Gamma ray burst
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Image:Gammarayburst-GRB990123.jpeg Gamma-ray bursts (GRBs) are the most luminous physical phenomena in the universe known to the field of astronomy. They consist of flashes of gamma rays that last from seconds to hours, the longer ones being followed by several days of X-ray afterglow. These flashes occur at apparently random positions in the sky about once per day.
The hypothetical objects that produce gamma-ray bursts are called gamma-ray bursters. There are two types of these bursters, long-duration bursters and short duration bursters.
There is now almost universal agreement in the astrophysics community that the long-duration bursts (> 2 sec) are associated with the beamed energy from a specific kind of hypernova event. The death of supermassive stars once its silicon "burning" is complete with a Zero Age Main Sequence (ZAMS) mass between 40 and 100 solar masses causes a direct collapse of the core to a black hole (vs. a slower "fall back" collapse if the ZAMS mass is between 25 and 40 and no black hole creation at all in stars with a ZAMS mass between 100-250 solar masses due to the resultant pair-instability supernova leaving no core remnant at all). The close connection between GRB's and Type Ib/c supernovae shows that the progenitor stars are almost exclusively low metallicity (at ZAMS), fast rotating Wolf-Rayet stars.
The angular momentum of the fast rotating Wolf-Rayet star causes the collapsing object to form a spinning accretion torus around the rapidly rotating black hole (rather like water spiraling round a plug-hole). The rotation axis area (polar regions) is quickly cleared of gas such that just 10 seconds after the "burning" ended it has around a tenth of the density of the equatorial region, allowing the energy to be released in two jets along the rotation axis. If Earth happens to lie along the rotation axis, it receives a huge burst of gamma-rays. If the Earth does not lie along the rotation axis then an x-ray flash will be observed as the relativistic jet and attendant shock interacts with the material expelled earlier in a stellar wind by the progenitor star.
The most promising model for the short duration bursts (< 2 sec but typically averaging 0.3 sec) is that developed by Martin Rees in the 1990s. In this model two neutron stars coalesce or a neutron star is devoured by a black hole. This causes an enormous release of gravitational potential energy. The lack of material around such a system means that the energy release stops as soon as the merger is complete (hence the short duration of the burst).
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Discovery
Cosmic gamma-ray bursts were discovered in the late 1960s by the US Vela nuclear test detection satellites. The Velas were built to detect radiation emitted by nuclear weapons tests, but they picked up occasional bursts of gamma rays from unknown sources. In 1973 researchers at the US Los Alamos National Laboratory were able to use the data from the satellites to determine that the bursts came from deep space.
Gamma ray bursts can only be observed directly from space, as the atmosphere blocks gamma rays. Astronomers believed that once better gamma-ray detectors were put in orbit, they would be able to quickly pin down the locations of the GRBs. This belief was based on prior experience with X-ray sources. However, improved sensors launched in the 1970s did not have sufficient spatial resolution to pinpoint the location of the bursts for detailed study, and optical searches of the indicated regions of origin showed nothing of interest.
Template:Unsolved Further information on the burst sources proved hard to obtain, and led to more questions than answers. The first question posed by the GRBs was: are they local to the Milky Way, or do they occur in the distant reaches of the Universe? The second was: what mechanism causes the bursts? If they do occur in the distant Universe, the mechanism must be capable of producing enormous amounts of energy.
Little progress was made on the matter through the 1980s, but in April 1991, NASA launched the Compton Gamma Ray Observatory. One of the instruments on board Compton was the Burst And Transient Source Experiment (BATSE). This device could detect gamma-ray bursts and locate their positions in the sky with reasonable accuracy. BATSE established that there were at least two categories of gamma ray burst: hard gamma ray bursts and soft gamma ray repeaters.
BATSE detected two or three GRBs each day, and found that they are randomly distributed over the entire sky. If the bursts were occurring in our own Galaxy, they would be preferentially distributed in the galactic plane. Even if they were associated with the galactic halo, they would still be preferentially distributed towards the galactic center. If that were the case, nearby galaxies would be expected to have similar haloes, but they did not show up as "hot spots" of faint gamma-ray bursts.
To many astronomers, this implied that the GRBs originated in the distant universe, but that led to the problem of finding a mechanism that could generate so much energy. Other theorists were also still able to come up with "local" models for the GRBs. One theory was that we were only seeing GRBs in the near vicinity, thus we should not expect any preferential distribution in the sky. One suggested possibility was that the GRBs originated in the Oort Cloud.
A debate between astronomers of the "local" model versus the extra-galactic model was set up on April 22, 1995. This was the 75th aniversary of the Curis - Shapley "Great Debate" over the "local" versus extra-galactic location of other galaxies. The debator for the "local" model was Dr. Donald Q Lamb of the University of Chicago, while the debator for the extragalactic model was Dr. Bohdan Paczynski of Princeton University.
