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

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No consumables onboard LAT Steady response over time expected. Standard operation in sky survey' mode allows almost flat exposure of the sky ... – PowerPoint PPT presentation

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


1
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

2
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
3
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
4
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

5
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)

6
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

7
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
8
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
9
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
10
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
11
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

12
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
13
Systematic uncertainties from foreground modeling
isotropic diffuse
isotropic diffuse
isotropic diffuse
isotropic diffuse
isotropic diffuse
14
SED of the isotropic diffuse emission (1 keV
100 GeV)
15
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
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