The Gamma Ray Large Area The Gamma Ray Large Area Space Telescope (GLAST)Space Telescope (GLAST)

Dalit Engelhardt

7/18/06Boston 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– f > 3× 1019 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 ProcessesCompton Scattering

100 keV < E < 10 MeV

Photoelectric Effect

E < 50 keV

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-Gamma-Ray 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

• Blazar-class active galactic nuclei (AGNs)

• Pulsars

• Solar flares

• Unidentified Gamma-ray sources

• Gamma-ray bursts

• Dark matter

Blazar-class AGNs

• 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

http://www.bu.edu/blazars

Pulsars

• 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

http://imagine.gsfc.nasa.gov/Images/basic/xray/pulsar.gif

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

• 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 (?)

http://www.gaengineering.com

Gamma-Ray Bursts

• 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)

http://www.spacedaily.com/images/grb70228.jpg http://csep10.phys.utk.edu

Dark Matter

• 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

It would be very nice if I could get a picture for this one…

InstrumentationGLAST Burst Monitor (GBM)

1 keV 10 keV 100 keV 1 MeV 10 MeV 100 MeV 1 GeV 10 GeV 100 GeV 1 TeV

Large Area Telescope (LAT)

http://wwwalt.tp4.ruhr-uni-bochum.de/tp4/experimente/glast_intro-eg.html

http://www.mpe.mpg.de/gamma/instruments/glast/GBM/

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 ground- or 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• Highly sensitive detectors of UV, visible, and near

infrared• Multiply signal from incident light by as much as a

factor of 108

• 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 kgTrigger Threshold: 0.61 ph/cm2/s

Telemetry Rate: 15-25 kbps

Low-Energy Detectors

Material NaI (Sodium Iodide)

Number 12

Area 126 cm2

Thickness 1.27 cm

Energy range 8 keV to 1 MeV

High-Energy Detectors

Material BGO (Bismuth Germanate)

Number 2

Area 126 cm2

Thickness 12.7 cm

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-uni-bochum.de/tp4/experimente/glast_intro-eg.html

•http://www-glast.stanford.edu/

Sources• 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-uni-bochum.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