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Search for Dark Matter Galactic Satellites with Fermi-LAT

Ping WangKIPAC-SLAC, Stanford

University

Representing the Fermi LAT Collaboration

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Outline• Gamma-rays produced from Weakly

Interacting Massive Particle (WIMP) annihilation and Final State Radiation (FSR)

• Fermi LAT sensitivity to Dark Matter (DM) satellites

• Criteria for DM satellite candidate selection

• Preliminary results for 3-month LAT data

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Gamma-rays from WIMP Annihilation and FSR

• Secondary photons mainly through decay; dominant if mainly annihilation into heavy quarks or gauge bosons – Non-power-law; a soft cutoff at

DM particle’s mass

• Photons directly radiated from the external legs; dominant if mainly annihilating into charged leptons – Very hard spectrum (~1/E); a

sharp cutoff at DM particle’s mass

Photon spectrum ~ 1/E

Birkedal et al., 2005

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Fermi LAT Sensitivity to DM Satellites

• Simulation of Milky Way dark matter satellites with truncated NFW profile from Taylor & Babul, 2004, 2005

• Background estimate using Galprop diffuse and isotropic power law extragalactic diffuse (Sreekumar et al.)

• Generic WIMP model: WIMP mass 100GeV, cross-section 2.3e-26 cm^3/s, bb-bar only– This model can give the required relic density

• Compare the signal, background flux inside the tidal radius• No. of sigma = signal / sqrt (bkgd + signal)

~3 with >5sigma for 1 year

~10 with >5sigma for 5 years

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Fermi LAT Sensitivity to DM Satellites

• Simulation of Milky Way dark matter satellites with truncated NFW profile from Taylor & Babul, 2004, 2005

• Background estimate using Galprop diffuse and isotropic power law extragalactic diffuse (Sreekumar et al.)

• Generic WIMP model: WIMP mass 100GeV, cross-section 2.3e-26 cm^3/s, bb-bar only

• Compare the signal, background flux inside the tidal radius• No. of sigma = signal / sqrt (bkgd + signal)

24 realizations

Galprop conv.

DM satellites with >5 sigma for 1yr LAT observation

Average of radial extension (observed by LAT) ~ 1 deg

Radial extension (deg)

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Criteria for DM Satellite Candidate Selection

• Source has spatial extension (can be resolved by the LAT)

• Source energy spectrum is non-power-law (if WIMP annihilation) or 1/E power-law (if FSR)

• Source is not variable• Source has no counterparts at other

wavelengths

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Analysis Method to Search for DM Satellites using LAT Data

• Blind search strategy– Optimize the analysis method using 3-month LAT

data – Fix the analysis method and analyze 1-year LAT data

• Analysis method– Search for 5 sigma detections with |b|>10deg, which

are not identified in Fermi LAT catalog– Test source extension

• Hypothesis testing: NFW model VS. point source model– Test source energy spectrum

• Hypothesis testing: WIMP annihilation spectrum VS. power law spectrum

– Test source variability• Light curve 1 week bins

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Preliminary Results for 3-month LAT Data

• 3-month data– Aug 8th – Nov 7th, 2008– 200 MeV – 60 GeV– “Diffuse” class

• One interesting example– Possibly extended source– Possibly non-power-law– Not variable based on light curve with time interval 1

week– No molecular cloud counterpart– No dSph counterpart

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TS Map and Residual TS Map for 200 MeV – 60 GeV: 3-month data

Residual TS>30 (>4sigma); like another source.

•Pixel size = 0.125 deg

•Grid size = 2 deg x 1 deg

TS Map: point source model Residual TS Map

PRELIMINARY

PRELIMINARY

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TS Map and Residual TS Map for 200 MeV – 300 GeV: 10-month data

Residual TS>40 (>5sigma); like another source.

•Pixel size = 0.125 deg

•Grid size = 2 deg x 1 deg

TS Map: point source model Residual TS Map

PRELIMINARY

PRELIMINARY

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Smoothed Counts Map for 200 MeV – 300 GeV: 10-month data

Also seems another source in this counts map

•Pixel size = 0.125 deg

•Grid size = 2 deg x 1 degSmoothed Counts Map

PRELIMINARY

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Preliminary Results for 3-month LAT Data

• One interesting example– Possibly extended source– Possibly non-power-law– Not variable based on light curve with time

interval 1 week– No molecular cloud counterpart– No dSph counterpart– Two close sources

• Confirmed by 10-month data

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Summary• The Fermi LAT offers a unique opportunity to

discover DM satellites by the gamma rays produced in WIMP annihilations– Energy Spectrum– Spatial extent– Steady source– No counterparts at other wavelengths

• No DM satellite found in 3-month LAT data, and this result is consistent with generic WIMP model and N-body simulations.

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Back up slides

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TS Map and Residual TS Map for 1.6GeV – 3.2 GeV: 10-month data

Residual TS~25 (~5sigma); like another source.

•Pixel size = 0.125 deg

•Grid size = 2 deg x 1 deg

TS Map: point source model Residual TS Map

PRELIMINARY

PRELIMINARY

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Light Curve•Light curve with time interval 1 week

•Within 2 deg radius region around the source

PRELIMINARY

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Test Statistic• TS = 2 * (ln(L1) – ln(L0))• TS approximately follows a chi-square

distribution if comparing two hierarchically nested models.– L1: bkgd and a source– L0: bkgd only

• Otherwise, need to define a threshold of TS to accept (or reject) hypothesis 0.– Gtlike: DM spectrum VS. power law spectrum

• L1: bkgd and a DM point source (DMFit model for bb-bar channel only)

• L0: bkgd and a pwl point source model (pwl spectrum model)– Sourcelike: NFW source VS. point source

• L1: bkgd and a NFW source• L0: bkgd and a point source

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DM Spectrum VS. PwlData 0: pwl index

Data 0: flux /m^2/s (10MeV-600GeV)

No. of samples

Significance level

deltaTS

2.0 1.0e-3 10,000 0.001 2.95

2.0 6.0e-4 10,000 0.001 6.35

1.5 7.0e-5 10,000 0.001 4.52

1.5 4.0e-5 10,000 0.001 3.48

2.5 1.0e-2 10,000 0.001 -0.06

2.5 5.0e-3 10,000 0.001 4.31

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NFW VS. Point sourceData 0: pwl index

Data 0: flux /m^2/s (10MeV-600GeV)

No. of samples

Significance level

deltaTS

2.0 1.0e-3 1,000 0.001 5.21

2.0 6.0e-4 1,000 0.001 6.82

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Simulation of Gamma-ray Flux from Neutralino Annihilation in the Milky Way

•Gamma-ray flux from WIMP annihilation

Milky Way Halo simulated by Taylor & Babul (2005)

All-sky map of DM gamma ray emission (Baltz 2006)

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