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Gamma-ray burst

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

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

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

Historical mass extinctions on Earth may have been caused by gamma ray bursts. The short duration of a gamma ray burst would limit the immediate damage to life; however, a nearby burst could alter atmospheric chemistry by reducing the ozone layer and generating acidic nitrogen oxides. These atmospheric changes could ultimately cause severe damage to the biosphere.

Discovery and history

Vela and the discovery of GRBs

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

BATSE

The discovery of GRBs was confirmed by many later space missions, including Apollo and the Soviet Venera probes. Many speculative theories about these events were presented, most of which involved nearby Galactic sources. There were no major advances until the launch of the Compton Gamma Ray Observatory and its Burst and Transient Source Explorer (BATSE) instrument, an extremely sensitive gamma-ray detector. This instrument provided crucial data that gamma-ray bursts are isotropic,[2] not biased towards any particular direction in space, such as the galactic plane or the Galactic center, ruling out nearly all galactic origins. BATSE data also showed that GRBs fall into two distinct categories: short-duration, hard-spectrum bursts ("short bursts"), and long-duration, soft-spectrum bursts ("long bursts").[3] Short bursts are typically less than two seconds in duration and are dominated by higher-energy photons; long bursts are typically more than two seconds in duration and dominated by lower-energy photons. The separation is not absolute and the populations overlap observationally, but the distinction suggested two different classes of progenitors.

BeppoSAX and the afterglow era

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

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

Swift and GRBs today

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

Distance scale and energetics

Galactic vs. extragalactic models

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

Extragalactic nature of GRBs

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

GRB Jets: collimated emission

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

The fact that GRBs are jetted also suggests that there are far more events occurring in the Universe than actually seen, even when factoring in the limited sensitivity of available detectors. Most jetted GRBs will "miss" the Earth and never be seen; only a small fraction happen to be pointed the right way to allow detection. Still, even with these considerations, the rate of GRBs is very small—about once per galaxy per 100,000 years.[17]

Short GRBs

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

Progenitors: what makes GRBs explode?

Main article: Gamma ray burst progenators

GRBs show an extraordinary degree of diversity. They can last anywhere from a fraction of a second to many minutes. Bursts could have a single profile or oscillate wildly up and down in intensity, and their spectra are highly variable unlike other objects in space. The near complete lack of observational constraint led to a profusion of theories, including evaporating black holes, magnetic flares on white dwarfs, accretion of matter onto neutron stars, antimatter accretion, exotic types of supernovae, and rapid extraction of rotational energy from supermassive black holes, among others.[21]

Emission mechanisms

Main article: Gamma ray burst emission mechanisms

Gamma ray burst emission mechanisms are debateable as of 2007.[22] A successful model of GRBs must explain not only the energy source, but also the physical process for generating an emission of gamma rays that matches the durations, light spectra, and other characteristics of observed GRBs.[23]

Mass extinction on Earth

One line of research has investigated the consequences of Earth being hit by a beam of gamma rays from a nearby (about 500 light years) gamma ray burst. This is motivated by the efforts to explain mass extinctions on Earth and estimate the probability of extraterrestrial life. The consensus seems to be that the damage that a gamma ray burst could do would be limited by its very short duration, and the fact that it would only cover half the Earth (the other half would be in its shadow). A sufficiently close gamma ray burst could do serious damage to atmospheric chemistry, perhaps instantly wiping out half the ozone layer, and causing nitrogen-oxygen recombination, generating acidic nitrogen oxides. These effects would diffuse across to the other side of the Earth and result in long-term climate and atmospheric changes and a mass extinction. The damage from a gamma ray burst would probably be significantly greater than a supernova at the same distance.

