Probing Dark Matter and pre-Galactic Lithium with Hadronic Gamma Rays

1. Probing Dark Matter and pre-Galactic Lithium with Hadronic Gamma Rays Tijana Prodanović University of Illinois at Urbana-Champaign Brian D. Fields, UIUC John F. Beacon, OSU

2. Outline • Cosmic Rays & Gamma Rays • Diffuse Hadronic Galactic Gamma Rays • What we know • What we don’t know: dark matter signal? • Diffuse Hadronic Extragalactic Gamma-Ray Background • Lithium-gamma-ray connection • Probe lithium nucleosynthesis • Structure Formation Cosmic Rays • Implications • Constraints • Conclusion Seminar @ McGill 01/19/2006

3. Cosmic Rays • High-energy charged particles • Accelerated in astrophysical (collisionless) shocks • Spectrum: • Strong shocks: Flux~E-2 • Measured (JACEE 1998) : Flux~E-2.75 • CR proton energy density in local interstellar medium: ~0.83 eV/cm3 (typical galactic magnetic field B~3μGauss has ε~0.25 eV/cm3) • Trivia: CR muon flux at sea level 1 cm-2 min-1 Seminar @ McGill 01/19/2006

4. Any shock source is a candidate! (Blandford & Eichler 1987) Sites Supernova remnants galactic cosmic rays Structure formation shocks structure formation cosmic rays (Inoue, Kang, Miniati) Cosmic Ray Acceleration Sites VLA image of Tycho SNR Reynolds & Chevalier Seminar @ McGill 01/19/2006

5. Why Cosmic Rays? • Galaxy/Universe is a “beam dump” for CRs ! • Probe acceleration sites • Probe particle physics beyond our reach • Probe dark matter (WIMP annihilation) • Understand processes difficult to access • Structure formation shock properties • Cosmological baryons • But CRs diffuse in the magnetic field… Seminar @ McGill 01/19/2006

6. Gamma Rays … • give away direction! • “Pionic” gamma-rays • Distinctive spectrum – pion “bump”; peaks at mπ/2 (Stecker 1971; Dermer 1986) • But no strong evidence for pion “bump” • Can use the shape of the spectrum (Pfrommer & Enßlin 2003) to find max “pionic” fraction (Prodanović & Fields 2004) • Relativistic electrons • Inverse Compton (off starlight & CMB) • Synchrotron • Bremsstrahlung Seminar @ McGill 01/19/2006

7. Seminar @ McGill 01/19/2006

8. 1. Galactic Cosmic Rays: 1.1 Galactic Gamma-Ray Sky Gaetz et al (2000) SNR E0102-72

9. Galactic “Pionic” Gamma Rays Prodanović and Fields (2004a) • Find max “pionic” fluxso that “pion bump” stays below observed Galactic spectrum • Galactic CRs: • Max “pionic” fraction • But notice the residual! Brems/IC Strong et al. (2004) Seminar @ McGill 01/19/2006 ???

10. Constraining Dark Matter • Possible dark matter annihilation gamma-ray signals • But first need to constrain gamma-ray foreground • Boer et al. 2005 (astro-ph/0508617) : “GeV excess” due to WIMP annihilation, mass ~ 50-100 GeV • Such signal requires low “pionic” component Seminar @ McGill 01/19/2006

11. Constraining Dark Matter Foreground Prodanović, Fields & Beacom (2006) In preparation • Though inverse Compton component at low end, cutoff at ~ TeV • Pionic gamma’s dominate? • Unconventional spectral index • Milagro: pionic dominates indeed • Observed spectral index • Milagro: pionic only ~10% ! • “TeV excess”? • Another component? Dark matter? Point sources? Need more data!!!! Milagro Water Cherenkov Detector Seminar @ McGill 01/19/2006

12. Beck et al. 1994 1. Galactic Cosmic Rays:1.2 Extragalactic Gamma-ray Background

13. Li-g-ray Connection • Any cosmic-ray source produces both gamma-rays and lithium • Connected essentially with ratio of reaction rates (Fields and Prodanović 2005) • Li abundance: local CR fluence • Diffuse extragalactic : CR fluence across Universe • Given one, constrain other Seminar @ McGill 01/19/2006

14. Extragalactic Gamma-Ray Background • Still emission at the Galactic poles • Subtract the Galaxy EGRB is the leftover(Strong 2004, Sreekumar 1998) • Guaranteed components(Pavlidou & Fields 2002) • Normal galaxies • Blazars(Stecker & Salamon 1996) • Any other cosmic-ray source Seminar @ McGill 01/19/2006

15. Estimating Galactic CR “pionic” g-rays from Extragalactic Gamma-Ray Background • “Pion bump” not observed in extragalactic gamma-ray background • Maximize pionic spectrum so that it stays below the observed extragalactic gamma-ray background • Galactic CRs – accelerated in supernova remnants; use propagated spectrum • Integrate over the redshift history of Galactic CR sources (cosmic star-formation rate) • Max “pionic” fraction Fields and Prodanović (2005) Seminar @ McGill 01/19/2006

16. Galactic CRs and 6Li • 6Li only made by cosmic rays • Standard assumption: 6LiSolarmade by Galactic CRs • Li-gamma-ray connection: 6LiSolar requires → but entire observed extragalactic gamma-ray background(Strong et al. 2004) • What’s going on? • Milky Way CR flux higher than average galaxy? • CR spectrum sensitivity (thresholds!; Li probes low E) • 6Li not from Galactic CRs? Seminar @ McGill 01/19/2006

