Diffuse γ-rays and pbar flux from dark matter annihilation --- a model for consistent results with

EGRET and cosmic ray data

Xiao-Jun Bi (IHEP, CAS)

Based on PLB668, 87 (2008) PRD78, 043001 (2008)

Diffuse gamma rays of the MW

• COS-B and EGRET (20keV~30GeV) observed diffuse gamma rays, measured its spectra.

• Diffuse emission comes from nucleon-gas interaction, electron inverse Compton and bremsstrahlung. Different process dominant different parts of spectrum, therefore the large scale nucleon, electron components can be revealed by diffuse gamma.

GeV excess of spectrum• Based on local sp

ectrum gives consistent gamma in 30 MeV~500 MeV, outside there is excess.

• Harder proton (electron) spectrum explain diffuse gamma, however inconsistent with antiproton and positron measurements.

Solutions to the GeV excess problem

A systematic error ? (Stecker, Hunter, Kniffen APP 2008)

Waiting for Fermi (GLAST)

GeV excess of EGRET

No anomaly.

Yes.No.

Confirm the anomaly.

CRs (Strong, Moskalenko, Reimer APJ 2004) DMA (de Boer et al. AA 2005)

Data of B/C, proton spec Pbar flux ……

Our model of CR propagation

The optimized model

• The local observed spectra of proton and electron can be different from the globally averaged spectra

• Strong et al introduced different spectra of p, e from the local ones to give best fit of the diffuse gamma data

• Problems: different spectra of heavy nuclei and proton so that B/C and other data are not upseted.

• Diffuse time scale of proton is much smaller than energy loss time scale. We may expect a universal spectrum of proton over the Galaxy (unless some new sources have lifetime smaller than diffuse time)

• Electron spectra can have large spatial fluctuation.

Fit the spectrum from DMA

• Reconstruct the DM profile, two rings at 4kpc and 14kpc (supported by rotation curve)

• B~100• Fi,j -----

Is the dark matter interpretation of the EGRET gamma excess compatible with antiproton measurements ?

• Astro-ph/0602632:

• Lars Bergstr\"om, Joakim Edsj\"o, Michael Gustafsson

• Universal large boost factor boost pbar flux greatly

Our model

• Universal spectra of proton and all nuclei, consistent with local ones, renormalization of electron spectrum

• Include contribution from DMA• Explain the boost factor of DMA by taking the subhalos i

nto account• No normalization factors for bkg from CRs by adjust prop

agation parameters• Consistent pbar flux with data (small diffusion region 1.5

kpc, new ring positions 16kpc, different boost factors of gamma and pbar)

• Source form is now consistent with SNR observation

Height of diffusion region and DM rings

• Fitting the CR data, especially the ratio of unstable secondary to stable secondary, the gas density the CR transversed is 0.2 atom/cm3, lower than gas density 1 atom/cm3. Then it is infered that the height of diffusion region is much larger than the height of the gas disk.

• Reflected by magnetic fields of the molecular clouds and transport in low density region, therefore the diffuse region is smaller than usual one. In our model the height is about 1.5kpc. We assume the average gas density that the CR transversed is about 1.5 times smaller than average gas density.

• Not universal boost; Large boost at large radius boost pbar smaller than gamma and move ring to outside

• Rotation curve from HI gas flaring (Kalberla et al. AA2008) show ring at large radius , ~17 kpc

• Consistent profiles of latitude and longitude data

The first generation object

Diemand, Moore & Stadel, 2005:• Depending on the nature of the

dark matter: for neutralino-like dark matter, the first structures are mini-halos of 10-6 M⊙.

• There would be zillions of them surviving and making up a sizeable fraction of the dark matter halo.

• The dark matter detection schemes may be quite different!

Boost for different radii

Results of different regionsGeVm 70~40

Our result

Source distribution

Conclusion

• We build a model to explain the GeV excess of Galactic diffuse gamma rays observed by EGRET.

• Our model overcomes difficulties of the optimized (Strong et al) and DMA (de Boer et al.) models. The model explains the diffuse gamma rays spectra and profile quite naturally. The CR data is also consistent with observation.

• The model need further test by the Fermi satellite.