GLAST Science and Instruments

1
GLAST GeV Astronomy in a Multiwavelength
Context Jonathan F. Ormes NASA Goddard Space
Flight Center GLAST Large Area Telescope
Collaboration Jonathan.F.Ormes_at_nasa.gov
Special thanks to Dave Thompson and Steve Ritz
for providing most of this material
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Introduction

• Multiwavelength observations are essential for
  GLAST science
• Both previous observations and theoretical
  considerations show that gamma-ray sources are
  usually broad-band objects.
• GLAST will address many important questions
• What is going on around black holes? How do
  Natures most powerful accelerators work? (are
  these engines really black holes?)
• What are the unidentified sources found by EGRET?
• What is the origin of the diffuse background?
• What is the high energy behavior of gamma ray
  bursts?
• What else out there is shining gamma rays? Are
  there further surprises in the poorly measured
  energy region?
• When did galaxies form?
• GLAST operations will contribute to a wide range
  of cooperative, multiwavelength observations.

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Coordinated Observations

• Most of our science depends on coordinated
  observations
• Pulsars
• AGN
• SNRs
• Bursts
• Unidentified sources
• Discovery finding and understanding new
  phenomena
• Learning from experience

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Pulsars Rotating Neutron Stars

• Science Topics
• Neutron star population studies.
• Location and nature of particle acceleration and
  interaction (model testing).
• Physics in extreme magnetic and electric fields.
• Matter state at high densities.
• Relativistic effects.

Except for the pulsations themselves, time
variability is not significant for pulsars.
Simultaneous multiwavelength observations are not
essential.
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AGN Multiwavelength Variety
Radio Optical X-ray
Gamma ray

• Science Topics
• Location and nature of particle acceleration and
  interaction in jets.
• Confirmation of unified models.
• Cosmological probes using high-energy cutoff due
  to absorption by Extragalactic Background Light.
• Contribution to the diffuse background.

Fossatis summary of blazar Spectral Energy
Distributions
Due to variability on short time scales, AGN
require simultaneous multiwavelength observations
for maximum scientific return.
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A Few Additional Remarks on Multiwavelength AGN
Observations

• Measure spectral energy distributions
• model fits can tell us (in the context of the
  models!) some of the features and parameters of
  the system, if error analysis (both statistical
  and systematic) is understood.
• Leverage complementary capabilities
• Gamma spatial resolution isnt great, but we can
  tell time well (especially from ground-based
  observatories).
• Use radio take advantage of superior spatial
  resolution. By measuring flare time lags between
  radio and gamma, can get an indication of where
  emission is taking place.
• Time lags relative to other wavelengths also can
  tell us the size of the emission region (again,
  in the context of models).
• Physics with sources. Use optical, radio, x-ray
  need follow-up measurements to ID sources and
  measure redshifts.
• enables EBL studies, luminosity function
  determination, evolution studies, early Universe
  probes (especially 1-50 GeV observatories). Work
  for 100s or 1000s of new sources.

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AGN Challenge

• Supermassive BH systems shine magnificently in
  gammas, but what physics are we learning?
• Are we just measuring the weather?
• Thats interesting, but can we learn more about
  the holes themselves? (e.g., find a measure of
  the hole mass, spin and do a population survey
  study GR effects, etc.)

from Sikora, Begelman, and Rees (1994), shown in
Greg Madejskis talk
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AGN Multiwavelength Variability
3C279 Gamma rays X-rays UV Optical IR Radio

• Strength and phasing of flaring at different
  wavelengths is a powerful tool for modeling
  emission.
• Also need observations before and after a flare
  to be sure it is the same flare.
• Test universality of the idea that high states
  have flatter spectra.

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Supernova Remnants Origin of Cosmic Rays?

• Science Topics
• Electron v. proton acceleration in SNR.
• Interactions with interstellar medium.
• Upper limit of SNR acceleration of cosmic rays.

