New 'Fermi' gamma-ray telescope makes first sky map


By Rachel Courtland (Image: NASA/DOE/International LAT Team) NASA’s recently launched GLAST gamma-ray observatory has made its first map of the sky, and now the agency has given it a new name: the Fermi Gamma-ray Space Telescope. It was named in honour of Nobel-laureate Enrico Fermi, who described how charged particles in space could be accelerated to high speeds. The $700 million telescope launched on 11 June from Cape Canaveral, Florida. It is expected to operate for 10 years, observing high-energy gamma-ray photons from violent supermassive black holes and mysterious cosmic explosions called gamma-ray bursts. It may even help pin down the nature of the dark matter that pervades the universe. As sometimes occurs after the successful launch and check-out of a space observatory, the telescope has been renamed. It was christened after the high-energy physicist Enrico Fermi (1901 – 1954). He won the Nobel Prize for Physics in 1938 for inducing radioactivity in matter by slamming neutrons into atoms. Fermi also devised a theory to explain how shock waves and magnetic fields can accelerate charged particles. The accelerated particles, called cosmic rays, produce gamma rays when they strike clouds of gas in space. “His theory provides the foundation for understanding the new phenomena his namesake telescope will discover,” says Paul Hertz of NASA Headquarters in Washington, DC. At a press conference on Tuesday, project scientists released the telescope’s first map of the sky at gamma-ray wavelengths. The map (pictured) is built up from 95 hours of observations and was made with an instrument called the Large Area Telescope, which can scan the entire sky once every 3 hours. “This is like the night sky at a Fourth of July celebration, but we’re seeing it on a cosmic scale,” says Peter Michelson of Stanford University in Palo Alto, California. A previous instrument called the Energetic Gamma Ray Experiment Telescope, which flew on NASA’s Compton Gamma-ray Observatory until it was de-orbited in 2000, took more than a year to build a similar map, Michelson said. Bright spots in the map include the Crab Nebula, which hosts a radiation-spewing stellar corpse called a pulsar, and several blazars, violent active galaxies where colossal black holes accelerate particles to more than 99% the speed of light. But the maps’ main feature is a long swath of gamma rays emitted by the disc of our Milky Way galaxy. Most of the gamma rays come from cosmic rays hitting interstellar gas. But astronomers hope that some may come from the annihilation of weakly interacting massive particles (WIMPs), a class of particles that may make up dark matter. Astronomers expect WIMPS to accumulate close to the centre of the galaxy. When two of them collide, they annihilate and produce gamma rays. So it is possible that observations of the Milky Way could reveal such a concentration of dark matter. But picking out that signal from the gamma-ray light emitted by garden-variety astronomical objects will likely take a year or more. “Being able to pull that signal out from all the other busy activity that is going on, all the other gamma-ray traffic, is going to take some time,” says project scientist Steve Ritz of NASA’s Goddard Space Flight Center in Greenbelt, Maryland. Fermi’s other instrument, the GLAST Burst Monitor (GBM), is already detecting gamma-ray bursts on a daily basis, Chip Meegan of NASA’s Marshall Space Flight Center in Huntsville, told reporters. These bursts, which have been detected in large numbers by NASA’s Swift telescope, are fleeting explosions thought to be caused when massive stars die or when neutron stars merge. Fermi’s two instruments will allow the telescope to observe the bursts at a wide – and largely unexplored – range of the energy spectrum of gamma rays, from 8000 to 30 million eV. Fermi also boasts a wide field of view, which should enable astronomers to catch bursts when they start and follow them as they peak and dim,
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