Search for Dark Matter Galactic Satellites with Fermi-LAT Ping Wang KIPAC-SLAC, Stanford University Representing the Fermi LAT Collaboration 1 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 2 Gamma-rays from WIMP Annihilation and FSR • Secondary photons mainly • Photons directly radiated from through p0 decay; dominant the external legs; dominant if if mainly annihilation into mainly annihilating into heavy quarks or gauge bosons charged leptons – Non-power-law; a soft cutoff at DM particle’s mass – Very hard spectrum (~1/E); a sharp cutoff at DM particle’s mass Birkedal et al., 2005 Photon spectrum ~ 1/E 3 Fermi LAT Sensitivity to DM Satellites ~3 with >5sigma for 1 year ~10 with >5sigma 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 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) 4 Fermi LAT Sensitivity to DM Satellites DM satellites with >5 sigma for 1yr LAT observation 24 realizations Galprop conv. • Average of radial Simulation of Milky Way dark matter satellites with truncated NFW extension (observed by profile from Taylor & Babul, 2004, 2005 LAT) ~ 1 deg • 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) 5 Radial extension (deg) 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 6 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 7 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 8 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 9 Residual TS>30 (>4sigma); like another source. 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 10 Residual TS>40 (>5sigma); like another source. 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 11 Also seems another source in this counts map 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 12 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. 13 Back up slides 14 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 15 Residual TS~25 (~5sigma); like another source. Light Curve •Light curve with time interval 1 week •Within 2 deg radius region around the source 16 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 17 DM Spectrum VS. Pwl Data 0: Data 0: flux No. of Significance deltaTS pwl index /m^2/s samples level (10MeV600GeV) 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 4.3118 NFW VS. Point source Data 0: Data 0: flux No. of Significance deltaTS pwl index /m^2/s samples level (10MeV600GeV) 2.0 1.0e-3 1,000 0.001 5.21 2.0 6.0e-4 1,000 0.001 6.82 19 Simulation of Gamma-ray Flux from Neutralino Annihilation in the Milky Way •Gamma-ray flux from WIMP annihilation d dN 1 annv E , l, b Bf ddE 4p 2 f dE f 2 (r ( , l , b)) LOS Milky Way Halo simulated by Taylor & Babul (2005) 2 m d All-sky map of DM gamma ray emission (Baltz 2006) 20