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

Tijana ProdanovićTijana ProdanovićUniversity of Illinois at Urbana-Champaign

Brian D. Fields, UIUCJohn F. Beacon, OSU

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

Tijana Prodanović Seminar @ North Carolina State University

12/13/2005

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


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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


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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 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

0 pppp ismcr


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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 Find max “pionic” flux

so that “pion bump” stays below observed Galactic spectrum

Galactic CRs: Max “pionic” fraction

But notice the residual!

2.75GCRp p

???

Prodanović and Fields (2004a)

Brems/ICStrong et al. (2004)

%50obs/)(


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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 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% !

Another component? Dark matter? Point sources?

75.2GCR )( EE

Prodanović, Fields & Beacom (2005)In preparation

61.2GCR )( EE

Milagro Water Cherenkov Detector

Need more

data!!!!

1. Galactic Cosmic Rays:

1.2 Extragalactic Gamma-ray Background

Beck et al. 1994


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Li--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

0 pppp ismcr

...Li7,6 ismcr

crism yy ,,

1

Li


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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” -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

75.2)()( EmE pp

Fields and Prodanović (2005)

%75~/)( obs


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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)

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?

112 srscm 5, 1022.3)GeV1.0( I

51011.1)GeV1.0( I112 srscm


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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)

2. Structure Formation Cosmic Rays:

Extragalactic Gamma-Ray Background

Miniati et al. 2000


12/13/2005

Structure Formation Cosmic Rays 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


12/13/2005

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


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Estimating SFCR“pionic” -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 -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

2.2)()( EpEp

Prodanović and Fields(2004a)

%40~/)( obs

%80~/)( obs


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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

0.4Li

Li3.0

,

BBNp

SFCR


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Searching for SFCRs in High Velocity CloudsClouds of gas falling onto our Galaxy (Wakker & van Woerden 1997)

Some high-velocity clouds show evidence for little or no dustOrigins:

Galactic fountain model (Shapiro & Field 1976)

Extragalactic Magellanic Stream-type objects or gas left over from formation of the

Galaxy (Oort, Blitz , Braun)

Some high-velocity clouds have metallicity 10% of solar

low-metallicity, HVCs with little dust are promising sites for testing pre-Galactic Li and SFCRs (Prodanović and Fields 2004b)


12/13/2005

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


12/13/2005

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)


12/13/2005

The End


12/13/2005

Strong et al. (2004)Diffuse Galactic continuum, EGRET data

Conventional model Optimized model


12/13/2005

BBN in the light of WMAP

Dark, shaded regions =WMAP + BBN predictions

Light, shaded and dashed regions = observations

Cyburt, Fields and Olive (2003)


12/13/2005

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.


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Cherenkov Radiation: Gamma-ray vs. Hadronic

More narrow cone!


12/13/2005

Milagro Gamma-Ray Observatory

Miracle telescope @ LANL

TeV Gamma-Ray Fishing!

Water Cherenkov Extensive Air Shower Array:

Low thresholdLarge field of viewObserving 24/7

Cherenkov light threshold lower in the waterGamma’s pair-produce faster450 + 273 PMTs


12/13/2005

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Documents

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