Can dark matter annihilation dominate the extragalactic gamma-ray background?

Annihilating dark matter (DM) has been discussed as a possible source of gamma rays from the galactic center and as a contribution to the extragalactic gamma-ray background. Assuming universality of the density profile of DM halos, we show that it is quite unlikely that DM annihilation is a main constituent of extragalactic gamma-ray background, without exceeding the observed gamma-ray flux from the galactic center. This argument becomes stronger when we include enhancement of the density profiles by supermassive black holes or baryon cooling. The presence of a substructure may loosen the constraint, but only if a very large cross section as well as the rather flat profile are realized.     

Dark matter annihilation in the Galaxy

The awaited dark matter (DM) neutralino annihilation signal from the galactic halo crucially depends on the presence of small-scale clumps. A mass function of the DM small-scale clumps is calculated in the standard cosmological scenario. The final distribution of clumps in the Galaxy is influenced by their tidal destruction. The basic sources of clump destruction are (i) clumps of larger scales, (ii) the gravitational field of the galactic disk, (iii) the stars in the galactic bulge, and (iv) the stars in the galactic halo. The destruction of clumps due to their mutual tidal interactions is important at earl stages of hierarchical clustering and the galactic halo formation. The clumps surviving through the hierarchical clustering are continuously destroyed by interactions with the galactic disk and stars. It is shown that, among the Moon or Earth mass DM clumps surviving through the hierarchical clustering, only 20% will further survive near the Sun’s position mainly because of the tidal destruction by the galactic disk. This reduction of DM clump density significantly diminishes the expected DM annihilation signal from the galactic halo. The text was submitted by the authors in English.     

Cosmic ray constraints on the annihilations of relic particles in the galactic halo

We re-evaluate the fluxes of cosmic ray antiprotons, positrons and gamma rays to be expected from the annihilations of relic particles in the galactic halo. We stress the importance of observational constraints on the possible halo density of relic particles, and specify their annihilation cross sections by the requirement that their cosmological density close the Universe. We use a Monte Carlo programme adapted to fit e⁺e⁻ data to calculate the p̄, e⁺ and γ spectra for some supersymmetric relic candidates. We find significantly smaller p̄ fluxes than previously estimated, and conclude that present upper limits on cosmic ray p̄ and e⁺ do not exclude any range of sparticle masses. We discuss the prospects for possible future constraints on sparticles from cosmic γ rays.     

A two-component dark matter model with real singlet scalars confronting GeV <Emphasis Type="BoldItalic">γ</Emphasis>-ray excess from galactic centre and Fermi bubble

We propose a two-component dark matter (DM) model, each component of which is a real singlet scalar, to explain results from both direct and indirect detection experiments. We put the constraints on the model parameters from theoretical bounds, PLANCK relic density results and direct DM experiments. The γ-ray flux is computed from DM annihilation in this framework and is then compared with the Fermi-LAT observations from galactic centre region and Fermi bubble.     

Searching for γ-ray emission from Reticulum Ⅱ by Fermi-LAT

Recently, many new dwarf spheroidal satellites(dSphs) have been discovered by the Dark Energy Survey(DES). These dSphs are ideal candidates for probing for gamma-ray emissions from dark matter(DM) annihilation.However, no significant signature has been found by the Fermi-LAT dSph observations. In this work, we reanalyze the Fermi-LAT Pass 8 data from the direction of Reticulum II, where a slight excess has been reported by some previous studies. We treat Reticulum II(DES J0335.6-5403) as a spatially extended source, and find that no significant gamma-ray signature is observed. Based on this result, we set upper-limits on the DM annihilation cross section.     

Anti-deuterons from heavy Dark Matter

We study the fluxes of anti-deuterons that could be produced by annihilations in the galactic halo of Dark Matter particles with multi-TeV mass and a large annihilation cross section, as indicated by the recent PAMELA results. The model of Minimal Dark Matter (MDM) is an example in this category. We find that the fluxes are well within the reach of planned experiments for DM candidates that annihilate mainly into quark pairs, and also extend into the multi-GeV range above the expected astrophysical background. They are instead less promising if the main annihilation channel is into gauge bosons.     

