Gamma-ray bursts are short-lived bursts of gamma-ray photons, the most energetic form of light. At least some of them are associated with a special type of supernovae, the explosions marking the deaths of especially massive stars.
Lasting anywhere from a few milliseconds to several minutes, gamma-ray bursts (GRBs) shine hundreds of times brighter than a typical supernova and about a million trillion times as bright as the Sun, making them briefly the brightest source of cosmic gamma-ray photons in the observable Universe. GRBs are detected roughly once per day from wholly random directions of the sky.
Until recently, GRBs were arguably the biggest mystery in high-energy astronomy. They were discovered serendipitously in the late 1960s by U.S. military satellites which were on the look out for Soviet nuclear testing in violation of the atmospheric nuclear test ban treaty. These satellites carried gamma ray detectors since a nuclear explosion produces gamma rays. As recently as the early 1990s, astronomers didn't even know if GRBs originated at the edge of our solar system, in our Milky Way Galaxy or incredibly far away near the edge of the observable Universe. (That is, they didn't know how far away GRBs were to within a factor of a few billion light years!) But now a slew of satellite observations, follow-up ground-based observations, and theoretical work have allowed astronomers to link GRBs to supernovae in distant galaxies.
In this series of articles we will explore what astronomers know about gamma-ray bursts, what they think causes them, the evidence for the theories, and the lingering mysteries. Along the way we'll encounter powerful hypernovas and strange Wolf-Rayet stars.
Gamma-ray bursts are separated into two classes: long-duration bursts and short-duration bursts. Long duration ones last more than 2 seconds and short-duration ones last less than 2 seconds. However, this doesn't tell the whole story. That is because short duration bursts range from a few milliseconds to 2 seconds with an average duration time of about 0.3 seconds (300 milliseconds). The long-duration bursts last anywhere from 2 seconds to a few hundreds of seconds (several minutes) with an average duration time of about 30 seconds.
Astronomers think that long and short duration GRBs are created by fundamentally different physical properties. And whereas they now are fairly confident of what drives the long GRBs, there are only theories when it comes to what drives short-duration bursts. Here we will concern ourselves with long-duration bursts and address short-duration bursts later on.
Astronomers now know that long-duration gamma-ray bursts can originate near the farthest edges of the observable Universe. The stars linked to them are typically on the order of billions of light years away. This means the light from them traveling at "the speed of light" (about 186,000 miles per second or 300,000 kilometers per second) took that many years to reach us. The Earth itself is about 4 billion years old, so some GRBs occurred when our planet was still a fiery newborn, before the first microbes formed, even before the oceans had formed. Some GRBs that we have observed actually originated while the universe was only a few billion years old.
These stars are so far away that we don't actually see the light from them before they explode. They belong to an early generation of stars (e.g. maybe the second or third generation of stars) in the Universe. Although such stars long ago died, only now is the light from their explosive deaths reaching us.
That's not to say astronomers have no idea what kind of stars produce gamma-ray bursts. Working with large amounts of data collected over the past 15 years with special instruments aboard satellites, such as NASA's Compton Gamma-Ray Observatory and the joint Italian-Dutch BeppoSAX, and using computer simulations, astronomers have developed a working model of the kind of star that produces a GRB.
The theory describing how gamma-ray bursts originate is called the "collapsar" model. Dr. Stan Woosley of the University of California, Santa Cruz, and one of the architects of the model, coined this term because the model involves the collapse of the core of a special kind of star. This core collapse occurs while the outer layers of the star explode in an especially energetic supernova dubbed a "hypernova" by astronomers. (Here we'll refer to the theory as the "collapsar/hypernova" model to keep in mind both the core collapse and the supernova explosion.
In looking for the stellar candidates capable of producing a hypernova, astronomers are confronted with the fact that gamma-ray bursts are so far away not even the most powerful telescopes can see the stars thought to be responsible for those observed so far. But proponents of the collapsar/hypernova model think they have an idea. The kind of star is very heavy, very hot, and prone to episodic fits in which large amounts of material is ejected from it. Such a star is called a "Wolf-Rayet" star after two 19th Century French astronomers, Charles Wolf and Georges Rayet, who studied the first example.