The idea that a nearby gamma-ray burst could significantly affect the Earth's atmosphere and potentially cause severe damage to the biosphere was introduced in 1995 by physicist Stephen Thorsett, then at Princeton University.[24] Scientists at NASA and the University of Kansas in 2005 released a more detailed study that suggests that the Ordovician-Silurian extinction events of 450 million years ago could have been triggered by a gamma-ray burst. The scientists do not have direct evidence that such a burst activated the ancient extinction, rather the strength of their work is their atmospheric modeling, essentially a "what if" scenario. The scientists calculated that gamma-ray radiation from a relatively nearby star explosion, hitting the Earth for only ten seconds, could deplete up to half of the atmosphere's protective ozone layer. Recovery could take at least five years. With the ozone layer damaged, ultraviolet radiation from the Sun could kill much of the life on land and near the surface of oceans and lakes, disrupting the food chain. While gamma-ray bursts in our Milky Way galaxy are indeed rare, NASA scientists estimate that at least one nearby event probably hit the Earth in the past billion years. Life on Earth is at least 3.5 billion years old. 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 do not 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."[25]

Comparative work in 2006 on galaxies in which GRBs have occurred suggests that metal-poor galaxies are the most likely candidates. The likelihood of the metal-rich Milky Way galaxy hosting a GRB was estimated at less than 0.15%, significantly reducing the likelihood that a burst has caused mass extinction events on Earth.[26]

Notable GRBs

GRBs of significant historical or scientific importance include:

  • 670702 – The first GRB ever detected.[27]
  • 970228 – The first GRB with a successfully detected afterglow. The location of the afterglow was coincident with a very faint galaxy, providing strong evidence that GRBs are extragalactic.[28]
  • 970508 – The first GRB with a measured redshift (distance). At z=0.835, it confirmed unambiguously that GRBs are extragalactic.[29]
  • 971214 – In 1997, this was believed by some to be the most energetic event in the universe. This claim has since been discredited.[30][31]
  • 980425 – The first GRB with an observed associated supernova (1998bw), providing strong evidence of the link between GRBs and supernovae. The GRB itself was very unusual for being extremely underluminous. Also the closest GRB to date.[32]
  • 990123 – This GRB had the optically brightest afterglow measured to date, momentarily reaching or exceeding a magnitude of 8.9, which would be visible with an ordinary pair of binoculars, despite its distance of nearly 10 billion light years. This was also the first GRB for which optical emission was detected before the gamma-ray emission had ceased.[33]
  • 030329A – An extremely close (z=0.168)[34], and therefore extremely bright GRB, with an unambiguous supernova association.[35] GRB 030329 was so bright that its gamma radiation ionized the Earth's upper atmosphere. [36]
  • 050509B - The first short GRB with a host association. Provided evidence that (some) short GRBs, unlike long GRBs, occur in old galaxies and do not have accompanying supernovae.[37]
  • 050724 – A thoroughly observed short gamma-ray burst with an afterglow suggesting the demise of a neutron star orbiting a black hole.[38]
  • 050904 – The most distant GRB observed as of 2005, at z=6.29 (13 billion light-years).[39]
  • 060218 – A low-redshift GRB with an accompanying supernova.[40]
  • 060505 - The first, well-observed, long duration GRB not accompanied by a bright supernova.[41]

