Comic Gamma-Ray Background from Dark Matter Annihilation

Transcription

1 TeV Particle Astrophysics II Madison (Aug. 29, 2006) Comic Gamma-Ray Background from Dark Matter Annihilation Shin ichiro Ando (California Institute of Technology) S. Ando & E. Komatsu, Phys. Rev. D 73, (2006) S. Ando, E. Komatsu, T. Narumoto & T. Totani, to be submitted

2 1. Introduction

3 Cosmic gamma-ray background (CGB) Extragalactic Diffuse γ-ray Background 0.0 HEAO A2,A4(LED) E2 dj/de (kev2/(cm2-s-kev-sr) ASCA HEAO-A4 (MED).0 COMPTEL EGRET 1.0 v Photon Energy (kev) Strong et al Fig. 3. Extragalactic X-ray and γ-ray spectrum. Data compilation from Sreekumar et al. (1998) except for (Weidenspointner et al. 2000) and EGRET 30 MeV 20 GeV (this work). (1998). Fig. 3 shows the extragalactic X- and ground, using the compilation by Sreekumar e but using our new EGRET values, and al COMPTEL results (Weidenspointner et al. 2 Discovered with EGRET at GeV region Same photon intensity coming from all the directions Table 3. EGRB intensity. 6 Eγ, MeV Intensity, cm 2 sr 1 s 1 MeV 1 Total error FURTHER CHECKS FOR SYSTEMATICS MODEL In order to check the robustness of our mates, we compare estimates based on data hemispheres and four quarter spheres (Tab tests for the presence of apparent anisotro EGRB values, and thus gives an independe

4 Origin of CGB at GeV region: Candidates 1. Blazars AGN population beaming towards Earth ~50 detected as point sources by EGRET 2. Galaxy clusters Either pp collisions or electron inverse-compton 3. Dark matter annihilation Good candidate motivated by particle physics (e.g., supersymmetric neutralino) Energy spectrum is characteristic

5 Blazar contribution Luminosity function constructed with the detected EGRET blazar properties he 68%, 95% and 99% C.L. regions for the PLE model parameters [the faint-end slope index γ1 and the mean gamma-ray 0 (Lγ /Lr )"]. The best-fit values, (!p",γ1 ) = (3.28,0.69), are shown by the cross. The dashed contours correspond to.33, respectively, where η is the ratio of the normalizations of the gamma-ray to radio luminosity functions. GAMMA-RAY LUMINOSITY FUNCTION OF BLAZARS Narumoto & Totani dn/d(logz) 1.2 LDDE model PLE model SS96 model EGRET blazars LDDE model PLE model SS96 model EGRET blazars dn/d(logl!) Redshift z log (L! [erg s"1]) F IG. data. 6. The Luminosity of the blazars. Thefor line are the same as Figure 5. The luminosity is νl EGRET blazars. The histogram is the EGRET solid anddistribution dashed curves areegret the best-fit models themarkings LDDE and 1σ Poisson error. elihood analysis. The dotted curve is obtained from the blazar GLF model of SS96. The error bars are 1σ Poisson error. Luminosity-dependent density evolution (LDDE) model is more prefered

6 Blazar contribution Best LDDE model explains only ~25% of the CGB above 0 MeV Most CGB still can only be explained, although such a model is disfavored at ~2σ level Still, the excess around 3 GeV cannot be attributed to blazars

7 [32]. The blazar model also would imply 00 sources with a > 300 GeV flux of the order of a typical Whipplesource, whereas the steeper power law alone corresponds to 40 sources. The first number seems worryingly large in view of the confirmed sources, in spite of excessive observation campaigns on candidate sources from Dark matter annihilation? Annihilation into gamma rays in all the observable dark halos Is the excess due to this signal? Constraint by the Galactic center (Ando 2005, PRL 94, ) can be avoided by Elsässer & Mannheim 2005 FIG. 2: Extragalactic gamma-ray background: spectrum as determined from EGRET data by Strong et al. (data points); substructure (Oda, Totani & Nagashima 2005), or minispikes around IMBH (Horiuchi & Ando 2006; see also, Bertone, Zentner & Silk 2005)

