Gammaray From Annihilation of Dark Matter Particles

1
Gamma-ray From Annihilation of Dark Matter
Particles

• Eiichiro Komatsu
• University of Texas at Austin
• AMS Meeting_at_CERN, April 23, 2007

K. Ahn EK, PRD, 71, 021303R (2005) 72, 061301R
(2005) S. Ando EK, PRD, 73, 023521 (2006) S.
Ando, EK, T. Narumoto T. Totani, MNRAS, 376,
1635 (2007) S. Ando, EK, T. Narumoto T. Totani,
PRD, 75, 063519 (2007)
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What Is Out There?
WMAP 94GHz
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What Is Out There?
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Deciphering Gamma-ray Sky

• Astrophysical Galactic vs Extra-galactic
• Galactic origin (diffuse)
• E.g., Decay of neutral pions produced by
  cosmic-rays interacting with the interstellar
  medium.
• Extra-galactic origin (discrete sources)
• Active Galactic Nuclei (AGNs)
• Blazars
• Gamma-ray bursts
• Exotic Galactic vs Extra-galactic
• Galactic Origin
• Dark matter annihilation in the Galactic Center
• Dark matter annihilation in the sub-halos within
  the Galaxy
• Extra-galactic Origin
• Dark matter annihilation in the other galaxies

Relativistic Jets
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Blazars

• Blazars A population of AGNs whose relativistic
  jets are directed towards us.
• Inverse Compton scattering of relativistic
  particles in jets off photons -gt gamma-rays,
  detected up to TeV
• How many are there?
• EGRET found 60 blazars (out of 100 identified
  sources)
• GLAST is expected to find thousands of blazars.
• GLASTs point source sensitivity (gt0.1GeV) is 2 x
  10-9 cm-2 s-1
• AMS-2s equivalent (gt0.1GeV) point source
  sensitivity is about 10 times larger, 10-8 cm-2
  s-1 (G. Lamanna 2002)

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Blazar Luminosity Function Update
Narumoto Totani, ApJ, 643, 81 (2006)
LDDE

• Luminosity-Dependent Density Evolution (LDDE)
  model fits the EGRET counts very well. This model
  has been derived from
• X-ray AGN observations, including the soft X-ray
  background
• Correlation between blazars and radio sources
• LDDE predicts that GLAST should detect 3000
  blazars in 2 years.
• This implies that AMS-2 would detect a few
  hundred blazars.

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Redshift distribution of blazars that would be
detected by GLAST

• LDDE1 The best-fitting model, which accounts for
  1/4 of the gamma-ray background.
• LDDE2 A more aggressive model that accounts for
  100 of the gamma-ray background.
• It is assumed that blazars are brighter than 1041
  erg/s at 0.1 GeV.

Ando et al. (2007)
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?-ray Background

• Un-resolved Blazars that are below the
  point-source sensitivity will contribute to the
  diffuse background.
• EGRET has measured the diffuse background above
  the Galactic plane.
• LDDE predicts that only 1/4 of the diffuse light
  is due to blazars!
• AMS-2 will do MUCH better than EGRET in the
  diffuse background

Ando et al. (2007)
(G. Lamanna 2002)
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Dark matter (WIMP) annihilation
GeV-?

• WIMP dark matter annihilates into gamma-ray
  photons.
• The dominant mode jets
• Branching ratios for line emission (two gamma
  gammaZ0) are small.
• WIMP mass is likely around GeVTeV, if WIMP is
  neutralino-like.
• Can GLAST or AMS-2 see this?

Ando et al. (2007)
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DM Annihilation in MW
Diemand, Khlen Madau, ApJ, 657, 262 (2007)

• Simulated map of gamma-ray flux by Diemand et
  al., as seen from 8kpc away from the center.
• Challenging for AMS-2 (Jacholkowska et al. 2006)

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Why MW? There are many more dark matter halos out
there!

• WIMP dark matter particles are annihilating
  everywhere.
• Why focus only on MW? There are so many dark
  matter halos in the universe.
• We cant see them individually, but we can see
  them as the background light.
• We might have seen this already in the background
  light the real question is, how can we tell,
  for sure, that the signal is indeed coming from
  dark matter?

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Gamma-ray Anisotropy From Dark Matter Annihilation
Ando EK (2006) Ando, EK, Narumoto Totani
(2007)

• Dark matter halos trace the large-scale structure
  of the universe.
• The distribution of gamma-rays from these sources
  must be inhomogeneous, with a well defined
  angular power spectrum.
• If dark matter annihilation contributes gt30, it
  should be detectable by GLAST in anisotropy.
• A smoking gun for dark matter annihilation.
• It would be very interesting to study if AMS-2
  would be able to detect anisotropy signal ---
  remember, the mean intensity will be measured by
  AMS-2 very well!

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HST for charged particles, and WMAP for
gamma-rays?
WMAP 94GHz
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Why Anisotropy?

