Depths of the Universe - Nailing the Gamma Ray Bursts
Nasas old Hete-2 has sent back a slew of evidence to put one over the recently launched Swift that was sent into space to perform precisely that assignment FLEETING bursts of gamma rays from the depths of the universe have puzzled astronomers for more than 30 years, but a slew of evidence has now confirmed that they originate from almighty collisions between the remnants of dead stars. Surprisingly, the data that clinched this discovery has come from an old Nasa probe the High Energy Transient Explorer-2 that has been in orbit since October 2000. Nasa recently launched another satellite, called Swift, explicitly to solve the mystery of short gamma ray (-rays) bursts. But Swift has been beaten to the best results by the lucky Hete-2.
Bursts of -rays come in two flavours. Long bursts lasting more than two seconds or so are generated by the collapse of young, massive stars as they give birth to black holes. But astronomers had very little information about the short bursts, which are difficult to observe because of their transient nature. The leading theory suggested that short bursts were released when a pair of neutron stars, or a neutron star and a black hole, crashed into each other. Neutron stars are the dense, burned-out cores of stars that are left behind after supernova explosions.
But short bursts might also be released from the same sources as long ones, or by flares from highly magnetized neutron stars called magnetars. Swift bagged the first direct evidence for the collision theory earlier this year, but researchers couldnt tell exactly where the burst was coming from, so they could not rule out collapsing stars or magnetic flares.
Although Swift is faster and more accurate than Hete-2, the older satellite got lucky on 9 July when it saw a short -ray burst, later named GRB050709, that lasted just 70 milliseconds. All the pieces of the puzzle came together for us with this burst, said Derek Fox of Pennsylvania State University, University Park, who is part of the Hete-2 team. Onboard X-ray cameras first traced the burst to a particular part of the sky. Then astronomers rushed to make follow- up observations using the orbiting Chandra X-Ray Telescope, the Hubble Space Telescope and the Danish 1.5-metre telescope at La Silla in Chile.
The last of these provided the first ever glimpse of the optical afterglow of a short -ray burst. Along with the other measurements, this allowed the scientists to pinpoint exactly where one of these explosions had come from: the outskirts of a galaxy lying about a billion light years from earth.
Knowing how distant it was, astronomers calculated that the short burst was 1,000 times less energetic than its longer counterparts. This was still too energetic to have come from a magnetar, the researchers concluded. The position of the burst, far away from the central star-forming region of its galaxy, also suggested that the burst came from relatively old objects, rather than young, collapsing stars. These observations are all consistent with models of the merger of two neutron stars, or of a neutron star with a black hole, said Luigi Piro, an astronomer from the Institute of Astrophysics and Cosmic Physics in Rome, Italy. Further analysis of the Swift observations bolster this idea. All of the analyses are reported in Nature.
Now that their source is well established, identifying future bursts could help in the hunt for gravitational waves, which are predicted by Einsteins general theory of relativity. Violent collisions between neutron stars should send ripples outwards through the fabric of space, and the Laser Interferometer Gravitational-Wave Observatory in Washington and Louisiana is looking for these ripples. If and when something is seen, it may be hard to tell whether its a real signal or just an accidental blip in the experiment. But confirming the signal by also seeing a -ray burst from a neutron-star collision will help to prove that gravitational waves have really been detected.
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