The Gamma Ray Large Area Space Telescope (GLAST)

1. The Gamma Ray Large Area Space Telescope (GLAST) Dalit Engelhardt 7/18/06 Observational Cosmology Lab Department of Physics University of Wisconsin-Madison Boston University

2. Outline • Gamma ray basics • Brief History of gamma-ray experiments • The Gamma-Ray Large Area Space Telescope (GLAST) • General mission information • Scientific goals • Instrumentation

3. 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

4. http://spacescience.nrl.navy.mil/images/

5. Ionization Processes Compton 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/

6. 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

7. 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.

8. 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

9. 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

10. The Gamma Ray Large Area Telescope (GLAST) - Mission Specifics http://www.dlr.de/rd/fachprog/extraterrestrik/Glast/glast.jpg

11. 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

12. Scientific Goals • Blazar-class active galactic nuclei (AGNs) • Pulsars • Solar flares • Unidentified Gamma-ray sources • Gamma-ray bursts • Dark matter

13. 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

14. 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

15. 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

16. 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

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

18. 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

19. Instrumentation Large Area Telescope (LAT) GLAST Burst Monitor (GBM) http://wwwalt.tp4.ruhr-uni-bochum.de/tp4/experimente/glast_intro-eg.html http://www.mpe.mpg.de/gamma/instruments/glast/GBM/ 1 keV 10 keV 100 keV 1 MeV 10 MeV 100 MeV 1 GeV 10 GeV 100 GeV 1 TeV

20. 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

21. http://f64.nsstc.nasa.gov/gbm/instrument/sciencegoals/spectroscopy.htmlhttp://f64.nsstc.nasa.gov/gbm/instrument/sciencegoals/spectroscopy.html

22. http://f64.nsstc.nasa.gov/gbm/

23. Scintillation Detectors (I) Incoming gamma rays (photons) • Basic idea: convert high-energy photons to low-energy photons (fluorescence), which can then be detected by photomultiplier tubes 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)

24. Scintillation Detectors (II) http://imagine.gsfc.nasa.gov/Images/science/scintillator.gif

25. 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”

26. 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

27. http://f64.nsstc.nasa.gov/gbm/

28. GBM Characteristics Total Mass: 115 kg Trigger Threshold: 0.61 ph/cm2/s Telemetry Rate: 15-25 kbps Low-Energy Detectors High-Energy Detectors

29. 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

30. http://imagine.gsfc.nasa.gov/docs/science/ http://wwwalt.tp4.ruhr-uni-bochum.de/tp4/experimente/glast_intro-eg.html

31. http://www-glast.stanford.edu/

32. 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