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Gamma-ray From Annihilation of Dark Matter Particles

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  1. Gamma-ray From Annihilation of Dark Matter Particles Eiichiro Komatsu University of Texas at Austin AMS Meeting@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)

  2. What Is Out There? WMAP 94GHz

  3. What Is Out There?

  4. 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

  5. Blazars • Blazars = A population of AGNs whose relativistic jets are directed towards us. • Inverse Compton scattering of relativistic particles in jets off photons -> 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. • GLAST’s point source sensitivity (>0.1GeV) is 2 x 10-9 cm-2 s-1 • AMS-2’s equivalent (>0.1GeV) point source sensitivity is about 10 times larger, ~ 10-8 cm-2 s-1 (G. Lamanna 2002)

  6. Narumoto & Totani, ApJ, 643, 81 (2006) Blazar Luminosity Function Update • 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. LDDE

  7. 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)

  8. -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)

  9. Dark matter (WIMP) annihilation GeV-γ • WIMP dark matter annihilates into gamma-ray photons. • The dominant mode: jets • Branching ratios for line emission (two gamma & gamma+Z0) are small. • WIMP mass is likely around GeV–TeV, if WIMP is neutralino-like. • Can GLAST or AMS-2 see this? Ando et al. (2007)

  10. Diemand, Khlen & Madau, ApJ, 657, 262 (2007) DM Annihilation in MW • 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)

  11. 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 can’t 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?”

  12. Ando & EK (2006); Ando, EK, Narumoto & Totani (2007) Gamma-ray Anisotropy From Dark Matter Annihilation • 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 >30%, 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!

  13. “HST” for charged particles, and “WMAP” for gamma-rays? WMAP 94GHz

  14. 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?

  15. A Simple Route to the Angular Power Spectrum Dark matter halo • 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. θ (= π / l)

  16. A Few Equations Gamma-ray intensity: Spherical harmonic expansion: Limber’s equation:

  17. 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?

  18. Ando, Komatsu, Narumoto & Totani (2007) Predicted Angular Power Spectrum 39% DM 61% DM • 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. 80% DM 97% DM

  19. What If Substructures Were Disrupted… 39% DM 61% DM • 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. 80% DM 97% DM

  20. “No Substructure”or “Smooth Halo” Limit 39% DM 61% DM Our Best Estimate: “If dark matter annihilation contributes > 30% of the mean intensity, GLAST should be able to detect anisotropy.” • A similar analysis can be done for AMS-2. 80% DM 97% DM

  21. Jean et al. (2003); Knoedlseder et al. (2005);Weidenspointner et al. (2006) Positron-electron Annihilation in the Galactic Center • 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.

  22. Churazov et al. (2005) INTEGRAL/SPI Spectrum • Ortho-positronium continuum is clearly seen (blue line) • Best-fit positronium fraction = (96 +- 4)% • Where do these positrons come from?

  23. 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 -> e+e- • Beamcom, Bell & Bertone, PRL, 94, 171301 (2005) • How about the extra-galactic background light?

  24. Ahn & EK, PRD, 71, 021303R; 71, 121301R; 72, 061301R (05) AGNs, Supernovae, and Dark Matter Annihilation… • 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 AGNs SMM COMPTEL SNe

  25. 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…

  26. 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 >30% 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…