Satellite Showcase
Fermi Gamma-Ray Space Telescope
|
The Fermi satellite being readied for
launch. The Large Area Telescope is on the top, and three of the Gamma-ray
Burst Monitors are on the facing side. Fermi is 2.8 m tall and 2.5 m wide. (Click for larger view)
|
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 know 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.
|
A cut-away of the Fermi LAT instrument.
A gamma-ray enters at the top of the stack, and electron-positron
pairs form within the stack. The LAT has 16 such stacks. (Click for larger view)
|
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)
|
One of the Fermi GBM detectors. (Click for larger view)
|
The secondary instrument onboard is the Gamma-ray Burst Monitor, or GBM,
built by Marshall Space Flight Center and the Max-Planck-Institut for
extraterrestrische Physik 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 will be able to 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 is devoting its
first year to conducting this survey,and will release this data to
astronomers all around the world. Fermi will then begin a program
of observations which will be proposed by astronomers.
During its first year, Fermi is also observing gamma-ray bursts, and using 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
|
The EGRET gamma-ray sources. Note the
number (and locations) of the unidentified sources (green circles). (Click for larger view)
|
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 the 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.
Publication Date: June, 2008
Updated: September, 2008
|