The Crab Nebula has stunned astronomers by emitting an unprecedented blast of gamma rays, the highest-energy light in the Universe, from a small area of the famous nebula. The cause of the 12 April gamma-ray flare, which lasted for some six days, hitting levels 30 times higher than normal and varying at times from hour to hour, described at the Third Fermi Symposium in Rome, is a complete and total mystery. The Crab’s recent outburst is more than five times more intense than any yet observed. Nasa’s Fermi space observatory is designed to measure only gamma rays, that emanate from the Universe’s most extreme environments and violent processes. Since its launch nearly three years ago, Fermi has spotted three such outbursts, with the first two reported earlier this year at the American Astronomical Society meeting. The Crab Nebula is composed mainly of the remnant of a supernova, which was seen on Earth to explode in the year 1054. At the core of the brilliantly coloured gas cloud is a pulsar – a rapidly spinning neutron star that emits radio waves which sweep past the Earth 30 times per second. But so far none of the nebula’s known components can explain the signal Fermi sees, said Roger Blandford, director of the Kavli Institute for Particle Astrophysics and Cosmology, US. “The origin of these high-energy gamma rays has to be some other source,” Blandford told BBC News.”It takes about six years for light to cross the nebula, so it must be a very compact region in comparison to the size of the nebula that’s producing these outbursts on the time scales of hours.” What has stumped astronomers is that these variations in gamma rays are not matched by changes in the emission of other light “colors”. Follow-up studies using the Chandra X-ray telescope, for example, showed no variations in the X-ray intensity.
Kavli Institute researcher Rolf Buehler outlined the details of the Crab’s flashes: “If you look in optical light, the Crab is very steady; in radio emission, it’s very steady; in very, very high-energy gamma rays it’s very steady. Only in this part between do we see it varying,” he told BBC News. “That’s why people hadn’t found this before; there was not an instrument like Fermi sensitive enough to capture it.” “To have something that puts almost all of its energy into gamma rays is an unusual thing,” Buehle added. “We’re looking at a big puzzle and are probably going to need a couple of years to understand it.” The best hunch so far is that in a region near the neutron star, intense magnetic fields become opposed in direction, suddenly re-organising themselves and accelerating close-by particles to near the speed of light, moving in curved paths, emiting the gamma rays seen by Fermi. The US-space-agency-managed telescope was launched in 2008. It honours Enrico Fermi, the great Italian-American physicist who worked on the development of the first nuclear reactor and who was awarded the Nobel Prize in Physics in 1938 for his work on radioactivity. (Source: http://www.sott.net/articles/show/228401-Huge-Gamma-ray-Flares-Mystify-World-s-Astronomers / Picture Credit: NASA/ESA/ASU/J. Hester)