GLAST: Gamma-Ray Large Area Space Telescope

Gamma radiation, the most energetic form of radiation in the electromagnetic spectrum, is invisible by the human eye. Among the entire electromagnetic spectrum, only the narrow band of optical wavelengths (to say: visible light) is directly accessible by the human eye. In order to observe gamma rays, one needs to develope and apply dedicated detector techniques, similarily as necessary to access other wavebands invisible for the human eye like radio waves, infrared and ultraviolett radiation and X-rays. Because the physical generation processes of electromagnetic radiation at other wavelengths are different, also the detection principles differ with the energy of the radiation.

What is ISU's role in GLAST? Here, at Iowa State University, Martin Pohl as one of the four GLAST Interdisciplinary Scientists selected by NASA conducts his long-term research program on modeling the diffuse galactic gamma-ray emission. This emission tells us about the physical conditions in the Galaxy as seen by cosmic ray particles, and it may also reveal previously hidden gas, i.e. baryonic dark matter. Galactic diffuse emission is both a burden as it provides background, and a valuable tracer of cosmic rays and the interstellar medium. He intends to provide a) an extensive physical analysis of diffuse Galactic gamma-ray emission, and b) a continuously refined model of the Galactic gamma-ray emission to be used for foreground estimation by the GLAST team and the GLAST guest observers.

An Executive Summary of the GLAST IDS project may be obtained from here: postscript   pdf


How do we observe gamma radiation? - The Instruments aboard GLAST The incident photon interacts with the converter material and generates a electron-positron pair. This pair of charged particles could be traced in the upper part of the detector and its energy can be measured in a calorimeter. Because of their high energies, gamma rays cannot be focused by a lens or mirror like visible light in an optical telescope. The gamma rays would pass directly through any such telescope. Therefore astronomers make use of special gamma-ray detectors. NASAs next major gamma ray astronomy mission GLAST will be actually equipped with two detector systems, the Large Area Telescope (LAT) and a Burst Monitor (GBM).

The LAT is GLAST's primary instrument. It provides a large field of view (roughly a quarter of the entire sky) and will be capable to detect gamma radiation with energy between twenty million eV (20 MeV) up to threehundert billion eV (300 GeV). For comparison: The energy of visible light is about one eV). GLAST will take advantage of the classic detection principle of gamma radiation: An incident photon interact in thick target material (in case of GLAST: thin lead foils), while doing so creating a pair of charged particles - an electron and its antiparticle, a position. The leptons will be subsequentely traced in a tracking system (in case of GLAST: silicon strip detectors). From the particle trajectories one can reconstruct the original direction of the incident photon. Electron and positron will be finally absorbed in a calorimeter, where the energy deposit of the pair is a characteritic of the energy of the incident photon. More information on this topic could be found within the internet pages of the LAT team at Stanford University

What are the scientific goals of the GLAST mission? An all-sky simulation of the one year in-flight performance of GLAST. The bright emission belt arises from our Milky Way. The GLAST mission will be the successor of the Compton Gamma Ray Observatory (CGRO). Because high energy gamma radiation could only be produced unter extreme conditions, GLAST will study the most energetic astromonical objects in the Universe. In the energy range of GLAST, the Universe is largely transparent to gamma rays. This way especially energetic sources near the edge of the visible Universe can be investigated. There is good reason to expect that GLAST will see known classes of sources deep of the early universe. Gamma rays point back to their sources, unlike cosmic rays, which are deflected by magnetic fields. GLAST will study such exotic objects like heavy black holes and neutron stars, but will also investigate the life cycle of stars, and trace the dark components of matter. But also our own Galaxy, the Milky Way, is a source of gamma radiation, which is generated by interactions of energetic particles of the cosmic radiation and interstellar matter. And even our Sun generate gamma rays at its surface during flare outbursts . And with the capabilities of the GLAST instrument there is a good chance given for unexpected and perhaps even more fascinating discoveries!


Even more Astrophysics with GLAST ?

Why must GLAST be a satellite experiment? There are a few hurdles astronomers must overcome in order to detect gamma rays. For one, there are not a lot of gamma rays out there to be detected. Scientists must be able to wait a long time to get enough information from a source. Another is the fact that most gamma rays are absorbed by the Earth's atmosphere. This is good since gamma rays are extremely energetic and harmful to life on Earth, but if you want to observe gamma-ray sources from the Universe it can pose a problem. Therefore gamma-ray observations are generally done with high-altitude balloons or satellites launched into space, above the atmosphere. It should be noted that not all gamma-ray detectors have to be launched to high-altitudes or into space. Scientists can actually use the atmosphere as part of a detector and obtain information about high-energy gamma rays from the ground with what are called Atmospheric Cherenkov Telescopes. For lower energy gamma rays it is still necessary to get above the atmosphere, and for a gamma-ray mission like GLAST, which will last for several years and detect gamma rays at both low and high energies, it is prudent to place the detectors on a satellite and launch it into space.

Links

GLAST Project Office at LHEA, GSFC
GLAST LAT at Stanford University
GLAST Education & Public Outreach Pages