Multi-component model of SNR W44 by De Jager and
Mastichiadis (1997). Components include
synchrotron, thermal X-ray, pulsar wind nebula,
relativistic bremsstrahlung, and inverse Compton.
Time scale for variability is long therefore
simultaneity is not critical for multiwavelength
observations.
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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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Some Lessons Learned from CGRO
No Real Surprises Here

• Observers at other wavelengths seem very helpful
  with service observations, as long as the
  observing time needed is relatively small.
• Organizing a true multiwavelength campaign,
  especially for extended observations, is VERY
  difficult.
• Relying on a few friends to carry out
  coordinated observations is risky (ground
  conditions vary).
• We were most successful in just announcing a
  target of interest to a large group and then
  seeing who was able to collect data.
• AGN flares seemed to be more easily seen first in
  gamma rays these could prove to be a useful
  trigger for a coordinated campaign.
• Unidentified gamma-ray sources are far less
  interesting than known objects to observers at
  other wavelengths (largely due to position
  uncertainties).

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Gamma-ray Observatories
Complementary capabilities ground-based
space-based ACT EAS Pair angular
resolution good fair good duty
cycle low high high area large large
small field of view small large largecan
reorient energy resolution good fair good,
w/ smaller
systematic uncertainties limiting
factor background photon
statistics
The next-generation ground-based and space-based
gamma-ray telescopes are well matched.
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Gamma-ray Observatories
sensitivity
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GLAST Mission Operating Plan
GLAST is considered a facility. A Users Group
will be formed to help the SWG determine the
optimal scientific operations of the facility.

• Working Concept for GLAST Operations
• The first year will be an all-sky survey. Unlike
  CGRO, the survey will be carried out primarily in
  a scanning mode, keeping the instruments always
  pointed away from the Earth (which is bright in
  gamma rays). This approach takes maximum
  advantage of the large fields of view of the LAT
  and the GBM, allowing a full sky survey on short
  (day) timescales.
• After the first year, the scientific program will
  be determined by peer-reviewed proposals.
  Proposals can request pointed observations, based
  on scientific requirements. A pointed
  observation can obtain about 3 times as many
  photons from a source per unit time as the
  scanning mode. Target of Opportunity proposals
  can also be included.

Constraints on GLAST Operations Essentially
none! Any non-occulted source can be observed
at any time.
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Mission Repointing Plan for Bursts
Summary of plan During all-sky scanning
operations, detection of a sufficiently
significant burst will cause the observatory to
interrupt the scanning operation autonomously and
to remain pointed at the burst region during all
non-occulted viewing time for a period of 5 hours
(TBR). There are two cases
1. The burst occurs within the LAT FOV.
If the burst is bright enough that an
on-board analysis provides gt90 certainty that a
burst occurred within the LAT FOV, the
observatory will slew to keep the burst direction
within 30 degrees (TBR) of the LAT z axis during
gt80 of the entire non-occulted viewing period
(neglecting SAA effects). Such events are
estimated to occur approximately once per week.
Only if the burst is exceptionally bright,
the observatory will slew to bring the burst
direction within 30 degrees (TBR) of the LAT z
axis during gt80 of the entire non-occulted
viewing period (neglecting SAA effects). Such
events are likely to occur a few times per year.
2. The burst occurs outside the LAT FOV.
After six months, this strategy will be
re-evaluated. In particular, the brightness
criterion for case 2 and the stare time will be
revisited, based on what has been learned about
the late high-energy emission of bursts.
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GLAST Mission Data Plans

• Working Concept for GLAST Data
• During the all-sky survey. The LAT team will use
  the early data for instrument calibration and
  preparation of an initial source catalog. At the
  end of the survey, all the data will be made
  public. The LAT team and the GLAST Science
  Support Center will be developing user-friendly
  software to enable all types of standard
  analysis.
• After the first year. All data (Level 1 gamma
  ray events and sensitivity) will be made public.
  Guest Investigator proposals will be awarded for
  scientific ideas, not images or other data. Use
  of data will be handled on an honor system. No
  data will be proprietary. The GLAST Science
  Support Center will be providing users access to
  the data and software tools.
• At all times, data from transient sources,
  including burst data from the GBM, will be made
  public immediately, in order to facilitate
  multiwavelength observations. On-board
  processing will produce alerts for bursts within
  seconds.

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Challenges for Multiwavelength Observations (1)
The large GLAST field of view changes the
situation, even compared to CGRO

• GLAST can monitor the sky for variable sources.
  
• Especially in scanning mode, but even if pointed,
  GLAST will see a large fraction of the sky with
  good sensitivity every day. GLAST can serve as a
  trigger for observations at other wavelengths.
  Flaring sources probably require some processing
  on the ground and will require a day or more
  (depending on intensity) for quicklook
  recognition. How the GLAST instrument teams
  communicate this information is under discussion.
  Proposals include a Web site showing bright
  sources, updated regularly.
• Bursts will give an on-board trigger that can be
  sent immediately to the ground. The current plan
  is to use the GCN.

