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The Gamma Ray Large Area Space Telescope - UW

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The Gamma Ray Large Area
Space Telescope (GLAST)
Dalit Engelhardt
Boston University
Observational Cosmology Lab
Department of Physics
University of Wisconsin-Madison
• Gamma ray basics
• Brief History of gamma-ray experiments
• The Gamma-Ray Large Area Space Telescope
– General mission information
– Scientific goals
– Instrumentation
Gamma Rays
• Highest-energy end of the electromagnetic spectrum
– E > 10 keV
– λ < 0.01 nm
– f > 3× 10 Hz
• Produced by nuclear transitions
• Ionizing radiation
– Photoelectric effect
– Compton Scattering
– Pair production
• Not bent by magnetic fields
Ionization Processes
Photoelectric Effect
Compton Scattering
E < 50 keV
100 keV < E < 10 MeV
Pair Production E > 1.02 MeV
(dominant method of photon interaction with
matter at E > 30 MeV)
Gamma Rays – Some History (I)
• 1900 – Paul Ulrich Villard observed a new type of rays
not bent by magnetic fields
• 1910 – William Henry Bragg showed that the rays
observed by Villard ionized gas in a similar way to x-rays
• 1914 – Ernest Rutherford and Edward Andrade showed
that the rays were a type of electromagnetic radiation by
measuring their wavelengths (crystal diffraction), coined
the term “gamma” rays
Gamma Rays – Some History (II)
• 1948-1958 – works by Feenberg and Primakoff (1948), Hayakawa
and Hutchinson (1952), and Morrison (1958) led scientists to believe
that a number of different processes which were occurring in the
universe would result in gamma-ray emission
– Cosmic ray interactions with interstellar gas, supernovae, interactions of
energetic electrons with magnetic fields
• 1961 – first gamma-ray telescope, carried into orbit by Explorer XI
– Picked up < 100 cosmic gamma-ray photons
– Apparent “uniform gamma-ray background”
• SAS-2 (1972), COS-B (1975-1982) satellites
– Confirmed earlier findings of gamma-ray background
– First detailed map of the sky at gamma-ray wavelengths
– Detection of a few point sources, but poor resolution prevented
identification of most of these with individual stars or stellar systems.
Gamma Rays – Some History (III)
• Late 1960’s – early 1970’s: Vela military satellite series
– Designed to detect gamma ray flashes from nuclear bomb
blasts, recorded gamma-ray bursts from outer space instead
• 1991 – launch of NASA’s Compton Gamma Ray
Observatory (CGRO)
– De-orbited in 2002 due to technical failure
• 2002 – launch of the ESA’s International Gamma-Ray
Astrophysics Laboratory (INTEGRAL). Achievements
– Spectral measurement of gamma-ray sources
– Detection of GRBs
– Mapping of the galactic plane in gamma-rays
Gamma Rays – Some History (IV)
• Ground-based experiments:
– Only very high-energy gamma ray permeate
through the earth’s atmosphere: currently
earth-based experiments can only detect
gamma-ray photons of energies greater than
1 TeV
– Imaging Atmospheric Cherenkov Telescope
• HESS, VERITAS, MAGIC, High-Energy-GammaRay Astronomy (HEGRA) telescopes
General Mission Information
• Space-based
– Lower-energy gamma rays are blocked by the earth’s atmosphere
• Joint venture of NASA and the U.S. Department of Energy and other
physics and astrophysics programs in the partner countries of France,
Germany, Italy, Japan, and Sweden
• Construction completed in May 2006
– Currently undergoing environmental testing in the U.S. Naval Laboratory
in Washington, D.C.
• Projected launch: September 2007 (on a Delta 2920H-10
launch vehicle)
– Low-earth circular orbit (565 km altitude) at 28.5 degree inclination,
period: 95 minutes
– Scan the entire sky every three hours
– Mission designed for a lifetime of 5 years, with a goal of 10 years of
– Mission will start with a one-year all-sky survey of gamma-ray sources,
after which guest observers will be able to apply for observation time
Scientific Goals
Blazar-class active galactic nuclei (AGNs)
Solar flares
Unidentified Gamma-ray sources
Gamma-ray bursts
Dark matter
Blazar-class AGNs
• Blazar = AGN with a relativistic jet pointing in earth’s
• GLAST could increase the number of known AGN
gamma-ray sources from about 70 to thousands
• All-sky monitor for AGN flares пѓ offer near-real-time
alerts for telescopes operating at other wavelengths
• Gamma-ray beams of pulsars are broader than their
radio beams пѓ GLAST will be able to search for many
more pulsars (radio-quiet)
– Will provide definitive spectral measurements that will distinguish
between the two primary models proposed to explain particle
acceleration and gamma-ray generation: outer cap and polar cap
Solar Flares
• Recent findings show that the sun is a source of
gamma rays in the GeV range
– GLAST will explore the acceleration of particles in the
Unidentified Gamma-Ray Sources
• More than 60% of recorded gamma-ray sources remain unidentified
(no known counterparts at other wavelengths)
– Likely less than a third are extragalactic (probably blazar AGNs)
– Possibilities: star-formation regions surrounding the solar
neighborhoods, radio-quiet pulsars, interactions of individual pulsars or
neutron binaries with the interstellar medium, Galactic microquasars,
supernova remnants, entirely new phenomenon (?)