• The shape of the power spectrum is determined by
  the structure formation, which is well known.
• Schematically, we have
• (Anisotropy in Gamma-ray Sky)
• (MEAN INTENSITY) x ?
• The mean intensity depends on particle physics
  annihilation cross-section and dark matter mass.
• The fluctuation power, ?, depends on structure
  formation.
• The hardest part is the prediction for the mean
  intensity. However Remember that the mean
  intensity has been measured already!
• The prediction for anisotropy is robust. All we
  need is a fraction of the mean intensity that is
  due to DM annihilation.
• Blazars account for 1/4 of the mean intensity.
  What about dark matter annihilation?

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A Simple Route to the Angular Power Spectrum

• To compute the power spectrum of anisotropy from
  dark matter annihilation, we need three
  ingredients
• Number of halos as a function of mass,
• Clustering of dark matter halos, and
• Substructure inside of each halo.

Dark matter halo
? ( p / l)
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A Few Equations
Gamma-ray intensity
Spherical harmonic expansion
Limbers equation
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Astrophysical Background Anisotropy from Blazars

• Blazars also trace the large-scale structure.
• The observed anisotropy may be described as the
  sum of blazars and dark matter annihilation.
• Again, three ingredients are necessary
• Luminosity function of blazars,
• Clustering of dark matter halos, and
• Bias of blazars the extent to which blazars
  trace the underlying matter distribution.
• This turns out to be unimportant (next slide)
• Is the blazar power spectrum different
  sufficiently from the dark matter annihilation
  power spectrum?

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Predicted Angular Power Spectrum
Ando, Komatsu, Narumoto Totani (2007)

• At 10 GeV for 2-yr observations of GLAST
• Blazars (red curves) easily discriminated from
  the DM signal --- the blazar power spectrum is
  nearly Poissonian.
• The error blows up at small angular scales due to
  angular resolution (0.1 deg) blazar
  contribution.

39 DM
61 DM
80 DM
97 DM
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What If Substructures Were Disrupted

• S/N goes down as more subhalos are disrupted in
  massive parent halos.
• In this particular example, the number of
  subhalos per halo is proportinal to M0.7, where M
  is the parent halo mass.
• If no disruption occurred, the number of subhalos
  per halo should be proportional to M.

39 DM
61 DM
97 DM
80 DM
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No Substructure or Smooth Halo Limit

• Our Best Estimate
• If dark matter annihilation contributes gt 30 of
  the mean intensity, GLAST should be able to
  detect anisotropy.
• A similar analysis can be done for AMS-2.

39 DM
61 DM
97 DM
80 DM
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Positron-electron Annihilation in the Galactic
Center
Jean et al. (2003) Knoedlseder et al.
(2005)Weidenspointner et al. (2006)

• INTEGRAL/SPI has detected a significant line
  emission at 511 keV from the G.C.
• Extended over the bulge -- inconsistent with a
  point source!
• Flux 10-3 ph cm-2 s-1
• Continuum emission indicates that more than 90
  of annihilation takes place in positronium.

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INTEGRAL/SPI Spectrum
Churazov et al. (2005)

• Ortho-positronium continuum is clearly seen (blue
  line)
• Best-fit positronium fraction (96 - 4)
• Where do these positrons come from?

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Light Dark Matter Annihilation

• Light (MeV) dark matter particles can produce
  non-relativistic positrons, which would produce
  line emission at 511keV. The required (S-wave)
  annihilation cross section (a few x 10-26 cm3
  s-1) is indeed reasonable!
• Boehm et al., PRL, 92, 101301 (2004)
• Hooper et al., PRL, 93, 161302 (2004)
• The fact that we see a line sets an upper limit
  on the positron initial energy of 3 MeV.
• Beacom Yuksel, PRL, 97, 071102 (2006)
• Continuum gamma-ray is also produced via the
  internal bremsstrahlung, XX -gt ee-?
• Beamcom, Bell Bertone, PRL, 94, 171301 (2005)
• How about the extra-galactic background light?

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AGNs, Supernovae, and Dark Matter Annihilation
Ahn EK, PRD, 71, 021303R 71, 121301R 72,
061301R (05)

• The extra-galactic background in 1-20MeV region
  is a superposition of AGNs, SNe, and possibly DM
  annihilation.
• SNe cannot explain the background.
• AGNs cut off at 1MeV.
• 20 MeV DM fits the data very well.

HEAO-1
DM
SMM
AGNs
COMPTEL
SNe
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Implications for AMS-2?

• Gamma-rays from DM annihilation of MeV dark
  matter, or possible positron excess, are out of
  reach.
• Too low an energy for AMS-2 to measure

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Summary

• Convincing evidence for gamma-rays from DM will
  have a huge impact on particle physics and
  cosmology.
• The Galactic Center may not be the best place to
  look. The extra-galactic gamma-ray background,
  which has been measured by EGRET and will be
  measured more precisely by AMS-2 and GLAST, may
  hold the key.
• The mean intensity is not enough the power
  spectrum of cosmic gamma-ray anisotropy is a very
  powerful probe.
• If gt30 of the mean intensity comes from dark
  matter annihilation (at 10 GeV), GLAST will
  detect it in two years.
• Prospects for detecting it in AMS-2 data remain
  to be seen.
• A possibility of MeV dark matter is very
  intriguing.
• But, it is out of reach for AMS-2