Wolf-Rayet stars are linked to hypernovae, which in turn are associated with gamma-ray bursts. Although the exact picture has not been worked out, astronomers think the gamma-ray photons are probably produced inside the star. The explosion originates at the center of these massive stars. While a black hole forms from the collapsing core, this explosion sends a blast wave moving through the star at speeds close to the speed of light. The gamma rays are created when the blast wave collides with stellar material still inside the star. These gamma rays burst out from the star's surface just ahead of the blast wave. Behind the gamma rays, the blast wave pushes the stellar material outward.
Erupting through the star surface, the blast wave of stellar material sweeps through space at nearly the speed of light, colliding with intervening gas and dust, producing additional emission of photons. These emissions are believed responsible for the "afterglow" of progressively less energetic photons, starting with X rays and then visible light and radio waves. (Whether additional gamma rays are also produced in this "afterglow" phase is still not settled, although some evidence indicates they are.) The afterglow phase can last for days or even weeks. Under the collapsar model, we detect both the GRB and the afterglow when the Earth happens to lie along or very near the axis of the blast. In general, there are many more GRBs than are detected simply because we are not favorably aligned to see them.
A dramatic development in the last several years has been the measurement and localization of fading x-ray signals a number of GRBs by the Beppo-SAX satellite . These afterglows, lasting typically for weeks, made possible the optical and radio detection of afterglows, which, as fading beacons, mark the location of the fiery and brief GRB event. These afterglows in turn enabled the measurement of redshift distances, the identification of host galaxies, and the confirmation that GRB were, as suspected, at cosmological distances of the order of billions of light-years, similar to those of the most distant galaxies and quasars. Even at those distances they appear so bright that their energy output during its brief peak period has to be larger than that of any other type of source, of the order of a solar rest-mass if isotropic, or some percent of that if collimated. This energy output rate is comparable to burning up the entire mass-energy of the sun in a few tens of seconds, or to emit over that same period of time as much energy as our entire Milky Way does in a hundred years.
The energy density in a GRB event is so large that an optically thick pair/photon fireball is expected to form, which will expand carrying with itself some fraction of baryons. The main challenge in the early 90's was not so much the ultimate energy source, but how to turn this energy into predominantly gamma rays with the right nonthermal broken power law spectrum with the right temporal behavior. To explain the observations, the relativistic fireball shock model was proposed by Rees and Meszaros (1992, 1994), following pioneering earlier earlier work by Cavallo & Rees, Paczynski, Goodman and Shemi & Piran. This model has been quite succesful in explaining the various features of GRB.
Astronomers have detected the strongest radiation blast from deep space ever known, exceeding the power of almost 9,000 exploding stars.
The gamma ray burst occurred 12.2 billion light years away in the constellation Carina. Its light has taken most of the age of the universe to reach us.
Jets of material powered by processes that are not yet fully understood are thought to blast outwards at nearly the speed of light, generating intense gamma rays.
The new explosion, designated GRB 080916C, was spotted last year by the American space agency Nasa's Fermi Gamma-ray Space Telescope, which is designed to detect gamma radiation.
Astronomers soon discovered that the gamma ray burst belonged in the record books.
The short-lived blast, described today in the online version of the journal Science, was more powerful than nearly 9,000 ordinary supernovae, or exploding stars.
Scientists calculated that the material emitting the gamma rays must have been moving at 99.9999% the speed of light.
The explosion was enigmatic as well as spectacular due to a curious time delay separating the highest-energy emissions from the lowest.
Scientists are still trying to understand the reason for the time delay, which may have a straightforward physical cause or be due to peculiar quantum effects.
Professor Peter Michelson, a member of the Fermi Gamma-ray Space Telescope team, said: 'Burst emissions at these energies are still poorly understood.
'This one burst raises all sorts of questions. In a few years, we'll have a fairly good sample of bursts, and may have some answers.
Lasting anywhere from a few milliseconds to several minutes, gamma-ray bursts (GRBs) shine hundreds of times brighter than a typical supernova and about a million trillion times as bright as the Sun, making them briefly the brightest source of cosmic gamma-ray photons in the observable Universe. GRBs are detected roughly once per day from wholly random directions of the sky.
Until recently, GRBs were arguably the biggest mystery in high-energy astronomy. They were discovered serendipitously in the late 1960s by U.S. military satellites which were on the look out for Soviet nuclear testing in violation of the atmospheric nuclear test ban treaty. These satellites carried gamma ray detectors since a nuclear explosion produces gamma rays. As recently as the early 1990s, astronomers didn't even know if GRBs originated at the edge of our solar system, in our Milky Way Galaxy or incredibly far away near the edge of the observable Universe. (That is, they didn't know how far away GRBs were to within a factor of a few billion light years!) But now a slew of satellite observations, follow-up ground-based observations, and theoretical work have allowed astronomers to link GRBs to supernovae in distant galaxies.
