Gamma ray burst Totally Explained

Everything about Gamma-ray Burst totally explained

Gamma-ray bursts (GRBs) are the most luminous electromagnetic events occurring in the universe since the Big Bang. They are flashes of gamma rays emanating from seemingly random places in deep space at random times. The duration of a gamma-ray burst is typically a few seconds, but can range from a few milliseconds to minutes, and the initial burst is usually followed by a longer-lived "afterglow" emitting at longer wavelengths (X-ray, ultraviolet, optical, infrared, and radio). Gamma-ray bursts are detected by orbiting satellites about two to three times per week, but the number of GRBs that could be observed from Earth is about three times as many and it's currently limited by the efficiency of the instruments.
Most observed GRBs appear to be collimated emission caused by the collapse of the core of a rapidly rotating, high-mass star into a black hole. A subclass of GRBs (the "short" bursts) appear to originate from a different process, the leading candidate being the collision of neutron stars orbiting in a binary system. All known GRBs originate from outside our own galaxy; though a related class of phenomena, SGR flares, are associated with Galactic magnetars. The sources of most GRBs are billions of light years away.
A nearby gamma ray burst could possibly cause mass extinctions on Earth. Though the short duration of a gamma ray burst would limit the immediate damage to life, a nearby burst might 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

Cosmic gamma-ray bursts were discovered in the late 1960s by the US Vela nuclear test detection satellites. The Velas were built to detect gamma-radiation pulses emitted by nuclear weapon tests 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. Any discoveries of weapon tests have never been publicly declared and details of the Vela Incident, an as-yet unidentified flash of light over the South Atlantic on September 22, 1979, 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, prompting the scientific study of GRBs.
   The presence of GRBs was confirmed later by many space missions such as Apollo and the Soviet Venera probes. To explain these events, many speculative theories were advanced, most of which posited nearby galactic sources. Little progress was made however 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 γ-ray bursts are isotropic (not biased towards any particular direction in space, such as toward the galactic plane or the galactic center), and therefore 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"). 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 isn't absolute and the populations overlap observationally, but the distinction suggests two different classes of progenitors. However, some believe there are a third type of GRBs
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   For decades after the discovery of GRBs astronomers couldn't find any counterpart or host to them, such as a star or galaxy, owing to poor resolution of their detectors. The best hope seemed to lie in finding a fainter, fading, longer wavelength emission after the burst itself, the "afterglow" of a GRB, as predicted by most models.
   In 1997 the Italian/Dutch satellite BeppoSAX detected a gamma-ray burst (GRB 970228), and when the X-ray camera was pointed in direction from which the burst had originated it detected a fading X-ray emission. Ground-based telescopes later identified a fading optical counterpart as well. The location of this event having been identified, once the GRB faded, deep imaging was able to identify a faint, very distant host galaxy in the GRB location, the first of many to come. Within only a few weeks the long controversy about the distance scale ended: GRBs were extragalactic events originating inside faint galaxies 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.
   As of 2007, a similar revolution in GRB astronomy is in progress, largely as a result of 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 towards the direction of a new burst in less than a minute. Swift's discoveries include the first observations of short burst afterglows and vast amount of data on the behavior of GRB afterglows at early stages during their evolution, even before the GRB's gamma-ray emission has stopped. The mission has also discovered huge X-ray flares appearing within minutes to days after the end of the GRB.

Distance scale and energetics

Galactic vs. extragalactic models

Prior to 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—coming from completely random directions—narrowed down these possibilities greatly, and by the mid 1990s only two theories were considered generally viable: 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 be an unobserved population of similar objects at greater distances, producing GRBs. 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.
   The discovery of afterglow emission associated with faraway galaxies definitively supported the extragalactic hypothesis. Not only are GRBs extragalactic events, but they're also observable 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 known (GRB 050904) has a redshift of 6.29 (12.3 billion light years). As observers are able to acquire spectra of only a fraction of bursts - usually the brightest ones - many GRBs may actually originate from even higher redshifts.

GRB Jets: collimated emission

Many GRBs have been observed to undergo a jet break in their light curve, during which the optical afterglow quickly changes from slowly fading to rapidly fading as the jet slows down. Furthermore, features suggestive of significant asymmetry have been observed in at least one nearby type Ic supernova, which may have the same progenitor stars as GRBs and have been observed to accompany GRBs in some cases (see ""). The jet opening angle (degree of beaming), however, varies greatly, from 2 degrees to more than 20 degrees. There is some evidence which suggests 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, or around 1/2000 of a solar mass. 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.
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.

Short GRBs

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

Progenitors: what causes GRBs?

