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

Observations of the extragalactic diffuse gamma-ray emission with the Fermi Large Area TelescopeMarkus Ackermann SLAC National Accelerator Laboratory

on behalf of the Fermi LAT collaboration

TeVPA 2009, SLAC National Accelerator Laboratory

TeVPA 2009, SLAC , 13.07.09 – 18.07.09 ● Markus Ackermann for the LAT collaboration ● 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 effective areafor vertically

incident -rays

LAT exposure @ 3GeV(1-year sim.)

2.8 1010 cm2s 3.8 1010 cm2s


Main contributions to the Fermi gamma-ray sky

EGRET EGB

LAT (E>100 MeV)

9 month observation

Inverse Compton 0-decay

Bremsstrahlung

Galactic diffuse emission(CR interactions with the interstellar medium)

Resolved sources

Isotropicdiffuseemission

• Residual cosmic rayssurviving background rejection filters

• misreconstructed -rays from the earth albedo


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

The isotropic diffuse gamma-ray emission

Dermer, 2007

Isotropic diffuse flux contribution from unresolved sources depends on LAT point source sensitivity

Contribution expected to decrease with LAT observation time


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

primary protonsalpha + heavy ion

EGRET EGB

sec. protonssec. positronssec. electrons albedo-gammasprim. electrons


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

LAT standard event classes:

Event class

Background contamination

transient <~ 100 x EGRET EGB flux

source <~ 20 x EGRET EGB flux

diffuse <~ 1 x EGRET EGB flux

MC study(Atwood et al. 2009)


Data selection for the analysis of the isotropic component

Example for improved background rejection: Transverse shower size in Calorimeter • clean dataset (observations with

high -ray flux, low CR flux)• contaminated dataset

(observations with low -ray flux, high CR flux)

• predicted distribution from LAT simulation

Improved residual background suppression: Factor 4-5 (above 1 GeV) compared to diffuse class

Retained effective area for -rays (relative to standard selection):

60-90%

Effective area rationew selection / standard selection

cleancontaminated simulation


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 A Analysis B

Isotropic spectrum shown here derived by analysis A

Resulting isotropic spectra agree within respective errors.


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 foreground -rays split into 3

Galactocentric annuli and into contributions from HI, H2 & radiation field

• Individual spectra of TS>200 (~>14) 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

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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

Sreekumar et al. 1998

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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 0-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

-ray emission model

HI (7.5kpc < r < 9.5kpc)

-ray emission model

Inverse Compton scattering


The LAT isotropic diffuse flux (200 MeV – 100 GeV)

10º < |b| < 20º 20º < |b| < 60º |b| > 60º galactic diffuse isotropic diffuse data sources

galactic diffuse isotropic diffuse data sources

galactic diffuse isotropic diffuse data sources


Systematic uncertainties from foreground modeling

isotropic diffuse

isotropic diffuse isotropic diffuse isotropic diffuse

isotropic diffuse


SED of the isotropic diffuse emission (1 keV – 100 GeV)


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 =2.45 between 200 MeV and 50 GeV

The spectrum as well as the characterization of the uncertainties from foreground modeling are preliminary

Documents

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