GLAST Science and Instruments

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GLAST Progress and Plans as of April
2003 Jonathan Ormes Project Scientist on behalf
of the GLAST team and the Large Area Telescope
Collaboration Jonathan.F.Ormes_at_nasa.gov
Second Veritas Symposium Chicago, April 25, 2003
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GLAST is an International Mission

• NASA - DoE Partnership on LAT
• LAT is being built by an international team
• Si Tracker Stanford, UCSC, Japan, Italy
• CsI Calorimeter NRL, France, Sweden
• Anticoincidence GSFC
• Data Acquisition System Stanford, NRL
• GBM is being built by US and Germany
• Detectors MPE

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Mission Objectives

• Understand the mechanisms of particle
  acceleration in astrophysical environments such
  as active galactic nuclei, pulsars and supernova
  remnants
• Determine the high energy behavior of gamma-ray
  bursts and other transients
• Resolve and identify point sources with known
  objects
• Probe dark matter and the extra-galactic
  background light in the early universe

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Science Requirements

• High Energy Gamma Rays 20 MeV - gt 300 GeV
• Source location lt0.5 arcmin
• High latitude source of 10-7 cm-2 s-1 flux, E-2
  spectrum,
• 1 s radius, after 1 yr survey
• Point source sensitivity lt 6 x 10-9 cm-2 s-1
• High latitude source after 1 yr survey, 5 s
  detection
• Background to be lt 10 of extragalactic high
  latitude diffuse emission
• Conduct broad band study of gamma ray bursts
• Determine burst spectra from lt10 keV to 30 GeV
• Determine burst locations lt15 degrees and send to
  the GRB notification network (GCN) within 7
  seconds

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GLAST Instruments
Large Area Telescope (LAT) PI Peter Michelson
Stanford University
GLAST Burst Monitor (GBM) PI Charles
Meegan Marshall Space Flight Center
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The GLAST Burst Monitor
Energy range 10 keV - 25 MeV FOV of 8 sr Notify
observers world-wide Recognize bursts in
realtime Positions to few degree
accuracy Transmit (within seconds)
GRB coordinates to the ground Re-point the main
instrument to GRB positions within 10 minutes
Side View
Top View
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GBM Capabilities
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GLAST Burst Monitoring

• LAT and GBM work synergistically to make new GRB
  observations

• GBM provides low-energy context measurements with
  high time resolution
• Broad-band spectral sensitivity
• Contemporaneous low-energy high-energy
  measurements
• Continuity with current GRB knowledge-base
  (GRO-BATSE)

• Provides rapid GRB timing location triggers
  w/FoV gt LAT FoV
• Improved sensitivity and response time for weak
  bursts
• Follow particularly interesting bursts for
  afterglow observations
• Provide rapid locations for ground/space follow-up

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Burst Alerts
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LAT Instrument
16 4x4 towers ? modularity height/width 0.4
? large field-of-view
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From EGRET to LAT
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LAT Sensitivity
200 ? bursts per year ? prompt emission
sampled to gt 20 µs AGN flares gt 2 month ?
time profile ?E/E ? physics of jets and
acceleration ? bursts delayed
emission all 3EG sources 80 new in 2 days ?
periodicity searches (pulsars X-ray binaries)
? pulsar beam emission vs. luminosity, age,
B 5-10 thousand sources in 1-yr survey ? AGN
logN-logS, duty cycle, emission vs.
type, redshift, aspect angle ? extragalactic
background light (? IR-opt) ? new ?
sources (µQSO,external galaxies,clusters)
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LAT Capabilities
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LAT Source Localizations
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LAT Instrument Triggering and Onboard Data Flow
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Large Area Telescope Parameters
CGRO/EGRET 20 MeV - 30 GeV 0.1 1500 cm2 0.5
sr 5.8 _at_ 100 MeV 0.5 _at_ 10 GeV 10-7 cm-2
s-1 100 ms 1810 kg 1991 - 1997
GLAST/LAT 20 MeV - gt 300 GeV 0.1 8,500 cm2 2.4
sr 3.5 _at_ 100 MeV 0.1 _at_ 10 GeV 3 ? 10-9
cm-2 s-1 lt10 ms 3000 kg 2006 - 2016
Change 10 to 300 GeV 5.6 4.8 Area 1/2.7 Area
1/25 1/30 gt 5

