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
Say: OK to interrupt and ask questions

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
@ NCSU: interested in CR acceleration, GCRs mostly
Me: interested in CRs as probes of cosmo structure formation and sources of diffuse gammas, an connections
Tijana Prodanović
McGill /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
Equipartition between CRs and mag. Field.
Collisionless shocks: shock thickness is much smaller than collision mean free path.
Emphasize: COLLISIONLESS/astrophysical shocks , as opposed to eg. planetary bow-shocks (shock between solar wind and planets magnetosphere)
Tijana Prodanović
McGill /19/2006

4. Cosmic Ray Acceleration Sites
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)
VLA image of Tycho SNR
Reynolds & Chevalier
When mention SFCRs here, say that you’ll come back to (explaining) that latter in the talk
Tijana Prodanović
McGill /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…
Comment along “beam dump” reference: use “fixed target” experiment to probe both beam and the target.
Magnetic field is a blessing for acceleration of charged particles but a curse for their propagation.
Tijana Prodanović
McGill /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
Pion bump symmetry: doppler shift- 2 photons, one boosted towards one boosted away
Tijana Prodanović
McGill /19/2006

7. EXPLAIN !!! Coordinates, brightness, sources, diffuse!
Tijana Prodanović
McGill /19/2006

8. 1. Galactic Cosmic Rays: 1.1 Galactic Gamma-Ray Sky
Gaetz et al (2000) SNR E
Remnant in SMC: X-ray (blue, hot gas million deg, shock heated), optical (green, oxygen), and radio (red, synchrotron)

9. Galactic “Pionic” Gamma Rays
Prodanović and Fields (2004a)
Find max “pionic” flux so that “pion bump” stays below observed Galactic spectrum
Galactic CRs:
Max “pionic” fraction
But notice the residual!
Bring attention to “GeV excess” !!!
Brems/IC
Strong et al. (2004)
Tijana Prodanović
McGill /19/2006
???

10. Constraining Dark Matter
Possible dark matter annihilation gamma-ray signals
But first need to constrain gamma-ray foreground
Boer et al (astro-ph/ ) : “GeV excess” due to WIMP annihilation, mass ~ GeV
Such signal requires low “pionic” component
WIMPs (cold (non-relativistic) dark matter ;massive and neutral; eg. Supersymetric neutralinos) can self-annihilate
Low pionic component: finite number of gama’s=> DM signal at the expense of pionic gamma’s
De Boer biggest problem: need huge DM core, up to 10 kpc!
Tijana Prodanović
McGill /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!!!!
Say: now check consistency of proposed WIMP signal with TeV observations.
CR knee: ~ 10^15 eV = 10^6 GeV ; CR ankle~10^19 eV
Point sources: do contribute but don’t know how much
Milagro Water
Cherenkov Detector
Tijana Prodanović
McGill /19/2006

12. 1. Galactic Cosmic Rays: 1.2 Extragalactic Gamma-ray Background
Beck et al. 1994
1. Galactic Cosmic Rays: Extragalactic Gamma-ray Background
Picture from Beck et al. (1994): total radio intensity (contures) , radio halo, traces CRs (electrons, synchrotron)
Emphasize: Now EGRB , so not Galactic gamma’s anymore

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
“First, let me show you why we care about EGREB, what can we learn from it”
Emphasize: EXTRAGALACTIC i.e. looking at what CRs do not in our Galaxy but in ALL galaxies together
Tijana Prodanović
McGill /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
Tijana Prodanović
McGill /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)
Tijana Prodanović
McGill /19/2006

16. Galactic CRs and 6Li
6Li only made by cosmic rays
Standard assumption: 6LiSolar made 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?
Tijana Prodanović
McGill /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…
BBN curves are 1 sigma. All things plotted with 1 sigma.
Primordial 4He measured in extragalactic H II regions in dwarf galaxies, with very active star formation cause need high temp.
3He is measured with something like 21 cm line but for He (forbiden transition) so need a lot of gas and so can measure only locally i.e. can’t get primordial
Cyburt 2005 (private communication)
Tijana Prodanović
McGill /19/2006

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

19. Structure Formation Cosmic Rays
Miniati et al. 2000
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
Bottom: bremsstrahlung X-ray from SCDM model (omega_matter=1) ; identifies galaxy clusters
Top: shock surface
Box: 30x40x30 (h MpC)^3 @z=0
Colors: blue=high intensity, red=low
Tijana Prodanović
McGill /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
Tijana Prodanović
McGill /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 formation CRs 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)
Tijana Prodanović
McGill /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
Tijana Prodanović
McGill /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)
Picture: image composite: MW drawing+HVC radio image (21 cm, rest frequency) from Hulsbosch & Wakker (1988)
low-metallicity, HVCs with little dust are promising sites for testing pre-Galactic Li and SFCRs (Prodanović and Fields 2004b)
Tijana Prodanović
McGill /19/2006

24. New Data on the Way! GLAST: Cherenkov experiments: TeV window
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
These will be able to sort out some of our questions.
HESS source: X-ray binary (normal star+ compact object) LS 5039 is a compact object though uncertain about it’s nature ; VHE source thought to be microquasar
H.E.S.S. Collaboration Science 309 (2005) 746
Tijana Prodanović
McGill /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)
As theorists we just raise more questions and make a mess of everything, but luckily observers can clean it up. Job security for everyone!
Irony: pion signal not even see yet, but already a powerful tool
Tijana Prodanović
McGill /19/2006

26. The End
Tijana Prodanović
McGill /19/2006

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

28. Detecting TeV Gammas Eth~200 GeV, small f.o.v., short duty cycle
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.
Tijana Prodanović
McGill /19/2006

29. Cherenkov Radiation: Gamma-ray vs. Hadronic
More narrow cone!
Tijana Prodanović
McGill /19/2006

30. Milagro Gamma-Ray Observatory
Miracle LANL
Water Cherenkov Extensive Air Shower Array:
Low threshold
Large field of view
Observing 24/7
Cherenkov light threshold
lower in the water
Gamma’s pair-produce faster
PMTs
TeV Gamma-Ray Fishing!
Tijana Prodanović
McGill /19/2006

31. Point sources in Milagro Region
J : Whipple , Fegan et al. (2005)
J : Whipple, Buckley (1998)
J : Whipple, Fegan (2005)
J : HEGRA, Aharonian et al. (2005)
Tijana Prodanović
McGill /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)
Tijana Prodanović
McGill /19/2006

33. Backup 1 : Li-gamma connection derivation
Tijana Prodanović
McGill /19/2006