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Challenges for Multiwavelength Observations (2)

• Selecting the sources for multiwavelength study
  will rely largely on other wavelengths.
• Availability of telescope time, coordinated
  plans, and observational constraints will limit
  the number and choice of sources. The GLAST
  science teams want to cooperate in
  multiwavelength programs with the broadest
  coverage. The LAT team is encouraging small
  optical observatories and even amateurs to help
  monitor sufficiently bright sources. We will try
  to support ongoing programs that contribute to
  multiwavelength studies.

The GLAST Telescope Network
http//www-glast.sonoma.edu/gtn/index.html The
Whole Earth Blazar Telescope http//www.to.astro.i
t/blazars/webt/homepage.html The Whole Year
Blazar Telescope
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Challenges for Multiwavelength Observations (3)

• We need multiwavelength observation programs for
  non-flaring sources.
• The broad-band shape of the spectrum is often
  critical in determining the nature of an
  unidentified source.
• The fluctuation (power density) spectrum is an
  important part of understanding flares.
• Determining blazar radio properties and redshifts
  is important, since GLAST expects to detect many
  blazars that are not yet identified as such. The
  proposed VLBA Imaging and Polarimetry Survey
  program could be extremely valuable.
• Monitoring of pulsars is important. The Parkes
  radio telescope in the southern hemisphere and
  several northern telescopes, perhaps including
  SETIs Allen Telescope Array are important.

Maximizing the science will require cooperation
among many instruments at all wavelengths.
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Exciting Times Are Ahead!

• MILAGRO
• Running! Outriggers complete, transient
  notification system.
• STACEE, CELESTE
• Running!
• VERITAS
• Building prototype. Northern hemisphere, array
  capabilities, great co-located optical
  opportunities
• HESS
• Building and commissioning phase 1. Southern
  hemisphere, array capabilities
• MAGIC
• Building. Simultaneous optical monitoring to 18m
  w/ dedicated and integrated 80cm-100cm telescope
  planned. Observing w/ INTEGRAL planned.

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Exciting Times Are Ahead!

• SWIFT
• Launch in 2003. Multi-wavelength. Primary
  mission is burst science (gt100/year, GCN alerts,
  locations), plus hard x-ray survey, sky monitor.
• GLAST
• Launch in 2006. Satellite capabilities, large GO
  program planned, GTN underway, GCN alerts.
• Cross-calibration and overlap between
  ground-based and space-based measurements
• Optical monitoring campaigns being organized
• GTN, WEBT, WYBT
• The promise of neutrinos.
• Amanda II preliminary limit for Mkn501
  approaching optimistic models (factor 10
  improvement over AMANDA B10). Putting
  performance parameters on the same footing (Aeff
  w/ energy).

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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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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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Transient policy

• The GLAST instrument teams have the duty to
  release data on transient gamma ray sources to
  the community as soon as practical. The
  decisions on which data are to be released will
  be based on advice from scientists analyzing the
  data and an evaluation of the scientific interest
  that the data might generate. They will follow
  the general guidelines suggested below
• 1) Gamma-ray bursts All data on gamma-ray bursts
  that trigger either the LAT or GBM will be
  released. The prompt data release will include
  direction, fluence estimate and other key
  information about the burst immediately on
  discovery. Individual photon data and technical
  information for their analysis will be released
  as soon as practical.
• 2) Blazars and some other sources of high
  interest 10-20 pre-selected sources from the 3rd
  EGRET catalog will be monitored continuously and
  the fluxes and spectral characteristics will be
  posted on a publicly accessible web site. Another
  10-20 scientifically interesting sources will be
  added to this list during the survey. The list
  will include some known or newly discovered AGN
  selected to be of special interest by the TeV and
  other communities as well as galactic sources of
  special interest discovered during the survey.
• 3) New transients The community will be notified
  when a newly discovered source goes above an
  adjustable flux level of about (2-5) x 10-6
  photons (gt 100 MeV) per cm2 s for the first time
  the flux level is to be adjusted to set the
  release rate to about 1-2 per week. A source
  exhibiting unusual behavior that is detectable on
  sub-day timescales, such as a spectral state
  change or a large flux derivative while the
  source is at elevated flux levels, will also
  trigger an alert to the community.

