Constraining Populations of Extragalactic Gamma-ray Sources with the Extragalactic Gamma-ray Background

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Constraining Populations of Extragalactic
Gamma-ray Sources with the Extragalactic
Gamma-ray Background

• Vasiliki PavlidouUniversity of Chicago
• Tonia Venters University
  of Chicago and APC (Paris 7)
• Jennifer Siegal-Gaskins University of
  Chicago
• Brian Fields
  University of Illinois
• Angela Olinto University
  of Chicago and APC (Paris 7)
• Carolyn Brown University of
  Chicago

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Extragalactic Gamma-ray Emitters1. Classes with
already detected members

• Blazars
• AGNs with jet aligned with line-of-sight
• Most populated class of EGRET identified sources
• Only class with identified members at high z
• Normal galaxies
• EGRET detected only 2 Milky Way, LMC
• GLAST will detect only 3 more SMC, M31, M33?
• Unidentified EGRET sources
• Likely to include many extragalactic sources
• More numerous than blazars!

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Extragalactic Gamma-ray Emitters2. Theorized
but yet-undetected classes

• Starburst galaxies
• GLAST will likely detect 10 (Torres et a
  2004)
• Merging / accreting clusters of galaxies and
  large-scale filaments
• Dark matter density peaks
• Insert your own here

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How do we make progress on extragalactic source
classes with no identified members?

• Theorize, then theorize some more.
• Use observations of the isotropic diffuse
  background to constraint properties of
  yet-undetected classes / exotic physics
• However
• unresolved members of established classes
  (blazars, normal galaxies, extragalactic
  unidetified sources) also have some guaranteed
  contribution to background
• These contributions need to be modeled and
  subtracted
• Task involves formidable uncertainties!

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How do we make progress on extragalactic source
classes with no identified members?

• Use information on background spectral shape

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How confident are we in the spectra of
unresolved emission from established source
classes?

• Unresolved emission spectra
• calculated from spectral index distributions of
  detected sources
• implicit assumption spectra of unresolved
  sources similar to spectra of resolved sources
• but also uncertainty involved in spectral index
  distribution of resolved sources (low number
  statistics, large measurement error in individual
  spectral indices)

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GLAST can help!

• GLAST will detect between 1,000 - 10,000 blazars
  (EGRET detected 60)
• v

Unresolved FSRQ emission using EGRET data
Unresolved FSRQ emission using GLAST data
(simulated)
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Conclusions

• Extragalactic diffuse emission encodes valuable
  information on (a) unresolved members of
  established classes(b) theorized, yet-undetected
  classes of extragalactic gamma-ray sources
• Spectral shapes of different components /
  observed emission powerful tool
  complementary to studies of relative emission
  intensity involves different, independent
  uncertainties
• GLAST will allow us to make much more confident
  predictions for spectral shape of known classes,
  and place much stricter constraints on
  properties of undetected classes