See also

References

  1. ^ Klebesadel, R.; et al. (1973). "Observations of Gamma-Ray Bursts of Cosmic Origin". Astrophysical Journal. 182: L85. {{cite journal}}: Explicit use of et al. in: |author= (help)
  2. ^ Meegan, C.A.; et al. (1992). "Spatial distribution of gamma-ray bursts observed by BATSE". Nature. 355: 143. {{cite journal}}: Explicit use of et al. in: |author= (help)
  3. ^ Kouveliotou, C.; et al. (1993). "Identification of two classes of gamma-ray bursts". Astrophysical Journal. 413: L101. {{cite journal}}: Explicit use of et al. in: |author= (help)
  4. ^ Fishman, C. J. and Meegan, C. A. (1995). "Gamma-Ray Bursts". Annual Review of Astronomy and Astrophysics. 33: 415–458.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  5. ^ GRBs are named based on the date on which they are named: the last two digits of the year, followed by the two-digit month and two-digit day. If two or more GRBs occur on a given day, the name is appended with a letter: 'A' for the first burst identified, 'B' for the second, etc.
  6. ^ van Paradijs, J.; et al. (1997). "Transient optical emission from the error box of the gamma-ray burst of 28 February 1997". Nature. 386: 686. {{cite journal}}: Explicit use of et al. in: |author= (help)
  7. ^ Not all scientists believed this association initially, and the exact redshift of this particular galaxy was not obtained until many years later. However, the next well-localized gamma-ray burst, GRB 970508, had a firm absorption redshift of 0.835 - a distance of 7 billion light years, and unambiguously far beyond our Galaxy.
  8. ^ For more on galaxies hosting GRBs, see the GHostS database http://www.grbhosts.org
  9. ^ Frontera, F. and Piro, L. (1998). Proceedings of Gamma-Ray Bursts in the Afterglow Era. Astronomy and Astrophysics Supplement Series.{{cite book}}: CS1 maint: multiple names: authors list (link)
  10. ^ Gehrels, N.; et al. (2004). "The Swift Gamma-Ray Burst Mission". The Astrophysical Journal. 611: 1005–1020. {{cite journal}}: Cite has empty unknown parameter: |1= (help); Explicit use of et al. in: |author= (help)
  11. ^ Lamb, D. Q. (1995). "The Distance Scale to Gamma-Ray Bursts". Publications of the Astronomical Society of the Pacific. 107: 1152.
  12. ^ Paczynski, B. (1995). "How Far Away Are Gamma-Ray Bursters?". Publications of the Astronomical Society of the Pacific. 107: 1167.
  13. ^ Haislip, J.; et al. (2006). "A photometric redshift of z = 6.39 ± 0.12 for GRB 050904". Nature. 440 (7081): 181–183. {{cite journal}}: Explicit use of et al. in: |author= (help)
  14. ^ Sari, R., Piran, T., Halpern, J. P. (1999). "Jets in Gamma-Ray Bursts". Astrophysical Journal. 519: L17 – L20.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  15. ^ Frail, D.A.; et al. (2001). "Beaming in Gamma-Ray Bursts: Evidence for a Standard Energy Reservoir". Astrophysical Journal. 562: L55 – L58. {{cite journal}}: Explicit use of et al. in: |author= (help)
  16. ^ Galama, T. J.; et al. (1998). "An unusual supernova in the error box of the gamma-ray burst of 25 April 1998". Nature. 395: 670–672. {{cite journal}}: Explicit use of et al. in: |author= (help)
  17. ^ Podsiadlowski; et al. (2004). "The Rates of Hypernovae and Gamma-Ray Bursts: Implications for Their Progenitors". Astrophysical Journal. 607L: 17P. {{cite journal}}: Explicit use of et al. in: |author= (help)
  18. ^ Prochaska; et al. (2006). "The Galaxy Hosts and Large-Scale Environments of Short-Hard Gamma-Ray Bursts". Astrophysical Journal. 641: 989. {{cite journal}}: Explicit use of et al. in: |author= (help)
  19. ^ Watson, D.; et al. (2006). "Are short γ-ray bursts collimated? GRB 050709, a flare but no break". Astronomy and Astrophysics. 454: L123 – L126. {{cite journal}}: Explicit use of et al. in: |author= (help)