• Energy Range
• Energy Resolution (DE/E)
• Effective Area (1GeV)
• Field of View
• Angular Resolution
• Sensitivity (gt 100 MeV)
• Deadtime
• Mass
• Lifetime
• 1 year survey at high latitudes

Increased area, field of view, angular
resolution, extended energy range and operational
efficiency provide a powerful combination!
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Science Topics

• Active Galactic Nuclei
• Isotropic Diffuse Background Radiation
• Cosmic Ray Production
• Molecular Clouds
• Supernova Remnants
• Normal Galaxies
• Endpoints of Stellar Evolution
• Neutron Stars/Pulsars
• Black Holes
• Unidentified Gamma-ray Sources
• Dark Matter
• Solar Physics
• Gamma-Ray Bursts

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From EGRET to GLAST (gt100 MeV)

• Map the gamma-ray sky with sensitivity gt 30
  times that of EGRET without becoming source
  confusion limited.

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AGN What GLAST will do
SRD
LAT

• EGRET detected 70-90 AGN. Extrapolating,
  GLAST should expect to see dramatically more
  many thousands.
• The GLAST energy range is broad, overlapping
  those of ground-based experiments for good
  multiwavelength coverage.
• The wide field of view will allow GLAST to
  monitor AGN for time variability on many scales.

Joining the unique capabilities of GLAST with
other detectors will provide a powerful tool.
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Blazars
Most EGRET blazars were seen only when they
flared

• What is the population of high-energy blazars?
• What is the nature of the quiescent emission?
• What is the relation to radio luminosity and
  variability?
• What are the high-latitude unidentifieds?

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Blazar Spectra
Mrk 501

• GLAST combined with TeV observatories will probe
  the complex spectra of blazars.
• Large FoV allows GLAST to monitor AGN over the
  whole sky for variability on many timescales.

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Gamma-ray Observatories
sensitivity
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AGN Multi-wavelength Variability

• Strength and phasing of flaring at different
  wavelengths is a powerful tool for modeling
  emission.
• Observations before and after a flare to be sure
  it is the same flare.
• Improved sensitivity will allow measurements of
  flares to shorter time scales and lower flux
  values.

3C279 Gamma rays X-rays UV Optical IR Radio
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EGRET surprise Gamma Ray Bursts

• EGRET was poorly designed for gamma-ray burst
  studies and yet produced exciting results
• 4 Bursts seen in the EGRET spark chamber
• Combined bursts fit a hard spectrum with no
  evidence of a cutoff to 10GeV.

Composite high-energy spectrum of five GRB seen
by EGRET (by Brenda Dingus). Fewer than 100
photons were seen in the five bursts combined.
Gamma Ray Burst Afterglows - Five to 200?
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GLAST brings new capabilities to studies of gamma
ray bursts

• What fraction of bursts have high-energy
  afterglows?
• What is the spectrum and does it have a cutoff?
• Can the bursts be used to test quantum gravity?

Greatly reduced deadtime will allow much more
complete sampling of the light curves.
FoV and area will give more bursts and on-line
analysis of superior angular resolution will
enable rapid follow-up observations.
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EGRET contributions to pulsar physics

• ? luminosity vs. spin-down power
• spectral hardness with age
• cut-off energy with magnetic field
• beaming fraction
• periodicity searches (ms ? s)

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LAT studies pulsars

• Up to 250 pulsars will be detectable, with half
  previously unknown in radio
• (McLaughlin and Cordes 2000)

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Outer gap vs. polar cap models

• Where are particles accelerated?
• How is particle beam energy converted into
  photons?
• What is shape of pulsar beam?
• How many pulsars are there?
• What is the pulsar birth rate?
• Where is most of the energy?