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Multi-wavelength campaigns

• Science requires broad band (radio to gamma-rays)
  study of these celestial sources. Therefore,
  following the survey, the observing program will
  be determined entirely by the astronomical and
  high energy physics communities based on
  proposals submitted.
• LAT and GBM team members can compete, but cannot
  win additional funding.
• Non-US investigators may apply
• Selection is based on peer reviewed proposals.
• The community will interface to the GLAST data
  through the GLAST Science Support Center.
• SSC mirror sites in Italy (LAT and GBM may have
  others)

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GLAST 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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Data release policies

• All-sky survey during the first year.
• LAT team to produce a point source catalog and an
  all sky map formal release 90 days following
  completion of the survey.
• Transient source locations are made public
  immediately with photon data (light curves,
  improved positions, etc.) to follow as practical.
  
• During first year photon data to include warning
  that the data may be unverified and uncalibrated
• Best efforts to release preliminary catalogs in
  time for AOs
• The first 3 months of observations will be
  delivered at 6 months
• The full 12 months of observations will be
  delivered 1 month after the end of the sky survey
• Guest investigators may propose for source
  studies, associated theory or key projects
• Data from these sources of interest are made
  available immediately to the GIs.
• Following the survey, it is being proposed that
  all GLAST data will be made public immediately.
• Comments on this policy may be sent to
  Jonathan.F.Ormes_at_nasa.gov or Donald.A.Kniffen_at_nasa
  .gov.
• We plan to conduct workshops on how to propose
  for and how to the use the tools to analyze the
  GLAST data

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Some Lessons Learned from CGRO - AGN (1)
Notes from Bob Hartman/Dave Thompson

• Even in the optical, it is difficult to get light
  curves over several weeks for more than 1-3
  objects at a time.
• For most blazars, the peak of the synchrotron
  bump is in the far IR, corresponding to a Compton
  peak in the GeV range.  It would be very useful
  to be able to correlate those, but there are
  essentially no observing resources there.  We got
  no real help from ISO. SIRTF has capability out
  to 160 m but is likely to be oversubscribed.
• Except for the brightest blazars, X-ray
  observations require focusing instruments.
  However, the big focusing instruments (e.g.
  Chandra, XMM) are generally hugely oversubscribed
  and very inflexible.  We got some good support at
  times from ASCA, BeppoSAX, and RXTE, but they
  will all be gone by 2007. Astro-E2 may help.
• Most of the X-ray telescopes need to have the
  target near 90 degrees from the Sun.  This means
  that the ground-based observers, (optical, TeV,
  etc.) must observe in poor conditions, if it is
  possible at all. Swift may be a big help.
• INTEGRAL, Swift, and perhaps EXIST will provide
  the hard X-ray coverage in the GLAST era that was
  offered by COMPTEL and OSSE in the CGRO time
  frame, although the medium-energy gamma-ray range
  (10-30 MeV) will be open.

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Some Lessons Learned from CGRO - AGN (2)

• We had very good radio support from U. Michigan
  (5, 8, 15 GHz) and Metsahovi (Finland 22, 37
  GHz). However, both of those programs have
  funding problems. We might be able to influence
  that (had some success while CGRO was active).
• At higher radio frequencies (90-600 GHz, the
  instruments are largely dedicated to topics like
  star formation, etc.  We have a few friends in
  key places, but there is no possibility for
  monitoring at intervals less than 1 month.
• The capabilities for VLBI observations at
  frequencies up to and beyond 100 GHz are
  improving rapidly.  This allows resolution much
  closer to the nucleus, and better correlation of
  blob ejections with gamma flares. This will only
  get better.
• The situation is improving in the optical with
  the increase in the number of 0.5-1 meter
  automated telescopes.  However, for monitoring
  bright flares on timescales lt1 day, the observers
  and instruments are largely concentrated in
  Europe and NA.  The Whole Earth Blazar Telescope
  and the Whole Year Blazar Telescope (WEBT WYBT
  Mattox Villata, et al.) are valuable approaches
  to organizing this.
• For bright objects and flares, the amateur
  astronomy community can make valuable
  contributions.  With a small amount of financial
  aid (filters, maybe some CCDs), they could be
  much more valuable.  They are a dedicated bunch -
  some of them make as many blazar observations as
  the professionals. The GLAST Telescope Network
  (GTN) involves them.