Theory Gamma-Ray Bursts (GRBs)

Gamma rays, the highest-energy form of light, were discovered in 1900 by the French physicist Paul Villard. NASA's Explorer 11 satellite, launched in 1961, detected the first gamma rays in space. In 1963, the U.S. Air Force began launching a series of satellites as part of Project Vela. These increasingly sophisticated satellites were designed to verify compliance with an international treaty that banned nuclear weapons tests in space or in the atmosphere. But starting in July 1967, scientists became aware the Vela satellites were seeing brief gamma-ray events that were clearly unrelated to weapons tests. GRBs are jetted explosions triggered either by the collapse of a massive star or by the merger of a neutron star with another neutron star or a black hole. If the jet is pointing towards the Earth, a burst of gamma rays is detected. As the jet expands, it loses energy and produces weaker, more isotropic radiation at X-ray and other wavelengths. Bright afterglows occur as the jets heat gas that was previously shed by the star. The following are more detailed views about, which surely however will get you away from that simple view. Most recent views, on a other hand, are that the study of GRBs must closely be linked with that of black holes

->Check That Easy Definition for GRBs!
Here is a good and easy definition for GRBs: gamma-ray bursts are the universe's most luminous explosions. GRBs come into two types. The short ones last less than 2 seconds as they result from the merger of two neutrons stars in a compact binary system, which trigger powerful jets of material and radiations with a power as strong as the Sun' energy produced during 10 billion years as the final result usually is a black hole! Most likely suspects are neutron stars and black holes. The long ones, lasting more than 2 seconds -to several minutes- with typical durations between 20 and 50 seconds, result from a supernova event from a star with 40 to 100 times the mass of our Sun, as the jets, in that case, are triggered by that the black hole being born from the star's collapse recapture the exploding gas and material released by the supernova explosion. The duration of the GRB is the one needed to that the star's layers collapse onto the forming black hole. Stars with more rapid rotation may be more likely to produce a long GRB. Metal content also plays a strong role in the development of long GRBs as astronomers have noted that long GRBs occur much more frequently in metal-poor galaxies. The astronomers found that 75 percent of long GRBs occurred among the 10 percent of star formation with the lowest metal content. That yields that the prevalence of long GRBs is decreasing as the Universe grows older. The few long GRBs found in high-metal environments might receive a assist from the presence of a nearby companion star, the mass of which accelerate the rotation of the parent star. Nearly daily such blasts occur. A neutron star be the result of the supernova, some GRB jets also can occur as a neutron star usually is found with both types. Such jets are able to reach about 99.9999% of the speed of light as, in some cases, bore all the way through the collapsing star and continue into space, where they interact with gas previously shed by the star and generate bright afterglows that fade with time. GRB events also emit gravitational waves as the resulting black hole just keeps wandering in a galaxy as they count by several millions in our Galaxy, for example. While the GRB itself is lasting minutes only, the fading afterglow remains observable for days or even weeks. About 1,000 GRBs occur yearly in the Universe as 1 lately was so strong it was observable naked eye. A GRB may be harmful to Earth should it occur at less than 6,000 light-years, with the jets aligned to our planet. Should the gamma rays then hit Earth, the ozone layer would be destroyed and damageable solar radiation let in. Azote dioxyde would massively develop at sea level and a large scale IEM, or electronic disturbance, occur. A nucler winter likely would occur too. Eta Carinae, a star on the verge of a supernova explosion and lying by 7,000 light-years from us might represent such a danger in the case its jets would explode in our direction. The question of the GRB originated due to the monitoring of nuke bombs arm race as gamma rays are released by those weapons and intrigued the observers
A still ill-known class of gamma-ray bursts (GRBs) might emerge from a supergiant star turning supernova as they have a duration of several hours, hence named 'ultra-long class.' The star's deep hydrogen envelope would take hours to complete its fall into the black hole, which would provide a long-lived fuel source
Most recent studies by NASA's Swift mission has found that GRBs likely existed as soon as in the early Universe, about 630 million years after the Big Bang only as the most distant GRB was found at the extreme distance of 13.14 billion light years, with a redshift 9.4. A variety of stars called 'carbon stars' similar to red giants stars, glowing brightly in longer wavelengths, and in the late stages of their lives, with unusually high amounts of carbon in their outer atmospheres which originate either from convection currents deep into the core, or from a nearby companion from which it is drawned is now considered the likely generators to gamma-ray bursts
Other gamma-rays objects in the Universe include active galaxies and pure mystery, a pinch of pulsars, some supernova remnants, and other celestial objects, such as globular star clusters and galaxies like our own Milky Way. In a galaxy gamma rays are mostly produced when high-energy cosmic rays smash into the gas between the stars
The family of gamma-ray bursts might turn more diverse than that as the so-called 'Christmas burst', by late 2011, was a unusually long 28 minutes and due to some exotic event like a binary system where a neutron star orbited a normal star that had just entered its red giant phase, enormously expanding its outer atmosphere. This expansion engulfed the neutron star, resulting in both the ejection of the giant's atmosphere and rapid tightening of the neutron star's orbit. Once the two stars became wrapped in a common envelope of gas, the neutron star may have merged with the giant's core after just five orbits, or about 18 months. The end result of the merger was the birth of a black hole and the production of oppositely directed jets of particles moving at nearly the speed of light, followed by a weak supernova. A alternative model would involve the tidal disruption of a large comet-like object and the ensuing crash of debris onto a neutron star located only about 10,000 light-years away (in the first case, the event would have occurred in a faraway galaxy)