8 Quick summary on CGB EGRET confirmed the existence of diffuse extragalactic gamma-ray emission (CGB) The CGB would contributed by either astrophysical sources such as blazars, or annihilating dark matter GLAST could pinpoint the origin Better understanding of blazars Potentially a smoking gun of dark matter annihilation Spectrum might not be sufficient for such a strong claim

9 2. Anisotropy as a Smoking Gun

10 Anisotropy from dark matter annihilation Rate of annihilation depends on density squared A characteristic very different from ordinary astrophysical objects The CGB anisotropy should also be quite different This could provide more stringent evidence of particle dark matter

11 Angular power spectrum Dark matter halo Projected along the line of sight is the CGB intensity Angular power spectrum, Cl, is related to the spatial power spectrum via Limber s equation 3D correlation can be modeled, using θ (= π / l) halo mass function, and density profile in each halo

12 A few equations..., if you want Gamma-ray intensity: I γ (ˆn, E γ ) = dr δ 2 (r, ˆnr)W ([1 + z]e γ, r) Spherical harmonic expansion: δi γ (ˆn) I γ = lm a lm Y lm (ˆn) C l = a lm 2 Limber s equation: dr I γ 2 C l = {W ([1 + z]e γ, r)} 2 P f r 2 f = δ 2 δ 2 (k = lr ; r )

13 3D power spectrum (z=0) 2 f (k) = k3 P f (k) 2π 2 δ 2 2 Ando & Komatsu, PRD 73, (2006)

14 Angular power spectrum M min is the lower mass cutoff, below which no dark halos are formed M min = 6 Msun is set by the free-streaming of the neutralino M min = 6 Msun is set by the Jeans mass of the baryon, below which halos might be tidally disrupted Ando & Komatsu, PRD 73, (2006)

15 3. Detectability

16 What are backgrounds? From Detector background!"##$%&'#($)*'+#,&-'.$,%*'%.+(&/',0' 1&2&-+,&/' $2/ Negligible, being 5% of the CGB above 0 MeV (even smaller at GeV) (01567'.0--&.,&/'58&97

17 -3 EB 6-4 What are backgrounds? energy, MeV bremss 1! " MeV -1 s -1 sr -2. intensity, cm 2 E MeV -1 s -1 sr -2. intensity, cm 2 E galdef ID 44_ IC galdef ID 44_ IC deg 1 total EB bremss! " 2 total EB 2! " deg <l<179.75, <l< <b<-20.25, 20.25<b< energy, MeV 0.25<l<179.75, <l< <b<-60.25, 60.25<b< energy, MeV Strong et al described in Table 2: top row H A B, middle row Galactic cosmic rays a foreground It strongly depends on the galactic latitude The flux is about one order of magnitude smaller than CGB for b > 20 deg, safely negligible

18 Detectability: Pure dark matter case Ando & Komatsu, PRD 73, (2006) Adopted GLAST parameters: Aeff 4 cm 2 Ωfov 2.4 sr θres T 1 yr

19 Detectability: Lower Cutoff Ando & Komatsu, PRD 73, (2006)

20 Mixed case with blazars Mmin = 6 Msun Mmin = 6 Msun Ando et al., to be submitted

21 4. Conclusions

22 The CGB anisotropy would be a key to revealing the origin of CGB, and potentially be a smoking gun of annihilating dark matter The resulting angular spectrum would be very different from the case of other sources We developed a new formalism for that calculation We showed that if the annihilating dark matter is a main CGB constituent, GLAST can detect anisotropy in a few years This is also true even with the existence of other sources like blazars, if the current dark matter contribution exceeds 30% at GeV