  20. ^ Grupe, D.; et al. (2006). "Jet Breaks in Short Gamma-Ray Bursts. I: The Uncollimated Afterglow of GRB 050724". Astrophysical Journal: In publication. {{cite journal}}: Explicit use of et al. in: |author= (help)
  21. ^ Ruderman, M. (1975). "Theories of gamma-ray bursts". Texas Symposium on Relativistic Astrophysics. 262: 164–180.
  22. ^ "Gamma-ray bursts from synchrotron self-Compton emission". blackwell-synergy.com. August 2004. Retrieved 2007-10-12.
  23. ^ Fishman, Gerald J. (May 22,1995). "Gamma-Ray Bursts: An Overview". nasa.gov. Retrieved 2007-10-12. {{cite web}}: Check date values in: |date= (help)
  24. ^ Thorsett, S. E. (05/1995). "Terrestrial implications of cosmological gamma-ray burst models". Retrieved 2007-09-15. {{cite journal}}: Check date values in: |date= (help); Cite journal requires |journal= (help); Unknown parameter |jjournal= ignored (help)
  25. ^ "Explosions in Space May Have Initiated Ancient Extinction on Earth". nasa.gov. June 4, 2005. Retrieved 2007-09-15. {{cite web}}: Check date values in: |date= (help)
  26. ^ "One Less Thing to Worry About". astrobio.net. April 19, 2006. Retrieved 2007-09-15. {{cite web}}: Check date values in: |date= (help)
  27. ^ Strong, Klebesadel, and Olson (February 15, 1974). "A Preliminary Catalog of Transient Cosmic Gamma-Ray Sources Observed by the Vela Satellites". The Astrophysical Journal. American Astronomical Society. {{cite journal}}: Check date values in: |date= (help)CS1 maint: multiple names: authors list (link)
  28. ^ Esin AA, Blandford R (2000). "Dust Echoes from Gamma-Ray Bursts". Astrophysical Journal. 534 (2): L151 – L154. PMID 10813670.
  29. ^ Reichart, Daniel E. (February 19,1998). "The Redshift of GRB 970508". The Astrophysical Journal. American Astronomical Society. {{cite journal}}: Check date values in: |date= (help); Cite has empty unknown parameter: |1= (help)
  30. ^ Sari, R.; Piran, T.; Halpern, J. P. (1999). "Jets in Gamma-Ray Bursts". Astrophysical Journal. 519: L17 – L20.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  31. ^ Frail, D.A.; et al. (2001). "Beaming in Gamma-Ray Bursts: Evidence for a Standard Energy Reservoir". Astrophysical Journal. 562: L55 – L58. {{cite journal}}: Explicit use of et al. in: |author= (help)
  32. ^ "Cosmic Cannon: How an Exploding Star Could Fry Earth". space.com. June 19, 2001. Retrieved 2007-10-10. {{cite web}}: Check date values in: |date= (help)
  33. ^ "GOTCHA! The Big One That Didn't Get Away". nasa.gov. January 27,1999. Retrieved 2007-10-10. {{cite web}}: Check date values in: |date= (help)
  34. ^ N. Caldwell; et al. "GCN 2053, GRB 030329, optical spectroscopy". {{cite web}}: Explicit use of et al. in: |author= (help)
  35. ^ T. Matheson; et al. "GCN 2120, GRB 030329: Supernova Confirmed". {{cite web}}: Explicit use of et al. in: |author= (help)
  36. ^ P.W. Schnoor; et al. "GCN 2176, GRB030329 observed as a sudden ionospheric disturbance (SID)". {{cite web}}: Explicit use of et al. in: |author= (help)
  37. ^ "Blast hints at black hole birth". bbc.co.uk. May 11, 2005. Retrieved 2007-10-10. {{cite web}}: Check date values in: |date= (help)
  38. ^ "Cosmic Explosion Could Be Black Hole Swallowing Neutron Star". nasa.gov. December 14,2005. Retrieved 2007-10-10. {{cite web}}: Check date values in: |date= (help)
  39. ^ "MOST DISTANT EXPLOSION DETECTED, SMASHES PREVIOUS RECORD". nasa.gov. September 12, 2005. Retrieved 2007-10-10. {{cite web}}: Check date values in: |date= (help)
  40. ^ "Strange Exploding Star Unlocks Supernova Secrets". August 30,2006. Retrieved 2007-10-10. {{cite web}}: Check date values in: |date= (help)
  41. ^ "Spatially resolved properties of the GRB 060505 host: implications for the nature of the progenitor". arxiv.org. march 15, 2007. Retrieved 2007-10-10. {{cite web}}: Check date values in: |date= (help)
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