Tijana Prodanovic

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

• Tijana Prodanovic
• University of Illinois at Urbana-Champaign
• Brian D. Fields, UIUC
• John F. Beacon, OSU

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Outline

• Cosmic Rays Gamma Rays
• Diffuse Hadronic Galactic Gamma Rays
• What we know
• What we dont know dark matter signal?
• Diffuse Hadronic Extragalactic Gamma-Ray
  Background
• Lithium-gamma-ray connection
• Probe lithium nucleosynthesis
• Structure Formation Cosmic Rays
• Implications
• Constraints
• Conclusion

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Cosmic Rays

• High-energy charged particles
• Accelerated in astrophysical (collisionless)
  shocks
• Spectrum
• Strong shocks FluxE-2
• Measured (JACEE 1998) FluxE-2.75
• CR proton energy density in local interstellar
  medium 0.83 eV/cm3 (typical galactic magnetic
  field B3µGauss has e0.25 eV/cm3)
• Trivia CR muon flux at sea level 1 cm-2 min-1

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Cosmic Ray Acceleration Sites

• Any shock source is a candidate! (Blandford
  Eichler 1987)
• Sites
• Supernova remnants (Blondin, Reynolds,
  McLaughlin)
• galactic cosmic
• rays (D. Ellison)
• Structure formation shocks
• structure formation cosmic rays (Inoue,
  Kang, Miniati)

VLA image of Tycho SNR Reynolds Chevalier
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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

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Gamma Rays

• give away direction!
• Pionic gamma-rays
• Distinctive spectrum pion bump peaks at mp/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
  (Prodanovic Fields 2004)
• Relativistic electrons
• Inverse Compton (off starlight CMB)
• Synchrotron
• Bremsstrahlung

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1. Galactic Cosmic Rays
1.1 Galactic Gamma-Ray Sky
Gaetz et al (2000) SNR E0102-72
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Galactic Pionic Gamma Rays
Prodanovic 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!

Brems/IC Strong et al. (2004)
???
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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

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Constraining Dark Matter Foreground
Prodanovic, Fields Beacom (2005) In preparation

• Though inverse compton component at low end,
  cutoff at TeV
• Pionic gammas dominate?
• Unconventional spectral index
• Milagro pionic dominates indeed
• Observed spectral index
• Milagro pionic only 10 !
• Another component? Dark matter? Point sources?

Need more data!!!!
Milagro Water Cherenkov Detector
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1. Galactic Cosmic Rays1.2 Extragalactic
Gamma-ray Background
Beck et al. 1994
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Li-g-ray Connection

• Any cosmic-ray source produces both gamma-rays
  and lithium
• Connected essentially with ratio of reaction
  rates (Fields
  and Prodanovic 2005)
• Li abundance local CR fluence
• Diffuse extragalactic CR fluence across
  Universe
• Given one, constrain other

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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

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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 Prodanovic (2005)
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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)
• Whats going on?
• Milky Way CR flux higher than average galaxy?
• CR spectrum sensitivity (thresholds! Li probes
  low E)
• 6Li not from Galactic CRs?

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The Lithium Problem

• 7Li made predominantly in the Big Bang
  Nucleosynthesis (Cyburt, Fields, McLaughlin,
  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)
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2. Structure Formation Cosmic Rays
Extragalactic Gamma-Ray Background
Miniati et al. 2000
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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

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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 a-particles ? only LiBeB
• Will contribute to halo star Li abundance
• ? Li problem even worse! (Suzuki Inoue 2002)
• Use Li-gamma-connection as probe

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Estimating SFCRpionic 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
• z0
• z10

Prodanovic and Fields(2004a)
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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

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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 (Prodanovic and Fields 2004b)
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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
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Final Thoughts

• Universal, model independent approach pionic
  spectrum
• Probe both Galactic and extragalactic sourcs
• 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)

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The End
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Strong et al. (2004) Diffuse Galactic continuum,
EGRET data
Conventional model
Optimized model
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BBN in the light of WMAP
Dark, shaded regions WMAP BBN predictions
Light, shaded and dashed regions observations
Cyburt, Fields and Olive (2003)
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Detecting TeV Gammas

• EM air shower
• TeV gammas hit top of the atmosphere
• ? pair production Compton Scat.
  Bremsstrahlung ? high-energy gammas ? 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
• Eth200 GeV, small f.o.v., short duty cycle
• Air Shower Array scintillators
• Eth50 TeV, large f.o.v., long duty cycle

H.E.S.S.
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Cherenkov Radiation Gamma-ray vs. Hadronic
More narrow cone!
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Milagro Gamma-Ray Observatory

• Miracle telescope _at_ LANL

• Water Cherenkov Extensive Air Shower Array

• Low threshold
• Large field of view
• Observing 24/7
• Cherenkov light threshold
• lower in the water
• Gammas pair-produce faster
• 450 273 PMTs

TeV Gamma-Ray Fishing!
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