Fermi Science Topics
The mission will start with a one-year all-sky survey of gamma-ray sources, after which, a scheduled observing plan will be derived for guest observers who can apply for observation time through a formal selection process. Scheduled observations will be interrupted at times for special "targets of opportunity", i.e., spontaneous events that occur for which it would be in the best interest of the science to repoint the spacecraft immediately to obtain gamma-ray data of these targets. Some of things Fermi will study are:
Active Galactic Nuclei and Blazars
Distant galaxies, some of which are intense and highly variable flaring sources of high-energy gamma rays. AGN are powered by supermassive black holes with up to billions of times the Sun's mass. The gamma rays seen from some AGN arise from relativistic jets of material that are aimed in our direction. The image to the right is a Hubble Space Telescope optical image of M87's jet.
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Example: 3C279
Distance: 7 billion light-years
Size: central region 3 light-days across
Power: at flare maximum near 1,000,000,000,000 (trillion) times the power output of the Sun
Gamma-Ray Bursts
Intense flashes of gamma rays lasting from fractions of a second to hours, some with fading afterglows visible for months. Apparently associated with star forming regions in galaxies, these are among the most powerful explosions in the universe. The image to the right is an artist's conception of a gamma-ray burst.
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Example: GRB 990123
Distance: 10 billion light-years
Size: emitting region is light-seconds across
Power: at maximum up to 1,000,000,000,000,000,000 (quintillion) times the Sun's power
Pulsars
Rotating neutron stars with strong magnetic fields. Neutron stars are collapsed stellar cores with the density of an atomic nucleus (about 1 billion tons per cubic centimeter!) and are slightly more massive than the Sun. The image to the right is a Chandra Observatory X-ray image of the Crab Nebula pulsar.
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Example: The Crab Nebula Pulsar
Distance: 7,000 light-years
Size: 10 kilometer diameter neutron star rotating 30 times per second
Power: 75,000 times the power output of the Sun
Solar Flares
Intense energetic solar explosions which produce gamma rays and accelerate atomic particles. Solar flares are generated within the Sun's highly magnetic active regions and can bombard Earth with high-energy radiation. The image to the right is an X-ray image of the flaring Sun from the Yohkoh satellite.
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Example: Solar Flare 11 June 1991
Distance: 8 light-minutes
Size: solar active regions are several times the size of planet Earth
Power: a typical flare has one-tenth the power output of the entire quiet Sun
Unidentified Gamma-Ray Sources
Of the 271 gamma-ray sources detected by the EGRET instrument onboard the Compton Gamma-Ray Observatory, 172 are unidentified sources. Possible candidates for these sources include active galactic nuclei, pulsars, supernova remnants, dense molecular clouds, and stellar-mass black holes within our Galaxy. It is even quite possible that entirely new phenomena could account for some portion of these unidentified sources. Fermi will localize the positions of the unidentified sources accurately enough to make searches for counterparts at other wavelengths much more feasible. The image to the right is a Compton Observatory all sky gamma-ray image of the unidentified sources.
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Cosmology and Particle Astrophysics
There is strong evidence that matter that radiates across the entire electromagnetic spectrum is only 10% of the total mass of the universe. In other words, 90% of the mass of the universe does not emit light at any wavelength. Currently the only way to detect this so-called dark matter is by its gravitational effects on luminous matter. One theoretical candidate for what dark matter might be is a type of hypothetical particle called a weakly interacting massive particle, or WIMP. The theory that predicts the existence of WIMPS also predicts that they may annihilate one another, producing gamma rays. If the theory is correct, then Fermi may be capable of observing radiation from these particles in the galactic halo, helping to unravel the mystery of dark matter. The image to the right is a Hubble Space Telescope optical image of a gravitationally lensed cluster of galaxies.
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