Transcript The Gamma Ray Large Area Space Telescope (GLAST)

The Gamma Ray Large Area
Space Telescope (GLAST)
Dalit Engelhardt
7/18/06
Boston University
Observational Cosmology Lab
Department of Physics
University of Wisconsin-Madison
Outline
• Gamma ray basics
• Brief History of gamma-ray experiments
• The Gamma-Ray Large Area Space Telescope
(GLAST)
– General mission information
– Scientific goals
– Instrumentation
Gamma Rays
• Highest-energy end of the electromagnetic spectrum
– E > 10 keV
– λ < 0.01 nm
19
– f > 3× 10 Hz
• Produced by nuclear transitions
• Ionizing radiation
– Photoelectric effect
– Compton Scattering
– Pair production
• Not bent by magnetic fields
http://spacescience.nrl.navy.mil/images/
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)
http://imagine.gsfc.nasa.gov
http://en.wikipedia.org/
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
satellite
– 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
include:
– 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
technique
• HESS, VERITAS, MAGIC, High-Energy-GammaRay Astronomy (HEGRA) telescopes
http://www.dlr.de/rd/fachprog/extraterrestrik/Glast/glast.jpg
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
operation
– 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
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Blazar-class active galactic nuclei (AGNs)
Pulsars
Solar flares
Unidentified Gamma-ray sources
Gamma-ray bursts
Dark matter
Blazar-class AGNs
http://www.bu.edu/blazars
• Blazar = AGN with a relativistic jet pointing in earth’s
direction
• GLAST could increase the number of known AGN
gamma-ray sources from about 70 to thousands
• All-sky monitor for AGN flares  offer near-real-time
alerts for telescopes operating at other wavelengths
Pulsars
http://imagine.gsfc.nasa
.gov/Images/basic/xray/
pulsar.gif
• Gamma-ray beams of pulsars are broader than their
radio beams  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
models
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
flares
Unidentified Gamma-Ray Sources
http://www.gaengineering.com
• 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
http://csep10.phys.utk.edu
•
http://www.spacedaily.com/images/grb70228.jpg
Nature and sources relatively unexplored and unknown
– Possible explanations: stars collapsing to form fast-rotating black holes,
supernovae
•
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
(LAT)
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  possible
connection with particle decay in the early universe
Instrumentation
GLAST Burst Monitor (GBM)
http://www.mpe.mpg.de/gamma/instruments/glast/GB
M/
1 keV
10 keV
Large Area Telescope (LAT)
http://wwwalt.tp4.ruhr-unibochum.de/tp4/experimente/glast_intro-eg.html
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
counterparts
• Sensitive to x-rays and gamma rays with 5 keV < E < 25 MeV
http://f64.nsstc.nasa.gov/gbm/instrument/sciencegoals/spectroscopy.html
http://f64.nsstc.nasa.gov/gbm/
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)
http://imagine.gsfc.nasa.gov/Images/science/scintillator.gif
Scintillation Detectors (III)
• Absorption of high energy (ionizing) electromagnetic or
particle radiation  fluorescence (at a Stokes-shifted
wavelength)
– When gamma rays pass through matter, high-energy electrons
or positrons are produced (compton scattering, photoabsorption,
pair production)  charged particles interact with scintillator 
emission of lower-energy photons
• Lower decay time (short duration of fluorescence
flashes)  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  useful for detection of high-energy
radiation.
– but longer decay times (order of hundreds of nanoseconds) than
organic materials  longer “dead time”
Photomultiplier Tubes
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Highly sensitive detectors of UV, visible, and near
infrared
Multiply signal
from incident light by as much as a
8
factor of 10
High gain, low noise, high frequency response
Large area of collection
http://en.wikipedia.org/wiki/Image:Photomultipliertube.svg
http://f64.nsstc.nasa.gov/gbm/
GBM Characteristics
Total Mass: 115 kg
Trigger Threshold: 0.61 ph/cm2/s
Telemetry Rate: 15-25 kbps
Low-Energy Detectors
High-Energy Detectors
Material
NaI (Sodium
Iodide)
Material
BGO (Bismuth
Germanate)
Number
12
Number
2
Area
126 cm2
Area
126 cm2
Thickness
1.27 cm
Thickness
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
origin
• 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  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
http://imagine.gsfc.nasa.gov/docs/science/
http://wwwalt.tp4.ruhr-unibochum.de/tp4/experimente/glast_intro-eg.html
•http://www-glast.stanford.edu/
Sources
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GLAST Stanford Home: http://www-glast.stanford.edu/
GLAST NASA Homepage: http://glast.gsfc.nasa.gov/
NASA’s Imagine the Universe: http://imagine.gsfc.nasa.gov/
The Space Science Division at the Naval Research Lab:
http://spacescience.nrl.navy.mil/
Max Planck Institute for Extraterrestrial Physics (Germany):
http://www.mpe.mpg.de
Boston University’s Institute for Astrophysical Research:
http://www.bu.edu/blazars
G & A Engineering: http://www.gaengineering.com
Ruhr-Universitat Bochum (Germany): http://wwwalt.tp4.ruhr-unibochum.de/tp4/experimente/glast_intro-eg.html
The Gamma Ray Astronomy Team at NASA: http://f64.nsstc.nasa.gov/gbm/
Space Daily: http://spacedaily.com
Wikipedia: http://www.wikipedia.org