Observations of Extragalactic Diffuse Gamma-Ray Emission with 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: > 8000 cm2 (standard event selection) • Field of view: 2.4 sr • Point source sensitivity(>100 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 @ 3GeV (1-year sim.) 3.8 1010 cm2s 2.8 1010 cm2s LAT effective area for vertically incident g-rays

3. Galactic diffuse emission (CR interactions with the interstellar medium) Inverse Compton p0-decay Isotropic diffuse emission Bremsstrahlung Resolved sources Main contributions to the Fermi gamma-ray sky LAT (E>100 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. 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 Residual CR produce unstructured, quasi-isotropic background(after sufficient observation time) Cosmic-ray background primary protons alpha + heavy ion EGRET EGB sec. protons sec. positrons sec. electrons albedo-gammas prim. electrons

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 |b|>10º 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 foregroundg-rays split into 3 Galactocentric annuli and into contributions from HI, H2 & radiation field • Individual spectra of TS>200 (~>14s) 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 • 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 < r < 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 g-ray emission model Inverse Compton scattering g-ray emission model HI (7.5kpc < r < 9.5kpc)

12. The LAT isotropic diffuse flux (200 MeV – 100 GeV) |b| > 60º 20º < |b| < 60º 10º < |b| < 20º 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 g=2.45 between 200 MeV and 50 GeV • The spectrum as well as the characterization of the uncertainties from foreground modeling are preliminary