Implications of the Fermi-LAT diffuse gamma-ray measurements on annihilating or decaying Dark Matter

Preprint typeset in JHEP style - HYPER VERSION 0000.0000 Implications of the Fermi-LAT diffuse gamma-ray measurements on annihilating or decaying Dark Matter Gert H¨utsi Tartu Observatory, T˜oravere 61602, Estonia E-mail: gert@aai.ee Andi Hektor National Institute of Chemical Physics and Biophysics, Tallinn 10143, Estonia E-mail: andi.hektor@cern.ch Martti Raidal National Institute of Chemical Physics and Biophysics, Tallinn 10143, Estonia, Department of Physics, P.O.Box 64, FIN-00014 University of Helsinki, Finland E-mail: martti.raidal@cern.ch Abstract: We analyze the recently published Fermi-LAT diffuse gamma-ray measurements in the context of leptonically annihilating or decaying dark matter (DM) with the aim to explain simul- taneously the isotropic diffuse gamma-ray and the PAMELA, Fermi and HESS (PFH) anomalous e± data. Five different DM annihilation/decay channels 2e, 2µ, 2τ, 4e, or 4µ (the latter two via an intermediate light particle φ) are generated with PYTHIA. We calculate both the Galactic and extragalactic prompt and inverse Compton (IC) contributions to the resulting gamma-ray spectra. To find the Galactic IC spectra we use the interstellar radiation field model from the latest release of GALPROP. For the extragalactic signal we show that the amplitude of the prompt gamma-emission is very sensitive to the assumed model for the extragalactic background light. For our Galaxy we use the Einasto, NFW and cored isothermal DM density profiles and include the effects of DM sub- structure assuming a simple subhalo model. Our calculations show that for the annihilating DM the extragalactic gamma-ray signal can dominate only if rather extreme power-law concentration- mass relation C(M) is used, while more realistic C(M) relations make the extragalactic component comparable or subdominant to the Galactic signal. For the decaying DM the Galactic signal always exceeds the extragalactic one. In the case of annihilating DM the PFH favored parameters can be ruled out by gamma-ray constraints only if power-law C(M) relation is assumed. For DM decaying into 2µ or 4µ the PFH favored DM parameters are not in conflict with the gamma-ray data. We find that, due to the (almost) featureless Galactic IC spectrum and the DM halo substructure, annihilating DM may give a good simultaneous fit to the isotropic diffuse gamma-ray and to the PFH e± data without being in clear conflict with the other Fermi-LAT gamma-ray measurements. Keywords: gamma ray theory, dark matter theory, gamma ray experiments. arXiv:1004.2036v2 [astro-ph.HE] 21 Jun 2010 1. Introduction During the last few years several experiments have shown an anomalous excesses in the cosmic electron and positron spectra. The PAMELA satellite has observed a steep rise of positron fraction e+/(e−+ e+) at energies above 10 GeV with no significant excess in the cosmic antiproton flux [1, 2]. The Fermi satellite and the HESS atmospheric Cherenkov telescope have measured an excess of high-energy (e−+ e+) flux with a cut-offof around 800 GeV [3, 4]. The ATIC and PPB-BETS balloon measurements indicate a similar excess [5, 6]. Most excitingly, the excess might originate from the annihilation or decay of the dark matter (DM) particles. The nature of those signatures requires the properties of DM to deviate strongly from the standard freeze-out predictions. The thermally averaged DM annihilation cross-section ⟨σAυ⟩has to be boosted some orders of magnitude over the standard freeze-out value ⟨σAυ⟩std ≃3 × 10−26 cm3s−1, which might be achieved, e.g., through the Sommerfeld effect [7] (see, e.g., [8, 9, 10] for the related phenomenological studies) or through the Breit-Wigner resonant enhancement [11, 12, 13, 14]. On the other hand, the decaying DM [15, 16, 17, 18, 19] can explain the excess independent of the freeze- out constraints. In both cases, the annihilation or decay of DM should favorably occur only through the leptonic channels [20, 8, 21, 22, 23], as no excess in the hadronic channels has been observed. Alternatively, the excess of e+ can potentially be explained by modifying or adding astrophysical sources, e.g. pulsars [24, 25, 26, 27]. The high energy leptons of the DM annihilation/decay are inevitably accompanied by the gamma-rays due to the final state radiation of charged leptons and decays of sub- products (“prompt gamma-rays”) and due to the upscattered background photons from the inverse Compton (IC) scattering (“IC gamma-rays”). Thus, the observed gamma-ray fluxes strongly constrain the above mentioned cosmic ray anomalies from DM annihila- tion/decay. The strongest gamma-ray constraints should arise from the observations of the Galactic center (GC) [28, 29], as the density of DM is very high and it is relatively close to us. Those analyses take into account both the prompt and IC gamma-ray contribu- tions [30, 31, 32, 33, 34, 35]. On the other hand, the GC is densely populated by different astrophysical objects, which contaminate the gamma-ray signal and introduce signific

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