In this series of articles we will explore what astronomers know about gamma-ray bursts, what they think causes them, the evidence for the theories, and the lingering mysteries. Along the way we'll encounter powerful hypernovas and strange Wolf-Rayet stars.
Gamma-ray bursts are separated into two classes: long-duration bursts and short-duration bursts. Long duration ones last more than 2 seconds and short-duration ones last less than 2 seconds. However, this doesn't tell the whole story. That is because short duration bursts range from a few milliseconds to 2 seconds with an average duration time of about 0.3 seconds (300 milliseconds). The long-duration bursts last anywhere from 2 seconds to a few hundreds of seconds (several minutes) with an average duration time of about 30 seconds.
Astronomers think that long and short duration GRBs are created by fundamentally different physical properties. And whereas they now are fairly confident of what drives the long GRBs, there are only theories when it comes to what drives short-duration bursts. Here we will concern ourselves with long-duration bursts and address short-duration bursts later on.
Astronomers now know that long-duration gamma-ray bursts can originate near the farthest edges of the observable Universe. The stars linked to them are typically on the order of billions of light years away. This means the light from them traveling at "the speed of light" (about 186,000 miles per second or 300,000 kilometers per second) took that many years to reach us. The Earth itself is about 4 billion years old, so some GRBs occurred when our planet was still a fiery newborn, before the first microbes formed, even before the oceans had formed. Some GRBs that we have observed actually originated while the universe was only a few billion years old.
These stars are so far away that we don't actually see the light from them before they explode. They belong to an early generation of stars (e.g. maybe the second or third generation of stars) in the Universe. Although such stars long ago died, only now is the light from their explosive deaths reaching us.
That's not to say astronomers have no idea what kind of stars produce gamma-ray bursts. Working with large amounts of data collected over the past 15 years with special instruments aboard satellites, such as NASA's Compton Gamma-Ray Observatory and the joint Italian-Dutch BeppoSAX, and using computer simulations, astronomers have developed a working model of the kind of star that produces a GRB.
The theory describing how gamma-ray bursts originate is called the "collapsar" model. Dr. Stan Woosley of the University of California, Santa Cruz, and one of the architects of the model, coined this term because the model involves the collapse of the core of a special kind of star. This core collapse occurs while the outer layers of the star explode in an especially energetic supernova dubbed a "hypernova" by astronomers. (Here we'll refer to the theory as the "collapsar/hypernova" model to keep in mind both the core collapse and the supernova explosion.
In looking for the stellar candidates capable of producing a hypernova, astronomers are confronted with the fact that gamma-ray bursts are so far away not even the most powerful telescopes can see the stars thought to be responsible for those observed so far. But proponents of the collapsar/hypernova model think they have an idea. The kind of star is very heavy, very hot, and prone to episodic fits in which large amounts of material is ejected from it. Such a star is called a "Wolf-Rayet" star after two 19th Century French astronomers, Charles Wolf and Georges Rayet, who studied the first example.
Wolf-Rayet stars are linked to hypernovae, which in turn are associated with gamma-ray bursts. Although the exact picture has not been worked out, astronomers think the gamma-ray photons are probably produced inside the star. The explosion originates at the center of these massive stars. While a black hole forms from the collapsing core, this explosion sends a blast wave moving through the star at speeds close to the speed of light. The gamma rays are created when the blast wave collides with stellar material still inside the star. These gamma rays burst out from the star's surface just ahead of the blast wave. Behind the gamma rays, the blast wave pushes the stellar material outward.
Erupting through the star surface, the blast wave of stellar material sweeps through space at nearly the speed of light, colliding with intervening gas and dust, producing additional emission of photons. These emissions are believed responsible for the "afterglow" of progressively less energetic photons, starting with X rays and then visible light and radio waves. (Whether additional gamma rays are also produced in this "afterglow" phase is still not settled, although some evidence indicates they are.) The afterglow phase can last for days or even weeks. Under the collapsar model, we detect both the GRB and the afterglow when the Earth happens to lie along or very near the axis of the blast. In general, there are many more GRBs than are detected simply because we are not favorably aligned to see them.