Explosive dark matter annihilation

In this Letter we study pair annihilation processes of dark matter (DM) in the Universe, in the case that the DM is an electroweak gauge nonsinglet. In the current Universe, in which the DM is highly nonrelativistic, the nonperturbative effect may enhance the DM annihilation cross sections, especially for that to two photons, by several orders of magnitude. We also discuss sensitivities in future searches for anomalous gamma rays from the galactic center, which originate from DM annihilation.     

Dark matter and generation of galactic magnetic fields

We look for possible spectral features and systematic effects in the Fermi LAT publicly available high-energy gamma-ray data by studying photons from the Galactic center, nearby galaxy clusters, nearby brightest galaxies, AGNs, unassociated sources, hydrogen clouds and from the Earth limb. Apart from the already known 130 GeV gamma-ray excesses from the first two sources, we find no statistically significant excesses from any of the cosmological sources nor from any control region. Therefore our main effort goes to the study of gamma rays appearing from the Earth limb. In the energy range of 30 to 200 GeV the Earth limb gamma-ray spectrum follows a power-law with spectral index 2.86±0.05 at 95 % CL, in a good agreement with the PAMELA measurement of the cosmic ray proton spectral index of 2.82–2.85, confirming the physical origin of the limb gamma-rays. In subsets of the Earth limb data at small photon incidence angle spectral features occur, including a feature at 130 GeV. We observe a systematic ～2σ-level difference in the Earth limb spectra with small and large incidence angles. The behavior of those spectral features as well as the background indicates that those may be statistical fluctuations or complicated unknown systematic effects of the Fermi LAT. In the latter case, only the Fermi LAT Collaboration can give the final answer having access to raw data and all details of the reconstruction.     

Is dark matter visible by galactic gamma rays?

The EGRET excess in the diffuse galactic gamma ray data above 1 GeV shows all features expected from dark matter WIMP annihilation: (a) It is present and has the same spectrum in all sky directions, not just in the galactic plane. (b) The intensity of the excess shows the 1/r ² profile expected for a flat rotation curve outside the galactic disc with an additionally interesting substructure in the disc in the form of a doughnut-shaped ring at 14 kpc from the centre of the galaxy. At this radius a ring of stars indicates the probable infall of a dwarf galaxy, which can explain the increase in DM density. From the spectral shape of the excess the WIMP mass is estimated to be between 50 and 100 GeV, while from the intensity the halo profile is reconstructed. Given the mass and intensity of the WIMPs the mass of the ring can be calculated, which is shown to explain the peculiar change of slope in the rotation curve at about 11 kpc. These results are model-independent in the sense that only the known shapes of signal and background were fitted with free normalization factors, thus being independent of model-dependent flux calculations. The statistical significance is more than 10σ in comparison with a fit of the conventional galactic model to the EGRET data. These signals of dark matter annihilation are compatible with supersymmetry including all electroweak constraints. The statistical significance combined with all features mentioned above provide an intriguing hint that the EGRET excess is indeed a signal from dark matter annihilation.     

Photons from dark matter in a (non-universal) extra dimension model

We study the multi-wavelength signal induced by pairs annihilations at the galactic center (GC) of a recently proposed dark matter (DM) candidate. The weakly interacting massive particle (WIMP) candidate, named A₋$A_{-}$, is the first Kaluza–Klein mode of a five-dimensional Abelian gauge boson. Electroweak precision tests and the DM cosmological bound constrain its mass and pair annihilation rate in small ranges, leading to precise predictions of indirect signals from what concerns the particle physics side. The related multi-wavelength emission is expected to be faint, unless a significant enhancement of the DM density is present at the GC. We find that in this case, and depending on few additional assumptions, the next generation of gamma-ray and wide-field radio observations can test the model, possibly even with the detection of the induced monochromatic gamma-ray emission.     

Dark matter annihilation in the milky way galaxy: effects of baryonic compression

If the dark matter (DM), which is considered to constitute most of the mass of galaxies, is made of supersymmetric particles, the central region of our Galaxy should emit gamma rays produced by their annihilation. We use detailed models of the Milky Way to make accurate estimates of continuum gamma-ray fluxes. We argue that the most important effect, which was previously neglected, is the compression of the dark matter due to the infall of baryons to the galactic center: it boosts the expected signal by a factor 1000. To illustrate this effect, we computed the expected gamma fluxes in the minimal supergravity scenario. Our models predict that the signal could be detected at high confidence levels by imaging atmospheric C erenkov telescopes assuming that neutralinos make up most of the DM in the Universe.     

