Gamma-ray flares from the Crab

The Crab nebula, shown here in a Hubble Space Telescope image, is the remnant of a supernova explosion so nearby that Chinese astrologers took note of it in AD 1054. Powered by the rapidly spinning neutron star at its heart, the nebula shines with extraordinary luminosity at all photon energies from radio to TeV gammas. The apparent stability of the Crab’s brightness over that entire range has made it a convenient reference for calibrating astronomical instruments; the brightness of other sources is often quoted in “millicrabs.” But now two teams that operate orbiting gamma-ray telescopes—the Italian Space Agency’s AGILE and NASA’s Fermi —have reported seeing powerful short-duration flaring of the Crab nebula at gamma energies ranging from 100 MeV to 1 GeV. Lasting a few days or weeks, and apparently occurring once or twice a year, the outbursts manifest synchrotron radiation by electrons and positrons in the nebula that have somehow been accelerated to extremely high energies on an astonishingly rapid time scale. The flares rudely challenge the prevailing theory of how charged particles are accelerated, very gradually, in shock fronts within supernova remnants. The theory goes back to Enrico Fermi’s 1949 attempt to explain the origins of cosmic rays. Now the Crab has been put under close surveillance by orbiting x-ray and optical telescopes as well as AGILE and Fermi, in hopes of pinpointing longer-wavelength clues within the nebula during the next gamma outburst. (M. Tavani et al., AGILE collaboration, Science, in press, doi:10.1126/science.1200083 ; A. A. Abdo et al., Fermi collaboration, Science, in press, doi:10.1126/science.1199705 .)—Bertram Schwarzschild