A dramatic development in the last several years has been the measurement and localization of fading x-ray signals a number of GRBs by the Beppo-SAX satellite . These afterglows, lasting typically for weeks, made possible the optical and radio detection of afterglows, which, as fading beacons, mark the location of the fiery and brief GRB event. These afterglows in turn enabled the measurement of redshift distances, the identification of host galaxies, and the confirmation that GRB were, as suspected, at cosmological distances of the order of billions of light-years, similar to those of the most distant galaxies and quasars. Even at those distances they appear so bright that their energy output during its brief peak period has to be larger than that of any other type of source, of the order of a solar rest-mass if isotropic, or some percent of that if collimated. This energy output rate is comparable to burning up the entire mass-energy of the sun in a few tens of seconds, or to emit over that same period of time as much energy as our entire Milky Way does in a hundred years.
The energy density in a GRB event is so large that an optically thick pair/photon fireball is expected to form, which will expand carrying with itself some fraction of baryons. The main challenge in the early 90's was not so much the ultimate energy source, but how to turn this energy into predominantly gamma rays with the right nonthermal broken power law spectrum with the right temporal behavior. To explain the observations, the relativistic fireball shock model was proposed by Rees and Meszaros (1992, 1994), following pioneering earlier earlier work by Cavallo & Rees, Paczynski, Goodman and Shemi & Piran. This model has been quite succesful in explaining the various features of GRB.
Astronomers have detected the strongest radiation blast from deep space ever known, exceeding the power of almost 9,000 exploding stars.
The gamma ray burst occurred 12.2 billion light years away in the constellation Carina. Its light has taken most of the age of the universe to reach us.
Jets of material powered by processes that are not yet fully understood are thought to blast outwards at nearly the speed of light, generating intense gamma rays.
The new explosion, designated GRB 080916C, was spotted last year by the American space agency Nasa's Fermi Gamma-ray Space Telescope, which is designed to detect gamma radiation.
Astronomers soon discovered that the gamma ray burst belonged in the record books.
The short-lived blast, described today in the online version of the journal Science, was more powerful than nearly 9,000 ordinary supernovae, or exploding stars.
Scientists calculated that the material emitting the gamma rays must have been moving at 99.9999% the speed of light.
The explosion was enigmatic as well as spectacular due to a curious time delay separating the highest-energy emissions from the lowest.
Scientists are still trying to understand the reason for the time delay, which may have a straightforward physical cause or be due to peculiar quantum effects.
Professor Peter Michelson, a member of the Fermi Gamma-ray Space Telescope team, said: 'Burst emissions at these energies are still poorly understood.
'This one burst raises all sorts of questions. In a few years, we'll have a fairly good sample of bursts, and may have some answers.
On 2008 March 19, the northern sky was the
stage of a spectacular optical transient that for a few seconds remained
visible to the naked eye. The transient was associated with GRB
080319B, a gamma-ray burst (GRB) at a luminosity distance of about 6 Gpc
(standard cosmology), making it the most luminous optical object ever
recorded by humankind. We present comprehensive sky monitoring and
multicolor optical follow-up observations of GRB 080319B collected by
the RAPTOR telescope network covering the development of the explosion
and the afterglow before, during, and after the burst. The extremely
bright prompt optical emission revealed features that are normally not
detectable. The optical and gamma-ray variability during the explosion
are correlated, but the optical flux is much greater than can be
reconciled with single-emission mechanism and a flat gamma-ray spectrum.
This extreme optical behavior is best understood as synchrotron
self-Compton model (SSC). After a gradual onset of the gamma-ray
emission, there is an abrupt rise of the prompt optical flux, suggesting
that variable self-absorption dominates the early optical light curve.
Our simultaneous multicolor optical light curves following the flash
show spectral evolution consistent with a rapidly decaying red component
due to large-angle emission and the emergence of a blue forward-shock
component from interaction with the surrounding environment. While
providing little support for the reverse shock that dominates the early
afterglow, these observations strengthen the case for the universal role
of the SSC mechanism in generating GRBs.
"This burst was a whopper," said Swift principal investigator Neil Gehrels of NASA's Goddard Space Flight Center in Greenbelt, Md. "It blows away every gamma ray burst we've seen so far."
Swift's Burst Alert Telescope picked up the burst at 2:12 a.m. EDT, March 19, and pinpointed the coordinates in the constellation Boötes. Telescopes in space and on the ground quickly moved to observe the afterglow. The burst is named GRB 080319B, because it was the second gamma ray burst detected that day.
