|At gamma-ray energies between 1 and 10 GeV, radio galaxy NGC 1275 is visible (left). At higher energies (10 to 100 GeV), another source (galaxy IC 310) can be faintly seen nearby (center). At energies from 100 to 300 GeV), only the new source is visible. (Credit: NASA/DOE/LAT collaboration and A. Nerenov et al.)
Greenbelt, MD--After more than three years in space, NASA's Fermi Gamma-ray Space Telescope is extending its view of the high-energy sky into a largely unexplored electromagnetic range. Yesterday, the Fermi team announced its first census of energy sources in this new realm.
Fermi's Large Area Telescope (LAT) scans the entire sky every three hours, continually deepening its portrait of the sky in gamma rays. The LAT is sensitive to photons with energies of 30 MeV to greater than 300 GeV and rejects 99.999% of signals generated by cosmic rays. Gamma rays interact with tungsten sheets in the LAT, producing electrons and positrons whose directions are analyzed to pinpoint the direction of the gamma ray. An "anticoincidence detector" produces a signal when a cosmic ray is detected, telling the data acquisition system to reject the signal.
"Before Fermi, we knew of only four discrete sources above 10 GeV, all of them pulsars," said David Thompson, an astrophysicist at NASA's Goddard Space Flight Center. "With the LAT, we've found hundreds, and we're showing for the first time just how diverse the sky is at these high energies."
Any object producing gamma rays at these energies is undergoing extraordinary astrophysical processes. More than half of the 496 sources in the new census are active galaxies, where matter falling into a supermassive black hole powers jets that spray out particles at nearly the speed of light. Only about 10% of the known sources lie within our own galaxy. They include rapidly rotating neutron stars (pulsars), the expanding debris from supernova explosions, and in a few cases, binary systems containing massive stars. More than a third of the sources are completely unknown, having no identified counterpart detected in other parts of the spectrum.
Just as bright infrared sources may fade to invisibility in the ultraviolet, some of the gamma-ray sources above 1 GeV vanish completely when viewed at higher, or "harder," energies. One example is the well-known radio galaxy NGC 1275, which is a bright, isolated source below 10 GeV. At higher energies it fades appreciably and another nearby source begins to appear. Above 100 GeV, NGC 1275 becomes undetectable by Fermi, while the new source, the radio galaxy IC 310, shines brightly.