Pinpointing a burst: GRB 970228
By the late 1990s, the local origin hypothesis for GRBs had been ruled out. The first clue came from the Italian-Dutch BeppoSAX satellite, which was launched in 1996 and operated until 2003.
BeppoSAX carried a gamma-ray detector that worked in conjunction with a pair of wide-field X-ray cameras. While the satellite's gamma-ray detector had poor angular resolution, a gamma-ray burst will generally have an X-ray component, which would allow the X-ray cameras to quickly pinpoint the source for observation by optical and other telescopes.
On February 28, 1997, BeppoSAX managed to rapidly pin down the precise location of a gamma ray burst, which allowed the team of J.van Paradijs of the University of Amsterdam for the first time ever, to detect the optical counterpart to a gamma-ray burst, using the William Herschel Telescope on the Canary Island La Palma. The optical afterglow of this burst, designated GRB 970228 was subsequently observed on March 26, 1997, with the Hubble Space Telescope, which showed that it was surrounded by a faint extended object, resembling the very distant galaxies in the Hubble Deep Field. By the time of its discovery the afterglow had, however, become too faint to detect its spectrum, which was needed to definitively prove its extragalactic nature.
BeppoSAX recorded another burst in the constellation Camelopardalis on May 8, 1997. The spacecraft's science team sent out an alert over the Internet and seven hours later, an optical source was detected by astronomer Howard Bond using the 90 centimeter telescope at Kitt Peak National Observatory. On May 11, astronomers used the 10 meter Keck II telescope to obtain a spectrum of the object. The absorption lines had undergone a massive Doppler shift; they showed a redshift of 0.835. According to Hubble's law (redshift is proportional to distance), the source of the burst must therefore be billions of light-years away.
Following these observations, astronomers were able to track down more faint visible-light and radio "afterglows" of GRBs, hours or days after the occurrence of the burst. More redshifts were obtained, and confirmed that the bursts occurred in the distant cosmos.
Visible light observations of several of these GRB locations in 1997 and 1998 identified possible links between the bursts and supernovae. The observations were not conclusive, but they were encouraging to astrophysicists who believed that the GRBs were associated with supernovae.
Caught in the act: GRB 990123
Astronomers first managed to obtain a visible-light image of a GRB as it occurred on January 23, 1999, using the Robotic Optical Transient Search Experiment 1 (ROTSE-1), sited in Los Alamos, New Mexico. ROTSE-1 consists of an array of four commercial 200 millimeter telephoto lenses focused on Charge-Coupled Device (CCD) electronic imaging arrays and mounted on an automated platform. While such lenses are modest even by the standards of amateur astronomy, ROTSE-1 has a wide field of view and can be quickly repositioned to scan any part of the visible sky.
In the dark hours of the morning of January 23, 1999, the Compton satellite recorded a gamma-ray burst that lasted for about a minute and a half. There was a peak of gamma and X-ray emission 25 seconds after the event was first detected, followed by a somewhat smaller peak 40 seconds after the beginning of the event. The emission then fizzled out in a series of small peaks over the next 50 seconds. Eight minutes after beginning, it had faded to a hundredth of its maximum brightness. The burst was so strong that it ranked in the top 2% of all bursts detected.
Compton reported the burst to its ground control facility at NASA's Goddard Space Flight Center, and Goddard immediately sent the data out over the Gamma Ray Burst Coordinates Network (GCN). The burst was designated "GRB 990123".
While Compton could not provide precise locations of bursts, the location was good enough for the wide-field ROTSE-1. The camera array automatically focused on the region of the sky and obtained an image of the burst 22 seconds after it was detected by Compton, with subsequent images obtained every 25 seconds after that. ROTSE-1 can image cosmic objects as faint as magnitude 16, and GRB hunters had expected the visible component of a GRB to be very faint. Instead, the visible component reached magnitude 9, so bright that it could have been seen with a good pair of binoculars. The object that produced it increased in brightness by a factor of 4,000 in less than a minute.
BeppoSAX had also seen the burst, and pinned down its location to within a few arcminutes. This data was sent out, and four hours after the burst the area was imaged with the 1.52 meter (60 inch) Schmidt camera at Palomar Observatory. The image revealed a magnitude 18 optical transient that was not on archive images of the same area.
The fading object, by now down to magnitude 20, was imaged the next night by the 10 meter Keck telescope and the 2.6 meter Nordic Optical Telescope at the Roque de los Muchachos Observatory. The observations revealed absorption lines with a redshift of 1.6, implying a distance of 9 billion light years.
