Search for Dark Matter Galactic Satellites with Fermi-LAT

1. Search for Dark Matter Galactic Satellites with Fermi-LAT Ping Wang KIPAC-SLAC, Stanford University Representing the Fermi LAT Collaboration

2. 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 Outline

3. Birkedal et al., 2005 Photon spectrum ~ 1/E 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 particle’s 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 particle’s mass

4. 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

5. DM satellites with >5 sigma for 1yr LAT observation 24 realizations Galprop conv. Average of radial extension (observed by LAT) ~ 1 deg 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) Radial extension (deg)

6. 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

7. 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

8. 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

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

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

11. 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

12. 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

13. 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.

14. Back up slides

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

16. Light Curve • Light curve with time interval 1 week • Within 2 deg radius region around the source PRELIMINARY

17. 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

18. DM Spectrum VS. Pwl

19. NFW VS. Point source

20. 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)