GRBs, Gamma-Rays Bursts are tremendous releases of energy, funnelled along jets, which unleash enormous and dangerous amounts of high-energy rays, the gamma-ray ones. The GRBs, which became an object of study lately only, are considered potentially dangerous to life, anywhere in the Universe, Earth included, as they may act like life-sterilizers. At a distance of 6,000 light-years from Earth, a wave of gamma-rays hitting Earth during 10 seconds would deplete the ozone layer by half, allowing harmful various radiations towards the surface. The ozone layer would need 5 years to replenish. A close GRB would too trigger a IEM, or general electromagnetic shortcut, the increase of nitrogen dioxyde and likely some ice age. It might that the first of the mass-extinctions at Earth be triggered by such an event. GRBs occur in our own Milky Way Galaxy, or down to the farthest reaches of the Universe, at some 7 billion light-years for example. 100 GRBs are observed yearly as the most worrying such potential event might occur with the explosion of the nearby, instable, giant star eta Carina

->Two Kinds of GRBs, Short and Long, and an Hybrid One
GRB's bursts typically fall into two categories, long or short. Long bursts are lasting more than 2 seconds and seem related to black hole-creating supernovae triggered from a red supergiant star with 40 to 60 solar masses, as they are seen at the very edges of the Universe. In that case, the GRB is triggered when the star's expelled material falls back unto the black hole! Short bursts are lasting less than 2 seconds and even often they last just a few milliseconds, as they are related to the merger of two neutron stars, or of a neutron star and a black hole, leading a new -or a bigger- black hole. It looks like some bursts are displaying some hybrid characteristics, lasting long but lacking a theoretical model of mergers to give them sense and explain how the merger could trigger a long-duration event. Scientists, until now, are left with that such hybrid GRBs are either long-short bursts from a merger, or long bursts from a star explosion without any supernova. Most conclude, however, that some new process must be at play -like what length of GRBs the merger model is creating, or some stars exploding in a radically new fashion than supernovae. A hybrid GRB seen in 2006 originated in a galaxy 1.6 billion light-years away. Short GRBs have been seen lately to occur in a more distant Universe than previously thought. A good evidence has been brought by April 2011 to that short GRBs indeed result from the collision of two neutron stars. A new supercomputer model show how orbiting neutron stars rapidly lose energy by emitting gravitational waves and merge after about three orbits, or in less than 8 milliseconds. The merger amplifies and scrambles the merged magnetic fields. A black hole forms and the magnetic field becomes more organized, eventually producing structures capable of supporting the jets that power short gamma-ray bursts. A swirling chaos of superdense matter with temperatures exceeding 18 billion degrees Fahrenheit surrounded the newborn black hole and it took 30 milliseconds in all for the jets to form. The ultimate proof of the merger model will have to await the detection of gravitational waves, those ripples in the fabric of space-time predicted by Einstein's relativity. Some supernovae at the origin of a GRB might be powered by the decay of super-strong magnetic fields around a magnetar