The Hubble Space Telescope performed observations on the location of GRB 990123 sixteen days after the event. It had faded by more than a factor of three million in that time. The Hubble was able to pick up the traces of a faint galaxy, whose blue color suggested it was forming new stars at a rapid rate. (later found to be a photographic flaw)
GRB 060218
In February 2006, NASA announced the discovery of the unusual GRB 060218. It was located in the constellation Aries and was a distance of 440 million light-years away. It lasted for 33 minutes and currently scientists have been unable to give a reason for it. They believe it may be the predecessor to a supernova.
What is a GRB?
Early theories
The combination of obvious brightness and implied distance of GRB 990123 led to two possibilities.
The first was that the radiation of the gamma ray burst was spread evenly. This implied that the gamma-ray energy released by the burst was equivalent to that which would be produced by converting the entire mass of a star 1.3 times the mass of our Sun completely into gamma radiation (see mass-energy equivalence). At visual wavelengths, if the burst had occurred 2,000 light years away within our own Galaxy, it would have shone twice as bright as the Sun.
Another possibility was that the gamma ray was not evenly distributed but was tightly beamed in a narrow region. While this would still imply a massive emission of energy, the energies would be on the order of supernova and thus would require less exotic physics. However, it would also require the events to be far more common, to make up for the fact that many would not point towards the Earth.
Astrophysicists have been challenged to come up with convincing mechanisms to explain the sheer power of these bursts. One line of thought proposed that collisions between neutron stars, or between a neutron star and a black hole, could do the job. Another proposed that the bursts were caused by supernova explosions of very large stars, or hypernovas.
The Hubble observations showed GRB 990123 to be associated with a young galaxy, which tended to discourage theorists who believed that the bursts were due to collisions between neutron stars or the like. The fairly high density of dead stars required for bursts is inconsistent with a young galaxy. Supernovae, on the other hand, occur frequently in younger star-forming galaxies, since the large stars that die in supernovae have short lifetimes.
Even the supernova model had trouble accounting for the energy output. One way around the problem was to assume that the burst energy was only sent out in specific directions, rather than in all directions. This would be similar to the directional high-energy cosmic jets emitted by some stars and galaxies during violent events. Another explanation for the great brightness of the burst was that its light had been focused by a gravitational lens, caused by the distortion of space by a large galaxy between Earth and the GRB.
The lensing theory was supported by observations that seemed to indicate there was a galaxy between Earth and the GRB. However, the "galaxy" turned out to be a photographic flaw. This didn't rule out gravitational lensing, but interest in the idea faded when Bradley E. Schafer of Yale pointed out that at a redshift of 1.6, the density of galaxies made the probability of lensing only about one in a thousand.
Astrophysicists Bohdan Paczynski of Princeton University and Stan Woosley of the University of California, Santa Cruz, independently suggested that a supernova might emit a narrow beam of gamma ray energy during its explosive collapse into a black hole, with the tightly focused beam giving the impression of a much more energetic event. Exactly how the collapse would generate such a beam remains a puzzle. However, an analysis of the afterglows of 17 GRBs that was published in the fall of 2001 did place limits on the width of the beam, stating that it was probably only a few degrees across. With such a narrow beam, the energy of a GRB amounts to several times the 1044 J which could be provided by a supernova only slightly more powerful than average.
With such a narrow beam, perhaps only one in 500 GRBs could be seen from Earth. If so, this could mean that they are actually a fairly common phenomenon in the Universe, probably occurring about once every minute. This means that astronomers might be able to observe "orphan afterglows" exactly like those following a GRB, but not associated with a detectable gamma-ray burst.
The brightness of GRBs varies rapidly, implying that their source objects are quite small: whatever causes the brightness variation cannot travel faster than the speed of light across the object. Very densely packed photons prevent each other from escaping, and astronomers therefore theorize that the energy initially leaves the object as a jet of matter, with gamma rays being created at a certain distance by internal shock waves.
There is some direct evidence of an association of a GRB with a supernova. A supernova synthesizes a wide range of heavy elements during its collapse, and many of these, particularly isotopes of nickel, are highly unstable and break down very quickly, releasing radiation. This means that a supernova actually gets brighter for a few days or weeks after its occurrence.
BeppoSAX targeted a GRB on 21 November 2001, and following the burst the Hubble Space Telescope tracked the evolution of GRB 011121 for an extended period of time. The light curve obtained matched that expected of a supernova. However, no valid spectrum was obtained of GRB 011121 that would have conclusively linked it to a supernova.
Modern ideas
Data on GRBs is still sketchy and they remain mysterious. Spectra have proven difficult to obtain, and only a few bursts have accurate distance measurements.
