2006
- Grupe 2006
- MacFadyen 1999
- Metzger 2007
- Plait 2008
- Stanek 2006
- Abbott 2007
- Kochanek
1993
- Vietri 1998
- MacFadyen
2006
- Blinnikov
1984
- Cline 1996
- Stern 2007
- Fishman, G.
1995
- Fan & Piran
2006
- Wozniak
2009
- Meszaros
1997
- Sari 1998
- Nousek 2006
- Guetta 2006
- Thorsett
1995
- Wanjek 2005
- Ejzak 2007
Books
- Gilbert Vedrenne, Jean-Luc Atteia, Gamma-Ray
Bursts: The brightest explosions in the Universe (Springer Praxis
Books / Astronomy and Planetary Sciences) (Hardcover) ISBN
3540390855 ISBN 978-3540390855.
http://www.springer.com/astronomy/practical+astronomy/book/978-3-540-39085-5
References
External links
- GRB Mission Sites
- GRB Follow-up Programs
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