Latest studies are now beginning to draw an effective picture of this phenomenon. GRBs part between long and short-duration ones. First ones are lasting between 2 and 10 seconds as they release relatively less energetic gamma rays, as the second ones last between milliseconds and 2 seconds, producing high-energy rays. Long duration GRBs are thought to originate at 20 solar-mass stars, in irregular galaxies with few heavy elements. Such heavy stars, which are not extremely high-mass ones do release enough material through their stellar lifes, albeit retaining enough to turn supernova, then black hole and GRB. Short-duration GRBs are believed to occur in any galaxy, as a result of the collision between two compact objects, like neutron stars, leading to a black hole. The jets seem to appear as the energy released is ramming into any possible envelopes of material remaining around the dying stars. X-ray flashes, which are less energetic events, have been seen occurring before the gamma-ray events, as they might be too GRBs seen at an angle, or events of their own, being softer GRBs produced by a baryonic (neutrons and protons) explosion instead of a leptonic (electrons) one. X-ray flashes, generally, seem to be good signals heralding a supernova explosion

GRBs which were once thought to be able to sterilize entire galaxies seem to have their effects limited to a radius about 200 light years wide instead. A less good new is that GRBs are maybe much more numerous than previously thought: those seen are seen only because of an appropriate angle of view as seen from Earth, as others would go unseen due to a bad angle. The next good new is that long-duration GRBs are occurring in irregular galaxies only, hence not in our Milky Way Galaxy. Short-duration GRBs are possible in our Galaxy, although their power is about 100 to 1,000 times less that long-duration ones. GRBs are occurring since the early beginnings of the Universe

->The current NASA's Swift mission which launched in Nov. 2004, is bringing much more knowledge about the GRBs. It's embarking three telescopes to relay any burst's location to ground- and space-based telescopes. Swift is able to monitor 1/6th of the sky at a time
The studies by Swift are showing that there likely is a rich variety of cosmic explosions in our local Universe, with the smaller gamma-ray bursts -termed 'X-ray flashes"- leaving behind a magnetar -a neutron star with a magnetic field which is 100-1000 times stronger than the one of an usual neutron star, and the larger GRBs. A hierarchy seems to build between ordinary supernova explosions (which leave behind them a neutron star) and gamma-ray bursts (which leave a black hole). What singularize GRBs and X-ray flashes from supernovae is that both have a disk of material rapidly rotating about the star they leave behind. X-ray flashes are in a ratio of ten-to-one to their powerful cousins
It seems like the GRBs be mostly the mark of the birth of a black hole. The gamma-rays explosions of the magnetars are due to that the combined effects of their swift rotation -like a neutron star- and their strong magnetic fields, intermittently yield a 'starquake', which fractures their crust and liberate the explosion -which is visible too in the X-rays and the visible light. One currently knows 12 magnetars only as they might be more numerous due to that they mostly are quiet, thus invisible
Swift discovered that the most ancient GRB event, the GRB 090423 occurred at more than 13 billion light-years, occurring just 630 million years after the Big Bang. Such events likely are associated with the formation of a black hole, consecutively to a supernova event, and part of the events which occurred among the earliest generations of stars, emerging from the Dark Ages

The extragalactic gamma-ray background, generally, is provided by active galaxies possessing central black holes containing millions to billions of times the sun's mass. As matter falls toward the black hole, some of it becomes redirected into jets of particles traveling near the speed of light. These particles can produce gamma rays in two different ways. When one strikes a photon of visible or infrared light, the photon can gain energy and become a gamma ray. If one of the jet's particles strikes the nucleus of a gas atom, the collision can briefly create a particle called a pion, which then rapidly decays into a pair of gamma rays. Closer sources, within our Milky Way galaxy are pulsars or gas clouds. The Fermi Gamma-ray Space Telescope, which launched by June 2008, has thus proved that active galaxies, albeit with between energies of 0.1 and 100 billion electron volts (GeV) -or from about 100 million to 30 billion times the energy of visible light- are minor players only in the production of the gamma-ray background of the Universe. Particle acceleration occurring in normal star-forming galaxies is a strong contender, or particle acceleration during the final assembly of the large-scale structure we observe today, for example, the mergers of clusters of galaxies. Dark matter too, the still hypothetic particles might interact together to produce gamma rays! High-energy gamma rays, generally, are produced when lower-energy light collides with accelerated particles. The 'inverse Compton process' makes that when a electron moving near the speed of light strikes a low-energy photon, the collision slightly slows the electron and boosts the light's energy into the gamma-ray regime. Such a process occurs at energetic galaxies called blazars and, as far as our own Milky Way Galaxy is concerned, at remnants of supernova explosions and pulsar wind nebulae, places where rapidly rotating neutron stars accelerate particles to near the speed of light