Gamma-Ray Bursts
Nature and sources relatively unexplored and unknown
– Possible explanations: stars collapsing to form fast-rotating black holes,
Because of high-energy response and short dead time GLAST will be better
equipped to investigate GRBs than current telescopes
– May permit gamma-ray-only distance determinations
– Will provide near-real-time location information to other observatories
– Can slew autonomously towards bursts for monitoring by its main instrument
Dark Matter
It would be very nice if I could get a picture for this one…
• Theory: weakly interacting massive particles (WIMPs)
annihilating each other, thus producing gamma rays
– Can expect a spatially diffuse, narrow emission line peaked
toward the galactic center
• GLAST will resolve the isotropic background detected by
earlier observations into discrete AGN sources
– Large area, low instrumental background
• Other possibility: diffuse, cosmic residual пѓ possible
connection with particle decay in the early universe
GLAST Burst Monitor (GBM)
1 keV
10 keV
Large Area Telescope (LAT)
100 keV 1 MeV 10 MeV 100 MeV 1 GeV 10 GeV 100 GeV 1 TeV
GLAST Burst Monitor (GBM)
• Collaborative effort between the National Space Science and
Technology Center in the U.S. and the Max Planck Institute for
Extraterrestrial Physics (MPE) in Germany
• Primary objective: to augment the GLAST LAT scientific return from
gamma-ray bursts
– Extend the energy range of burst spectra down to 5 keV
– providing real time burst location data over a wide field-of-view
(FOV) with sufficient accuracy to repoint the GLAST spacecraft
– Provide near-real-time burst data to observatories (either groundor space-based operating at other wavelengths) to search for
• Sensitive to x-rays and gamma rays with 5 keV < E < 25 MeV
Scintillation Detectors (I)
• Basic idea: convert high-energy photons to low-energy photons
(fluorescence), which can then be detected by photomultiplier tubes
Incoming gamma rays (photons)
rxn with Matter (e.g. scintillator crystals)
Compton scattering
Photoelectric Effect
Pair production
High-energy charged particles (electrons or positrons)
rxn with scintillator crystals
Lower-energy photons
Detection in photomultiplier tubes (PMTs)
Scintillation Detectors (II)
Scintillation Detectors (III)
• Absorption of high energy (ionizing) electromagnetic or
particle radiation пѓ fluorescence (at a Stokes-shifted
– When gamma rays pass through matter, high-energy electrons
or positrons are produced (compton scattering, photoabsorption,
pair production) пѓ charged particles interact with scintillator пѓ emission of lower-energy photons
• Lower decay time (short duration of fluorescence
flashes) пѓ shorter “dead time”
• Collection of emitted photons usually done by
photomultiplier tubes (PMTs)
• Types of scintillators: organic crystals, liquids, or plastics;
inorganic crystals
– Gamma-ray detection usually uses inorganic crystals, which
have high stopping powers пѓ useful for detection of high-energy
– but longer decay times (order of hundreds of nanoseconds) than
organic materials пѓ longer “dead time”
Photomultiplier Tubes
Highly sensitive detectors of UV, visible, and near
Multiply signal
from incident light by as much as a
factor of 10
High gain, low noise, high frequency response
Large area of collection
GBM Characteristics
Total Mass: 115 kg
Trigger Threshold: 0.61 ph/cm2/s
Telemetry Rate: 15-25 kbps
Low-Energy Detectors
High-Energy Detectors
NaI (Sodium
BGO (Bismuth
126 cm2
126 cm2
1.27 cm
12.7 cm
Energy range
8 keV to 1 MeV
Energy range
150 keV to 30 MeV
The Large Area Telescope (LAT)
• Employs the techniques of a pair telescope
– Alternating converter and tracking layers to calculate ray direction and
• Precision tracker consisting of an array of tower modules of 19 xy pairs of
silicon-strip detectors and lead converter sheets
• SSDs will have the ability to determine the location of an object in the sky to
within 0.5 to 5 arc minutes
– Absorption of e+/e- pair by scintillator detector or calorimeter to
determine initial ray energy
• LAT uses CsI calorimeters пѓ scintillation reactions with CsI blocks result in
flashes of light that are photoelectrically converted to voltage
– Anti-coincidence shields covering the entire telescope with a charged
particle detector to prevent the system from triggering due to other types
of cosmic rays
• LAT uses segmented plastic scintillator tiles
• Also uses a data acquisition system that provides further detection of false
(non-gamma) signals
• Sensitive to gamma rays of 20 MeV < E < 300 GeV
GLAST Stanford Home:
GLAST NASA Homepage:
NASA’s Imagine the Universe:
The Space Science Division at the Naval Research Lab:
Max Planck Institute for Extraterrestrial Physics (Germany):
Boston University’s Institute for Astrophysical Research:
G & A Engineering:
Ruhr-Universitat Bochum (Germany):
The Gamma Ray Astronomy Team at NASA:
Space Daily:
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