Swift's other two instruments, the X-ray Telescope and the Ultraviolet/Optical Telescope, also observed brilliant afterglows. Several ground-based telescopes saw the afterglow brighten to visual magnitudes between 5 and 6 in the logarithmic magnitude scale used by astronomers. The brighter an object is, the lower its magnitude number. From a dark location in the countryside, people with normal vision can see stars slightly fainter than magnitude 6. That means the afterglow would have been dim, but visible to the naked eye.
Later that evening, the Very Large Telescope in Chile and the Hobby-Eberly Telescope in Texas measured the burst's redshift at 0.94. A redshift is a measure of the distance to an object. A redshift of 0.94 translates into a distance of 7.5 billion light years, meaning the explosion took place 7.5 billion years ago, a time when the universe was less than half its current age and Earth had yet to form. This is more than halfway across the visible universe.
"No other known object or type of explosion could be seen by the naked eye at such an immense distance," said Swift science team member Stephen Holland of Goddard. "If someone just happened to be looking at the right place at the right time, they saw the most distant object ever seen by human eyes without optical aid."
GRB 080319B's optical afterglow was 2.5 million times more luminous than the most luminous supernova ever recorded, making it the most intrinsically bright object ever observed by humans in the universe. The most distant previous object that could have been seen by the naked eye is the nearby galaxy M33, a relatively short 2.9 million light-years from Earth.
Analysis of GRB 080319B is just getting underway, so astronomers don't know why this burst and its afterglow were so bright. One possibility is the burst was more energetic than others, perhaps because of the mass, spin, or magnetic field of the progenitor star or its jet. Or perhaps it concentrated its energy in a narrow jet that was aimed directly at Earth.
GRB 080319B was one of four bursts that Swift detected, a Swift record for one day. "Coincidentally, the passing of Arthur C. Clarke seems to have set the universe ablaze with gamma ray bursts," said Swift science team member Judith Racusin of Penn State University in University Park, Pa.
"This burst was a whopper," said Swift principal investigator Neil Gehrels of NASA's Goddard Space Flight Center in Greenbelt, Md. "It blows away every gamma ray burst we've seen so far."
Swift's Burst Alert Telescope picked up the burst at 2:12 a.m. EDT, March 19, and pinpointed the coordinates in the constellation Boötes. Telescopes in space and on the ground quickly moved to observe the afterglow. The burst is named GRB 080319B, because it was the second gamma ray burst detected that day.
Swift's other two instruments, the X-ray Telescope and the Ultraviolet/Optical Telescope, also observed brilliant afterglows. Several ground-based telescopes saw the afterglow brighten to visual magnitudes between 5 and 6 in the logarithmic magnitude scale used by astronomers. The brighter an object is, the lower its magnitude number. From a dark location in the countryside, people with normal vision can see stars slightly fainter than magnitude 6. That means the afterglow would have been dim, but visible to the naked eye.
Later that evening, the Very Large Telescope in Chile and the Hobby-Eberly Telescope in Texas measured the burst's redshift at 0.94. A redshift is a measure of the distance to an object. A redshift of 0.94 translates into a distance of 7.5 billion light years, meaning the explosion took place 7.5 billion years ago, a time when the universe was less than half its current age and Earth had yet to form. This is more than halfway across the visible universe.
"No other known object or type of explosion could be seen by the naked eye at such an immense distance," said Swift science team member Stephen Holland of Goddard. "If someone just happened to be looking at the right place at the right time, they saw the most distant object ever seen by human eyes without optical aid."
GRB 080319B's optical afterglow was 2.5 million times more luminous than the most luminous supernova ever recorded, making it the most intrinsically bright object ever observed by humans in the universe. The most distant previous object that could have been seen by the naked eye is the nearby galaxy M33, a relatively short 2.9 million light-years from Earth.
Analysis of GRB 080319B is just getting underway, so astronomers don't know why this burst and its afterglow were so bright. One possibility is the burst was more energetic than others, perhaps because of the mass, spin, or magnetic field of the progenitor star or its jet. Or perhaps it concentrated its energy in a narrow jet that was aimed directly at Earth.
GRB 080319B was one of four bursts that Swift detected, a Swift record for one day. "Coincidentally, the passing of Arthur C. Clarke seems to have set the universe ablaze with gamma ray bursts," said Swift science team member Judith Racusin of Penn State University in University Park, Pa.
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