search for dark matter in the galactic center

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


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)


The Large Area Telescope LAT

LAT

GBM

e+

γ

e -Calorimeter

1.80mTracker

ACD


Gamma Rays from Dark Matter

simulation simulation

simulation simulation


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


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!


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


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


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


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


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


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.

search for dark matter in the galactic center

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