• polar cap
• N(radio-loud) N(radio-quiet)
• long-lived
• outer gap
• N(radio-loud) ltlt N(radio-quiet) lifetime lt 1 or 2
  Myr

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LAT studies of SNR and CRs
LAT provides spatial separation

• What part of SNR emission is from shocked
  acceleration regions?
• What is the relative number densities of
  electrons and nucleons in cosmic rays?
• How do the cosmic ray intensities vary throughout
  the galaxy?
• Is cosmic ray production in other galaxies the
  same as in ours?
• How is the H2 distributed throughout the galaxy?

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Galactic Diffuse Emission
Giant Molecular Clouds in Cygnus region (galactic
arm structure?)
p0 flux measurement by GLAST will determine the
total mass in the GMCs and their C/H.
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EGRET to GLAST galactic diffuse gamma rays
EGRET
GLAST
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Cosmic Rays in the Magellenic Clouds

• Spatially spectrally resolve LMC, SMC and M31
• ? cosmic-ray distribution gt propagation and
  H2/CO mass conversion

Explore past SN rate, history and stability of
galaxies
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An Important Energy Band for Cosmology
Photons with Egt10 GeV are attenuated by the
diffuse field of UV-Optical-IR extragalactic
background light (EBL)
Opacity (Salamon Stecker, 1998)
opaque
No significant attenuation below 10 GeV.
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Searching for dark matter

• The lightest super-symmetric particle c is a
  leading candidate for non-baryonic CDM
• It is neutral (hence neutralino) and stable if
  R-parity is not violated
• It self-annihilates in two ways
• c c ? gg where Eg Mc c2
• c c ? Zg where Eg Mc c2(1-Mz2/4Mc2)
• Gamma-ray lines possible
• 30 GeV - 10 TeV

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EGRET unidentified sources

• Why care about number-flux relations?
• Detectability studies
• Diagnosis of source populations, e.g.
  completeness of samples
• Assessment of unidentified and unresolved
  sources and the backgrounds

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Unidentified Sources
Spectrum of 3EG J18355918 compared to that of
Geminga, indicating a probable isolated neutron
star (Halpern et al., Reimer et al. )

• Science Topics
• Discovery science.
• New sources or new insight about known objects.
• Nature of non-blazar transients.

Spectrum of 3EG J1714-3857/SNR RXJ1713-3946.
With the limited multiwavelength coverage, no
simple model explains the source (Reimer and
Pohl).
For transients or other variable unidentified
gamma-ray sources, having simultaneous
observations may be the only viable means of
positive identification.
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GLAST sources
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GLAST Project Master Schedule

• Instrument preliminary Design Reviews completed
• Spacecraft contractor selected Spectrum-Astro
• S/C PDR March 2003
• S/C CDR fall 2003
• Critical Design Reviews for instruments will be
  April or May this year
• Instrument deliveries in 2005
• GBM spring
• LAT summer
• Launch in 2006
• September (God willin and the creek dont
  rise.)

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Mission Requirements and Observing Plan

• Spacecraft
• Pointing knowledge lt 10 arcseconds (1 s)
• Observatory is designed to point anywhere,
  anytime
• Operate without pointing at the Earth
• Reorient quickly and autonomously to follow a
  transient
• 3 normal operational modes
• Scan (baseline)
• Inertial pointing
• Scan pointing - takes advantage of the wide field
  of view to optimize time on sky
• Mission Lifetime 5 years, Goal 10 years
• Observatory checkout 30-60 days
• First year is scanning to make all sky survey
• Planned observations subject to interruption for
  extraordinary transients
• Second year and beyond - operational mode driven
  by competitive proposals