17. The “Lithium Problem” • 7Li made predominantly in the Big Bang Nucleosynthesis (Cyburt, Fields, Schramm) • Measurements in low-metallicity halo stars: lithium plateau (Spite & Spite 1982) → indicate primordial lithium • But WMAP (2003)result: primordial Li~ 2 times higher than observed in halo-stars lithium problem • Any pre-Galactic sources of Li would contribute to halo-stars and make problem even worse! • And then there were cosmological cosmic rays… Cyburt 2005 (private communication) Seminar @ McGill 01/19/2006

18. 2. Structure Formation Cosmic Rays: Extragalactic Gamma-Ray Background Miniati et al. 2000

19. Structure formation shocks - cosmological shocks that arise from baryonic infall and merger events during the growth of large-scale structures (Miniati) Diffusive shock acceleration mechanism structure formation/cosmological cosmic rays X-ray observations of galaxy clusters: non-thermal excess(see eg. Fusco-Femiano et al. 2004) A large reservoir of energy and non-thermal pressure Miniati et al. 2000 Structure Formation Cosmic Rays Seminar @ McGill 01/19/2006

20. How To Find Them? • Implications still emerging • Will contribute to extragalactic gamma-ray background (Loeb & Waxman 2000) • CRs make LiBeB (Fields, Olive, Ramaty, Vangioni-Flam) • But structure formation CRs are mostly protons and α-particles → only LiBeB • Will contribute to halo star Li abundance → “Li problem” even worse! (Suzuki & Inoue 2002) • Use Li-gamma-connection as probe Seminar @ McGill 01/19/2006

21. Estimating SFCR“pionic” g-rays from Extragalactic Gamma-Ray Background • Structure Forming Cosmic Rays – assume all come from strong shocks with spectrum (about the same source spectrum as for Galactic CRs, but does not suffer propagation effects) • Assume all pionic g-rays are from structure formationCRs and all come from single redshift (unlike for Galactic CRs, history not known in this case) • Max “pionic” fraction z=0 z=10 Prodanović and Fields(2004a) Seminar @ McGill 01/19/2006

22. Structure Formation CRs and Lithium • Use Li-gamma-ray connection • From observed extragalactic gamma-ray background estimate maximal pionic contribution, assign it to structure formation CRs → estimate LiSFCR production (depending on the assumed redshift) • Structure formation CRs can be potentially significant source of pre-Galactic Lithium! • Need constrain structure formation CRs Seminar @ McGill 01/19/2006

23. Searching for SFCRs in High Velocity Clouds • Clouds of gas falling onto our Galaxy (Wakker & van Woerden 1997) • Some high-velocity clouds have metallicity 10% of solar • Some high-velocity clouds show evidence for little or no dust • Origins: • Galactic fountain model(Shapiro & Field 1976) • Extragalactic • Magellanic Stream-type objects or • gas left over from formation of the Galaxy(Oort, Blitz , Braun) low-metallicity, HVCs with little dust are promising sites for testing pre-Galactic Li and SFCRs(Prodanović and Fields 2004b) Seminar @ McGill 01/19/2006

24. New Data on the Way! • GLAST: • A view into “unopened window” (up to 300 GeV) • Better extragalactic background determination • Pion feature? • Cherenkov experiments: TeV window • H.E.S.S. • Southern hemisphere • Observing since 2004 • VERITAS • Northern hemisphere • upcoming H.E.S.S. Collaboration Science 309 (2005) 746 Seminar @ McGill 01/19/2006

25. Final Thoughts… • Universal, model independent approach: pionic spectrum • Probe both Galactic and extragalactic sources • Li-gamma connection • Need to disentangle diffuse gamma-ray sky! • Probe CR populations and implications through their diffuse gamma-ray signatures • A foreground for possible dark matter signal • Structure Formation Cosmic Rays • A large energy reservoir • Can have important effects (e.g. even worse lithium problem) • The key: need to use different measurements in concert (e.g. TeV measurements: lever arm on pionic component) Seminar @ McGill 01/19/2006

26. The End Seminar @ McGill 01/19/2006

27. Strong et al. (2004) Diffuse Galactic continuum, EGRET data Conventional model Optimized model Seminar @ McGill 01/19/2006

28. Detecting TeV Gammas • EM air shower: • ~TeV gamma’s hit top of the atmosphere • → pair production; Compton Scat. + Bremsstrahlung → high-energy gamma’s → pair production … = electron + photon cascade • Cherenkov light • Particle travels “superluminously” • “Pool” of faint bluish light (~250m diameter) • For 1 TeV photon: 100 photons/m2 • Extensive air shower detectors: • Air Cherenkov: reflectors • Eth~200 GeV, small f.o.v., short duty cycle • Air Shower Array: scintillators • Eth~50 TeV, large f.o.v., long duty cycle H.E.S.S. Seminar @ McGill 01/19/2006

29. Cherenkov Radiation: Gamma-ray vs. Hadronic More narrow cone! Seminar @ McGill 01/19/2006

30. Milagro Gamma-Ray Observatory • Miracle telescope@ LANL • Water CherenkovExtensive Air Shower Array: • Low threshold • Large field of view • Observing24/7 • Cherenkovlight threshold • lower in the water • Gamma’spair-produce faster • 450 + 273PMTs TeV Gamma-Ray Fishing! Seminar @ McGill 01/19/2006

31. Point sources in Milagro Region Seminar @ McGill 01/19/2006

32. BBN in the light of WMAP Dark, shaded regions = WMAP+ BBN predictions Light, shaded and dashed regions = observations Cyburt, Fields and Olive (2003) Seminar @ McGill 01/19/2006

33. Seminar @ McGill 01/19/2006