search for dark matter in the galactic center
TRANSCRIPT
First constraints onDark Matter from Fermi
observations of thegalactic center
Christine Meurer on behalf of the Fermi LAT collaboration
July 14, 2009TeVPA 2009 at SLAC
TeVPA 2009/14/07, SLAC Christine Meurer (Stockholm) 2/13
Outline
• LAT instrument
• Gamma rays from dark matter annihilation
• Fermi sky map of the galactic center region
• Analysis strategy
• Fermi spectral distribution of galactic center region
• Upper limit for dark matter
• Conclusions, ongoing work and outlook
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France: IN2P3, CEA/Saclay
Italy: Universities and INFN of Bari, Perugia, Pisa, Roma Tor Vergata, Trieste, ASI, INAF
Japan: Hiroshima University, ISAS, RIKEN
United States: CSU Sonoma. UC Santa Cruz, Goddard, NRL, OSU, Stanford (SLAC and HEPL), Washington, St. Louis
Sweden: Royal Institute of Technology (KTH), Stockholm University, Kalmar University
Fermi Large Area Telescope Collaboration
Principal Investigator: Principal Investigator:
Peter MichelsonPeter Michelson (Stanford & SLAC)
~407 Members (includes ~97 Affiliated Scientists,71 Postdocs,
and 123 Graduate Students)
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The Large Area Telescope LAT
LAT
GBM
e+
γ
e -Calorimeter
1.80mTracker
ACD
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Gamma Rays from Dark Matter
simulation simulation
simulation simulation
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Fermi sky map of the galactic center region
• The Fermi data of the galactic center region look very promising!• However, the region is very crowded by many sources (point sources and extended once).• Therefore, the analysis of the galactic center is extremely complicated.
Counts map with radius = 5deg around GCSources from bright source list are included, seeA.A. Abdo et al., The Astrophysical Journal Supplement Series, 183:46–66, 2009 July
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Strategy of Fermi GC data analysis
• At the moment the analysis of the galactic center region with focus on dark matter can not be done in a final way due to the limited amount of data.• To understand the data in detail, we need a very advanced analysis.• We are working forcefully on a maximum likelihood method which does a combined spectral and spatial fit. • The method has already been tested on simulations for a while, now we apply it to real data. Therefore, all point sources in the region of interest around the galactic center are modeled in detail as well as the diffuse galactic component.
Be aware:This is the analysis method we will focus on for publication, but this is NOT the method used for the analysis shown in this presentation!
TeVPA 2009/14/07, SLAC Christine Meurer (Stockholm) 8/13
Analysis shown in this presentation
● The goal of this analysis is to give an estimation of an upper limit of the Dark Matter flux and cross section. The spectrum you will see on the next slides (not background subtracted) is fitted with a broken power law.● The goal of this analysis is NOT to explain the spectrum of the galactic center source. ● For this analysis the LAT is not used to full capacity.
(1) Convolving a model function (in this case a broken power law) with the instrument response function(2) Fitting this to the data spectrum by minimizing the -log(Likelihood) function (only the spectral distribution of the data is used and not the spatial information)
Used method
Data selection• 8 month Fermi data (August 2008 – April 2009) in a 1deg x 1deg square around the galactic center (RA = 266.46deg, Dec= -28.97deg) • Energy range: 200MeV-40GeV, IRFs = P6_V3_DIFFUSE
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Uncorrected spectral event distribution, broken power law convolved with IRF
Residuals
Fit results for brokenPL2 (v07.3.3.3.2)
Integral (100MeV-100GeV) = (1.22 ± 0.02)* 10-6 cm-2 s-1
Index1= -1.38 ± 0.04Index2= -2.60 ± 0.05BreakValue=(1623 ± 107) MeV
chi2/dof=0.87
Preliminary
Preliminary
syst. uncertainty (eff. area) is taken
into account in chi2 calculation
* IRF
stat. error bars only
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Estimation of unfolded spectrum
residual=counts rate−broken power law∗IRFbroken power law∗IRF
* IRF = convolved with IRFs
Estimation of unfolded data:Estimation of unfolded data:
unfolded data=broken power lawresidual⋅broken power law
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Estimated corrected spectral event distribution
Fit results for brokenPL2 (v07.3.3.3.2)
Integral (100MeV-100GeV) = (1.22 ± 0.02)* 10-6 cm-2 s-1
Index1= -1.38 ± 0.04Index2= -2.60 ± 0.05BreakValue=(1623 ± 107) MeV
Preliminary
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Upper limit for DM flux (100MeV-50GeV)andDM cross section (100MeV-50GeV)
Confidence level 95% (v18.6)flux(DM) = 2.43 * 10-7 cm-2 s-1
σ v (DM) = 3.98 * 10-25cm3s-1
Confidence level 99% (v18.8)flux(DM) = 3.39 * 10-7 cm-2 s-1
σ v (DM) = 5.55 * 10-25cm3s-1
Upper limit for dark matter
chi2/dof=0.9
chi2/dof=2.6 chi2/dof=3.8
Confidence level 95% (v18.6) Confidence level 99% (v18.8)
(v07.3.3.3.2)
brokenPL2
brokenPL2brokenPL2
DM(bb)50GeV
DM(bb)50GeV
The given upper limits for Dark Matter are conservative ones.
dof =7-4 = 3
A fit of the GC Fermi spectrum with a Dark Matter model only gives a very bad fit (chi2/dof=66.5!).
w/o DM
For dark matter DMFit is used, see Tesla E. Jelterna, Stefano ProfumoJCAP 0811:003,2008 arXiv:0808.2641[astro-ph]
Preliminary
Preliminary
Preliminary
Preliminary
N/b
inN
/bin
N/b
in
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Conclusions and outlook
• The Fermi LAT has successfully taken 11 month of data until today.• The Fermi LAT data of the galactic center region look very promising (many details are visible).• However, the analysis of the galactic center region is extremely complicated due to many sources (point like and diffuse) in this area.• The 8 month Fermi LAT data of a 1degx1deg square at the galactic center region can be well described by a broken power law.• A first study for an upper limit of dark matter flux and cross section at the galactic center has been performed.• The Dark Matter and New Physics Group of the LAT collaboration is looking forward to publish a more advanced and detailed analysis of the galactic center region soon.