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Guest Investigator Program

• GI program starts during the survey
• 10-15 GIs
• Will grow to 100 Guest Investigations funded by
  NASA each year.
• GLAST Fellows program
• Continue Interdisciplinary Scientist (IDS)
  Program
• C. Dermer (NRL) - non-thermal universe
• B. Dingus (Wisconsin) - transients
• M. Pohl (Ruhr U.) - diffuse galactic
• S. Thorsett (UCSC) - pulsars
• Program of Education and Public Outreach
  continues throughout the mission

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Transients (AGN Flares)

• PLAN FOR THE FIRST YEAR
• Most AGN science can be best addressed by the
  all-sky scan.
• Unusually large flares will be treated as Targets
  of Opportunity, and studied in a coordinated
  multiwavelength campaign, for those where a
  multiwavelength campaign is feasible.
• Thus, autonomous repointing of the spacecraft is
  not required for AGN science during the first
  year.
•  
• This approach will be re-evaluated after the
  first year, as new knowledge about AGN might
  demand a new strategy.

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Mission Repointing Plan for Bursts

• Summary of plan
• Detect a sufficiently significant burst
• Interrupt the scanning operation
• Remain pointed at the burst region for 5 hours
  (TBR).
• There are two cases

Reevaluate strategy based on what has been
learned about delayed high-energy emissions. the
brightness criterion the stare time
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Gamma-ray science requires multi-wavelength
approach

• In the MeV range and above, sources are
  non-thermal
• ? produced by interactions of energetic
  particles
• Nature rarely produces mono-energetic particle
  beams. Broad range of particle energies leads to
  a broad range of photon energies.
• Example po production
• Charged particles rarely interact by only one
  process. Different processes radiate in
  different energy bands.
• Example synchrotron-Compton processes
• High-energy particles, as they lose energy, can
  radiate in lower-energy bands.
• Contrast non-thermal X-ray source can have
  high-energy cutoff
• Due to variability on short time scales, AGN
  require simultaneous multiwavelength observations
  for maximum scientific return.
• For other science, the time scale for variability
  is long (e.g. SNR, plereons) therefore
  simultaneity is not critical for multiwavelength
  observations.
• For transients or other variable unidentified
  gamma-ray sources, having simultaneous
  observations may be the only viable means of
  positive identification.

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GLAST Ground System
User community
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Requirements Involving Slew Maneuvers

• All of the remaining requirements involve
  slewing
• Observing modes the all-sky scan involves
  North- South rocking for uniform coverage,
  and the pointed observation periods
• will have slews on/off
  target during occultation to continue sky
  viewing.
• Sky coverage uniformity depends on the rocking
  strategy in the all-sky scan.
• Upon detection of a transient (e.g., a GRB)
  meeting certain conditions, the observatory will
  repoint to maintain coverage of the transient
  source.

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Exposures, Two Orbits
Work done by S. Digel
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Earth Avoidance for Pointed Observations
Rotation of Earths Disk
Rotation of Earths Disk
Spacecraft Centered Celestial Sphere
Orbit Plane
134o dia Earth Disk
After Occultation
Before Occultation

• Earths disk is receding to the right
• FOV is picking up inertial target

• Earths disk is approaching from the left
• FOV is losing inertial target

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Scan-pointing
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Scan Pointed Observations
One day observation trade 20 exposure on source
for sky coverage
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GLAST Mission Overview
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Summary of Capabilities

• Huge FOV (20 of sky) allows primarily scanning
  operations
• Opens unexplored region gt 10 GeV
• Unprecedented PSF for gamma rays (factor 5 better
  than EGRET at 10 GeV)
• Expect to find new classes of gamma-ray sources
  with the improved sensitivity
• No expendables ? potential for long mission
  without degradation
• Large sensitive area (gt 6? EGRET) for transients
• Quick reaction to gamma-ray bursts and other
  transients