National Aeronautics and Space Administration

Fermi Gamma-Ray Space Telescope

The Mission

The Fermi Gamma-Ray Space Telescope is the latest high energy gamma-ray observatory launched by NASA. It is designed to study energetic phenomena from a variety of celestial sources. Fermi is a collaboration between NASA, the Department of Energy, and science communities in six other nations.

The key scientific objectives of Fermi are to:

• understand how particles are accelerated in pulsars, supernovae, and active galaxies
• identify currently unidentified gamma-ray sources in the sky
• determine the high energy behavior of gamma-ray bursts
• study phenomena which may shed light on dark matter and particle physics.

While under development, the satellite was known as the Gamma-ray Large Area Space Telescope (GLAST). With the release of its first-light image of the gamma-ray sky, NASA renamed the satellite to honor Enrico Fermi.

Instruments

Fermi uses two instruments to observe the gamma-ray universe: the Large Area Telescope and the Gamma-ray Burst Monitor.

Large Area Telescope (LAT)

The primary instrument on Fermi is the Large Area Telescope, or LAT, built at Stanford University. It has a wide field-of-view, allowing it to see about 20% of the sky at the same time. It will detect gamma rays with energies ranging from 20 MeV to 300 GeV (10 million to 150 billion times the energy of the light detected by the human eye). With the resolution and sensitivity of its imaging capabilities, the LAT represents a major advancement over previous gamma-ray telescopes.

The LAT detects gamma rays by using a technique known as pair-conversion. When a gamma ray slams into a layer of tungsten in the detector, it creates an electron and positron pair. These particles in turn hit another, deeper layer of tungsten, each creating further particles and so on. The direction of the incoming gamma ray is determined by tracking the direction of these cascading particles back to their source using high-precision silicon detectors. Furthermore, a separate detector counts up the total energy of all the particles created. Since the total energy of the particles created depends on the energy of the original gamma ray, counting up the total energy determines the energy of that gamma ray. In this way, Fermi is able to make gamma-ray images of astronomical objects, while also determining the energy for each detected gamma ray.

Gamma-ray Burst Monitor (GBM)

The secondary instrument onboard is the Gamma-ray Burst Monitor, or GBM, built by Marshall Space Flight Center and the Max Planck Institute for Extraterrestrial Physics in Germany. The GBM is designed to observe gamma ray bursts (GRBs), which are sudden, brief flashes of gamma rays that occur about once a day at random positions in the sky. While NASA's Swift satellite has been studying gamma-ray bursts since 2004, there is still much to learn about them. Fermi is adding to our knowledge by studying GRBs to a much higher energy than Swift. The GBM has such a large field-of-view that it can see bursts from over 2/3 of the sky at one time, providing locations for follow-up observations of these enigmatic explosions. The GBM is composed of two sets of detectors - 12 sodium iodide (NaI) scintillators and two cylindrical bismuth germanate (BGO) detectors. When gamma rays interact with these crystalline detectors, they produce flashes of visible light, which the detector can use to locate the gamma-ray burst on the sky. The GBM works at a lower energy range than the LAT, so together they provide the widest range of energy detection in the gamma-ray regime for any satellite ever built.

Observing Plan

Since the LAT can see 20% of the sky at any time and can cover the sky every three hours, the primary observing objective of Fermi is to conduct a detailed survey of the entire sky. Fermi devoted its first year to conducting this survey. Fermi then began a program of observations which has been proposed annually by astronomers.

During its first year, Fermi also observed gamma-ray bursts, and used the LAT to study bursts in detail. This will continue throughout the mission.

Science Areas that Fermi will Study

Fermi will study not only a wide array of objects, but also attempt to solve some fundamental unsolved issues.

Unidentified Objects

In the 1990s, the EGRET instrument aboard the Compton Gamma-Ray Observatory observed 271 gamma-ray sources. Interestingly, two-thirds of them have not been identified because their positions in the sky were not known precisely enough. That is, we don't know whether these gamma-rays objects are stars or black holes or neutron stars or supernovae or distant galaxies. Astronomers expect many of them could be in other galaxies. The LAT on Fermi will observe thousands of sources (including the ones seen by EGRET), and will be able to pinpoint their locations well enough so other telescopes can look at them. These observations should help to identify these objects. Some may be pulsars or supernova remnants in our galaxy, while others may be in other galaxies. And some may hold suprises!

Looking for Dark Matter

Fermi will also look for annihilations of postulated weakly-interacting massive particles (WIMPs) in the halo of the Milky Way, and around other galaxies. These particles could possibly be dark matter, which so far makes its presence known only by its gravitational pull on matter that we can see. Recent theoretical work suggests that annihilation of WIMPs could be detectable with Fermi. The signature would be spatially diffuse, narrow line emission peaked toward the Galactic center, and around other galaxies. It would not be detected as a point source, but from an area possibly as large as the full moon. In addition, the gamma-ray light would be continuous, not short like a gamma-ray burst.

Published: June 2008
Text Reviewed: September 2018