Observations of the extragalactic diffuse gammaray emission with the Fermi Large Area Telescope

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Observations of the extragalactic diffuse
gamma-ray emission with the Fermi Large Area
Telescope

• Markus Ackermann
• SLAC National Accelerator Laboratory
• on behalf of the Fermi LAT collaboration
• TeVPA 2009, SLAC National Accelerator Laboratory

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The Fermi Large Area Telescope

• Energy range 100 MeV 300 GeV
• Peak effective area gt 8000 cm2 (standard event
  selection)
• Field of view 2.4 sr
• Point source sensitivity (gt100 MeV) 3x10-9 cm-2
  s-1
• No consumables onboard LAT ? Steady response over
  time expected

• Standard operation in sky survey mode allows
  almost flat exposure of the sky

LAT exposure _at_ 3GeV (1-year sim.)
3.8 1010 cm2s
2.8 1010 cm2s
LAT effective area for vertically incident g-rays
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Main contributions to the Fermi gamma-ray sky
LAT (Egt100 MeV) 9 month observation

• Residual cosmic rays
• surviving background rejection filters
• misreconstructed g-rays from the earth albedo

EGRET EGB
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The isotropic diffuse gamma-ray emission

• Potential contributions to the isotropic diffuse
  continuum gamma-ray emission in the LAT energy
  range (100 MeV-300 GeV)
• unresolved point sources
• Active galactic nuclei
• Star-forming galaxies
• Gamma-ray bursts
• diffuse emission processes
• UHE cosmic-ray interactions with the
  Extragalactic Background Light
• Structure formation
• large Galactic electron halo
• WIMP annihilation

Dermer, 2007

• Isotropic diffuse flux contribution from
  unresolved sources depends on LAT point source
  sensitivity
• ? Contribution expected to decrease with LAT
  observation time

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Cosmic-ray background

• Primary cosmic-rays secondary CR produced in
  earth atmosphere
• Charged and neutral cosmic-rays outnumber
  celestial gamma-rays by many orders of magnitude
• CR contamination strongly suppressed by
  Anti-coincidence detector (ACD) veto and
  multivariate analysis of event properties

primary protons alpha heavy ion
EGRET EGB
sec. protons sec. positrons sec. electrons
albedo-gammas prim. electrons

• Residual CR produce unstructured,
  quasi-isotropic background (after sufficient
  observation time)

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Data selection for the analysis of the isotropic
flux

• 3 event classes defined in standard LAT event
  selection
• LAT isotropic flux expected to be below EGRET
  level (factor 10 improvement in point source
  sensitivity)
• More stringent background rejection developed for
  this analysis
• Event parameters used
• Shower shape in Calorimeter
• Charge deposit in Silicon tracker
• Gamma-ray probability from classification
  analysis
• Distance of particle track from LAT corners

MC study (Atwood et al. 2009)

• LAT standard event classes

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Data selection for the analysis of the isotropic
component

• Example for improved background rejection
  Transverse shower size in Calorimeter
• clean dataset (observations with high g-ray flux,
  low CR flux)
• contaminated dataset (observations with low g-ray
  flux, high CR flux)
• predicted distribution from LAT simulation

clean contaminated simulation

• Improved residual background suppression
• Factor 4-5 (above 1 GeV) compared to diffuse
  class
• Retained effective area for g-rays (relative to
  standard selection)
• 60-90

Effective area ratio new selection / standard
selection
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Dataset and analysis techniques

• Analysis of 10 month of LAT data (Aug 2008 Jun
  2009)
• Total observation time 1.9 x 107
  s
• Events classified as gamma-rays 7.3 x 106
• Two independent analyses performed to extract
  isotropic diffuse component

Analysis B
Analysis A
Resulting isotropic spectra agree within
respective errors.
Isotropic spectrum shown here derived by analysis
A
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Analysis A

• Pixel-by-pixel max. likelihood fit of bgt10º sky
  
• equal-area pixels with 0.8 deg2 (HEALPIX grid)
• sky-model compared to LAT data
• point source and diffuse intensities determined
  simultaneously
• Energy range 200 MeV - 100 GeV
• Sky model
• Maps of Galactic foreground g-rays split into 3
  Galactocentric annuli and into contributions from
  HI, H2 radiation field
• Individual spectra of TSgt200 (gt14s) point
  sources from LAT catalog
• Map of weak sources from LAT catalog
• Spectrum of isotropic component
• Subtraction of residual background (derived from
  Monte Carlo simulation) from isotropic component

LAT sky

gal. diffuse

point sources

isotropic
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Analysis B

• Analysis technique used for EGRET (Sreekumar et
  al, 1998)
• Source flux and residual background subtracted
  from the data
• Isotropic spectrum derived from the offset of the
  measured flux to the galactic diffuse foreground

LAT sky
-
point sources
-
CR contamination
Sreekumar et al. 1998
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Model of the Galactic foreground
g-ray emission model Inverse Compton
scattering
g-ray emission model HI (7.5kpc lt r lt
9.5kpc)

• Diffuse gamma-ray emission of Galaxy modeled
  using GALPROP
• Spectra of dominant high-latitude components fit
  to LAT data
• Inverse Compton emission (isotropic ISRF
  approximation)
• Bremsstrahlung and p0-decay from CR interactions
  with local (7.5kpc lt r lt 9.5kpc) atomic hydrogen
  (HI)
• HI column density estimated from 21-cm
  observations and E(B-V) magnitudes of reddening
• 4 kpc electron halo size for Inverse Compton
  component

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The LAT isotropic diffuse flux (200 MeV 100 GeV)
10º lt b lt 20º
20º lt b lt 60º
b gt 60º
galactic diffuse isotropic diffuse data
sources
galactic diffuse isotropic diffuse data
sources
galactic diffuse isotropic diffuse data
sources
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Systematic uncertainties from foreground modeling
isotropic diffuse
isotropic diffuse
isotropic diffuse
isotropic diffuse
isotropic diffuse
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SED of the isotropic diffuse emission (1 keV
100 GeV)
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Summary

• The spectrum of the isotropic diffuse emission
  was measured by Fermi LAT between 200 MeV and 100
  GeV
• It is compatible with a power law of index g2.45
  between 200 MeV and 50 GeV
• The spectrum as well as the characterization of
  the uncertainties from foreground modeling are
  preliminary