Distance can be measured from the redshift of the GRB. However, gamma ray measurements do not have a distinctive line structure and so a redshift measurement can rarely be obtained from them. Sometimes only distance estimates can be made using absorption lines from the gas along the line of sight to the GRB. Some astrophysicists believe that the rate at which a GRB flickers may provide a useful index of its distance, and might even be a useful standard candle for determining distances to the far reaches of the cosmos.
There is also the puzzle that the burst durations fall into distinct "long" and "short" categories. The long bursts are generally agreed to be associated with hypernovae, while the short bursts are thought most likely to be the result of the mergers of pairs of neutron stars, or of a neutron star with a black hole.
Despite the fuzzy data and many questions, astronomers now feel they are closing in on a solution to the mystery, and remain very excited. They are making good use of the tools available for the job. The hypernovae model relies on the energy of the collapse being beamed along the rotation axis of the star, due to the torus of material spiraling into the black hole. Simulations of coalescing neutron stars also provide a very realistic explanation for the short duration bursts.
The astronomers have obtained much more information from the US High Energy Transient Explorer 2 (HETE-2) satellite, launched on 9 October 2000. The first HETE satellite had been launched on 4 November 1996, but it had been trapped in orbit in its payload shroud. Burst hunters were disappointed, but they were able to obtain a replacement. HETE-2 is specifically designed to quickly and precisely locate gamma-ray bursts, permitting other observatories, such as the NASA Chandra X-Ray Observatory, to obtain more details of the bursts.
A new mission to investigate GRBs has now started. The Swift Gamma Ray Burst Explorer satellite became fully operational in April 2005. Swift includes a "burst alert telescope" to alert the spacecraft of any gamma-ray burst. The satellite will then quickly realign itself to focus more sensitive instruments on the burst. Swift can shift 50 degrees in less than 50 seconds to focus on a precise sky coordinate.
On May 5, 2005, Swift spotted and followed a burst that was also scrutinized by other observatories. Its data suggest it might have been created by two neutron stars colliding. The investigation of this object is ongoing at this time.
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 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 less than a supernova at the same distance.
Scientists at NASA and the University of Kansas in 2005 released a 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." [1]
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 this planet [2].
List of GRBs
Notable GRBs
- GRB 060218, in 2006, a nearby gamma-ray burst with unusual characteristics
- GRB 990123, in 1999, was the first seen in the optical waveband while bursting.
- GRB 971214, in 1998, was found to be the brightest observed GRB in the universe. As it is likely that the radiation beamed in our direction, it is impossible to say what the total energy release was.
- GRB 970228, in 1997, was the first pinpointed. It was pinpointed using its X-ray afterglow.
Extreme GRBs
| Gamma Ray Bursts | |
| Title | GRB |
|---|---|
| Most distant | GRB 050904 |
| Least distant | GRB 980425 |
| Most energetic | GRB 971214 |
| Least energetic | |
| Longest duration | |
| Shortest duration | |
| First located | GRB 970228 |
| First optically observed | GRB 970228 |
Soft Gamma Repeater
A Soft gamma repeater is a type of magnetar which emits large bursts of gamma rays and X-rays at irregular intervals. The photons are less energetic than in a normal gamma ray burst (in the soft gamma ray and hard X-ray range), and repeated bursts come from the same region.
SGR 1806-20 had the most massive burst yet recorded, with an absolute magnitude of -29.
See also
References
- Neil Gehrels et al. "The Brightest Explosions in the Universe," Scientific American, Vol 287, No. 6, December 2002
- Originally based on the document [v1.1.0 / 01 jul 02 / gvgoebel@earthlink.net / public domain]
External links
- PBS NOVA: Death Star (gamma-ray bursts)
- Animation of Gamma Ray Burst (Quicktime)
- GRB 971214: Most energetic event in the universe
- GRB 971214: Space Science Update Webcast (RealMedia)
- Gamma-ray Burst Real-time Sky Map based on Swift data
- Gamma ray bursters segment of Science Friday, 3 June 2005 (RealAudio)
- Gamma-ray burst FAQ from CalTech
- Most distant cosmic blast sighted (BBC reports a registered GRB from about 13 billion light years away)
- Cosmological Gamma-Ray Bursts and Hypernovae Conclusively Linked (ESO)
- Official NASA Swift Homepage: The Swift Gamma-Ray Burst Mission
- Swift Mission Operations Center at Penn State
- M A S T E R Mobile Astronomical System of the Telescope-Robots
- Gamma Ray Burst Coordinates Network
- GRBlog: A Gamma-Ray Burst Database at University of Texas
- gamma ray bursts and mirror matter
Non-mainstream external links
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