Search for Dark Matter Galactic Satellites with Fermi-LAT
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Transcript of Search for Dark Matter Galactic Satellites with Fermi-LAT
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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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