The immense distances of most gamma-ray bursts has made pinning down the nature of the system that produces these explosions extremely difficult. The currently favored model for the origin of most observed gamma-ray bursts is the collapsar model, in which the core of an extremely massive, low-metallicity, rapidly-rotating star collapses into a black hole, and the infall of material from the star onto the black hole powers an extremely energetic jet that blasts outward through the stellar envelope. When the jet reaches the stellar surface, a gamma-ray burst is produced.
While the collapsar model has enjoyed a great deal of success, many other models exist that are still seriously considered. Winds from highly magnetized, newly-formed neutron stars (protomagnetars), accretion-induced collapse of older neutron stars, and the mergers of binary neutron stars have all been proposed as alternative models. The different models are not mutually exclusive, and it's possible that different bursts have different progenitors. For example, there's now good evidence that some short gamma-ray bursts (GRBs with a duration of less than about two seconds) occur in galaxies without massive stars

Emission mechanisms

The means by which gamma-ray bursts convert energy into radiation remains poorly understood, and as of 2007 there's still no generally accepted model for how this process occurs. A successful model of GRBs must explain not only the energy source, but also the physical process for generating an emission of gamma rays which matches the durations, light spectra, and other characteristics of observed GRBs. The nature of the longer-wavelength (X-ray through radio) afterglow emission that follows gamma-ray bursts has been modeled much more successfully as synchrotron emission from a relativistic shock wave propagating through interstellar space
, but this model has had difficulty explaining the observed features of some observed GRB afterglows (particularly at early times and in the X-ray band), and may be incomplete, or in some cases even inaccurate.

Mass extinction on Earth

Research has been conducted to investigate 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. A gamma ray burst at 6000 light years would result in mass extinction; a 1000 light year distant burst would be equivalent to a 100,000 megaton nuclear explosion. A burst 100 light years away would blow away the atmosphere, create tidal waves, and start to melt the surface of the Earth. A consensus seems to have been arrived at the fact that damage by a gamma ray burst would be very limited because of its very short duration, and the fact that it would only cover half the Earth, the other half being in its shadow. A sufficiently close gamma ray burst would however, result in serious damage to the atmosphere, shutting down communications (due to electro-magnetic disturbances), perhaps instantly wiping out half the ozone layer, and causing nitrogen-oxygen recombination, thereby generating acidic nitrogen oxides. These effects could diffuse across to the other side of the Earth, severely diminish the global food supply, and result in long-term climate and atmospheric changes and a mass extinction, reducing the global population to perhaps 10% of what it can now support. However, 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. In 2005, scientists at NASA and the University of Kansas released a more detailed study which suggested that the Ordovician-Silurian extinction events which occurred 450 million years ago could have been triggered by a gamma-ray burst. The scientists don't have direct evidence to suggest that such a burst resulted in 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, the recovery for which would take at least five years. With the ozone layer damaged, ultraviolet radiation from the Sun would 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 has 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. He said, "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."
   Comparative work in 2006 on galaxies in which GRBs have occurred suggests that metal-deficient 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 had caused mass extinction events on Earth.
   The Wolf-Rayet star WR 104, located 8000 light years from Earth, has been found to have a rotational axis aligned within 16° of the solar system. The chances of it producing a gamma ray burst are small, but if it did, the GRB would be close enough to have a significant impact on Earth.

Notable GRBs

GRBs of significant historical or scientific importance include:

670702: The first GRB ever detected.
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.
970508: The first GRB with a measured redshift (the extent to which the radiation is redshifted allows astronomers to calculate an estimate of the distance of the event), 0.835. This confirmed unambiguously that GRBs are extragalactic.
971214: In 1997, this was believed by some to be the most energetic event in the universe. This claim has since been discredited.
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.
990123: This GRB had the optically brightest afterglow measured before 080319B, 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.
030329A: An extremely close (z=0.168), and therefore extremely bright GRB, with an unambiguous supernova association. GRB 030329 was so bright that its gamma radiation ionized the Earth's upper atmosphere.
050509B: The first short GRB with a host association. Provided evidence that (some) short GRBs, unlike long GRBs, occur in old galaxies and don't have accompanying supernovae.
050724: A thoroughly observed short gamma-ray burst with an afterglow suggesting the demise of a neutron star orbiting a black hole.
050904: The most distant GRB with a securely measured distance, at a redshift of 6.29 (13 billion light-years).
060218: A low-redshift GRB with an accompanying supernova.
060505: The first, well-observed, long duration GRB not accompanied by a bright supernova.
060614: Another recent gamma-ray burst not accompanied by an observable supernova.
080319B: Extremely intense GRB with the brightest visible afterglow ever (at 5th magnitude, bright enough to see with the naked eye). It is the most energetic event detected by Swift so far and the most luminous optical source ever seen.