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)

What Is Out There?

WMAP 94GHz

What Is Out There?



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 Relativistic Jets  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

Blazars

QuickTimeý Dz TIFFÅià• ÅB  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)

Narumoto & Totani, ApJ, 643, 81 (2006)

Blazar Luminosity Function Update



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.

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 10 41 erg/s at 0.1 GeV.

Ando et al. (2007)



-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 (G. Lamanna 2002)

Ando et al. (2007)

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+Z 0 ) 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)

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)

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?”

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!

“HST” for charged particles, and “WMAP” for gamma-rays?

WMAP 94GHz

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?

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

: θ (= π / l) 1.

2.

3.

Number of halos as a function of mass, Clustering of dark matter halos, and Substructure inside of each halo.

Gamma-ray intensity:

A Few Equations

Spherical harmonic expansion: Limber’s equation:

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.

1.

Again,

three ingredients

are necessary: Luminosity function of blazars, 2.

3.

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?

Ando, Komatsu, Narumoto & Totani (2007)

Predicted Angular Power Spectrum

39% DM 61% DM  At 10 GeV for 2-yr observations of GLAST 80% DM 97% DM 

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.

What If Substructures Were Disrupted

…

39% DM 61% DM • S/N goes down as more subhalos are disrupted in massive parent halos.

80% DM 97% DM • In this particular example, the number of subhalos per halo is proportinal to M 0.7

, where M is the parent halo mass.

• If no disruption occurred, the number of subhalos per halo should be proportional to M.

“

No Substructure

”

or

“

Smooth Halo

”

Limit

Our Best Estimate: 39% DM 61% DM 80% DM 97% DM “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.

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. QuickTimeý Dz TIFFÅiLZWÅj êLí£ÉvÉçÉOÉâÉÄ Ç™Ç±ÇÃÉsÉNÉ`ÉÉǾå©ÇÈǞǽDžÇÕïKóvÇ­Ç• QuickTimeý Dz TIFFÅiLZWÅj êLí£ÉvÉçÉOÉâÉÄ Ç™Ç±ÇÃÉsÉNÉ`ÉÉǾå©ÇÈǞǽDžÇÕïKóvÇ­Ç•

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?

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 cm 3 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?

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 AGNs SMM SNe DM COMPTEL

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…

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…