Search for Dark Matter Galactic Satellites with FermiLAT

1
Search for Dark Matter Galactic Satellites with
Fermi-LAT

• Ping Wang
• KIPAC-SLAC, Stanford University
• Representing the Fermi LAT Collaboration

2
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

• Photons directly radiated from the external legs
  dominant if mainly annihilating into charged
  leptons
• Very hard spectrum (1/E) a sharp cutoff at DM
  particles mass

• Secondary photons mainly through p0 decay
  dominant if mainly annihilation into heavy quarks
  or gauge bosons
• Non-power-law a soft cutoff at DM particles mass

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Fermi LAT Sensitivity to DM Satellites
3 with gt5sigma for 1 year 10 with gt5sigma for 5
years

• 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 cm3/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)

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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 cm3/s, bb-bar only
• Compare the signal, background flux inside the
  tidal radius
• No. of sigma signal / sqrt (bkgd signal)

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 bgt10deg,
  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

• Pixel size 0.125 deg
• Grid size 2 deg x 1 deg

TS Map point source model
Residual TS Map
PRELIMINARY
PRELIMINARY
Residual TSgt30 (gt4sigma) like another source.
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TS Map and Residual TS Map for 200 MeV 300 GeV
10-month data

• Pixel size 0.125 deg
• Grid size 2 deg x 1 deg

TS Map point source model
Residual TS Map
PRELIMINARY
PRELIMINARY
Residual TSgt40 (gt5sigma) like another source.
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Smoothed Counts Map for 200 MeV 300 GeV
10-month data

• Pixel size 0.125 deg
• Grid size 2 deg x 1 deg

Smoothed Counts Map
PRELIMINARY
Also seems another source in this counts map
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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

• Pixel size 0.125 deg
• Grid size 2 deg x 1 deg

TS Map point source model
Residual TS Map
PRELIMINARY
PRELIMINARY
Residual TS25 (5sigma) like another source.
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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. Pwl
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NFW VS. Point source
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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)