Gamma-Ray Bursts (GRBs)

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. It might that one of the mass-extinction at Earth be triggered by such an event

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, as they are seen at the very edges of the Universe. 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

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

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