View
25
Download
0
Category
Tags:
Preview:
DESCRIPTION
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. The Fermi Large Area Telescope. - PowerPoint PPT Presentation
Citation preview
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
TeVPA 2009, SLAC , 13.07.09 – 18.07.09 ● Markus Ackermann for the LAT collaboration ● 3
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
TeVPA 2009, SLAC , 13.07.09 – 18.07.09 ● Markus Ackermann for the LAT collaboration ● 4
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
TeVPA 2009, SLAC , 13.07.09 – 18.07.09 ● Markus Ackermann for the LAT collaboration ● 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 Residual CR produce unstructured, quasi-isotropic background (after sufficient observation time)
primary protonsalpha + heavy ion
EGRET EGB
sec. protonssec. positronssec. electrons albedo-gammasprim. electrons
TeVPA 2009, SLAC , 13.07.09 – 18.07.09 ● Markus Ackermann for the LAT collaboration ● 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
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)
TeVPA 2009, SLAC , 13.07.09 – 18.07.09 ● Markus Ackermann for the LAT collaboration ● 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 -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
TeVPA 2009, SLAC , 13.07.09 – 18.07.09 ● Markus Ackermann for the LAT collaboration ● 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 A Analysis B
Isotropic spectrum shown here derived by analysis A
Resulting isotropic spectra agree within respective errors.
TeVPA 2009, SLAC , 13.07.09 – 18.07.09 ● Markus Ackermann for the LAT collaboration ● 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 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
+
+
=
LAT
skygal.
diff
use
poin
t so
urce
siso
tropi
c
TeVPA 2009, SLAC , 13.07.09 – 18.07.09 ● Markus Ackermann for the LAT collaboration ● 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
Sreekumar et al. 1998
-
-
LAT
skypoin
t so
urce
sC
R
conta
min
atio
n
TeVPA 2009, SLAC , 13.07.09 – 18.07.09 ● Markus Ackermann for the LAT collaboration ● 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 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
TeVPA 2009, SLAC , 13.07.09 – 18.07.09 ● Markus Ackermann for the LAT collaboration ● 12
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
TeVPA 2009, SLAC , 13.07.09 – 18.07.09 ● Markus Ackermann for the LAT collaboration ● 13
Systematic uncertainties from foreground modeling
isotropic diffuse
isotropic diffuse isotropic diffuse isotropic diffuse
isotropic diffuse
TeVPA 2009, SLAC , 13.07.09 – 18.07.09 ● Markus Ackermann for the LAT collaboration ● 14
SED of the isotropic diffuse emission (1 keV – 100 GeV)
TeVPA 2009, SLAC , 13.07.09 – 18.07.09 ● Markus Ackermann for the LAT collaboration ● 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 =2.45 between 200 MeV and 50 GeV
The spectrum as well as the characterization of the uncertainties from foreground modeling are preliminary
Recommended