Scientific tasks and present status of the GAMMA-400 project

The GAMMA-400 telescope is designed to investigate discrete high-energy gamma-ray sources in the energy range of 0.1–3000 GeV, to measure the energy spectra of galactic and extragalactic diffuse gammaray emissions, and to study gamma-ray bursts and gamma-ray emissions from an active Sun. The gamma-ray telescope has an angular resolution of ～0.01°, an energy resolution of ～1%, and a proton rejection factor of ～10⁶. Its special assignment is to measure fluxes of gamma rays, electrons, and positrons that could be associated with the annihilation or decay of dark matter particles.     

MeV dark matter: has it been detected?

We discuss the possibility that the recent detection of 511 keV gamma rays from the galactic bulge, as observed by INTEGRAL, is a consequence of low mass (1-100 MeV) particle dark matter annihilations. We discuss the type of halo profile favored by the observations as well as the size of the annihilation cross section needed to account for the signal. We find that such a scenario is consistent with the observed dark matter relic density and other constraints from astrophysics and particle physics.     

Primordial black hole as a source of the boost factor

Primordial black holes (PBHs) accumulate weakly interacting massive particles (WIMPs) around them and form ultracompact minihalos (UCMHs), if the WIMP is a dominant component of the dark matter (DM). In this Letter, we discuss that the UCMHs seeded by the PBHs with sub-earth mass enhance the WIMP annihilation in the present Universe and can successfully explain the positron and/or electron excess in cosmic ray observed by PAMELA/Fermi experiments. The signal is very similar to that from a decaying dark matter, which can explain the PAMELA and/or Fermi anomaly without conflict with any constraints as long as the decay mode is proper. In this scenario, the boost factor can be as large as 10⁵. In addition, we discuss testability of our scenario by gamma-ray point source and gravitational-wave experiments.     

Particle dark matter in the galactic halo

The presence of Dark Matter (DM) particles in the galactic halo can be unambiguously pointed out in a model-independent way by exploiting the DM annual modulation signature with an apparatus of specific features placed deep underground. At present DAMA/LIBRA is running at the Gran Sasso National Laboratory of the I.N.F.N. and has presented so far the results obtained with the data collected in the first six annual cycles (exposure 0.87 ton × yr). When including the exposure of the former DAMA/NaI experiment (0.29 ton × yr), the total exposure is 1.17 ton × yr for 13 annual cycles, a value order of magnitude larger than those typically released in the field. These DAMA/LIBRA data have further confirmed the model-independent evidence of the presence of Dark Matter particles in the galactic halo on the basis of the exploited DM signature (8.9 σ C.L. for the cumulative exposure).     

Supersymmetric dark matter and the extragalactic gamma ray background

We trace the origin of the newly determined extragalactic gamma-ray background from EGRET data to an unresolved population of blazars and neutralino annihilation in cold dark matter halos. Using results of high-resolution simulations of cosmic structure formation, we calculate composite spectra and compare with the EGRET data. The resulting best-fit value for the neutralino mass is m(chi) = 515(+110)(-75) GeV (systematic errors approximately 30%).     

Interpretation of the Galactic gamma-ray excess with the dark matter indicated by ~8Be and ~4He anomalous transitions

The long-standing Galactic center gamma-ray excess could be explained by GeV dark matter(DM) annihilation,but the DM interpretation seems to conflict with recent joint limits from different astronomical scale observations such as dwarf spheroidal galaxies,the Milky Way halo,and galaxy groups/clusters.Motivated by 8 Be and~4 He anomalous transitions with possible new interactions mediated by a vector boson X,we consider a small fraction of DM mainly annihilating into a pair of on-shell vector bosons XX followed by X→ e~+e~-in this paper.The Galactic center gamma-ray excess is explained by this DM cascade annihilation.The gamma rays are mainly from inverse Compton scattering emission,and the DM cascade annihilation could be compatible with joint astrophysical limits and meanwhile be allowed by AMS-02 positron observation.The direct detection of this model is also discussed.     

Gamma ray line astronomy as a probe of cold dark matter

Some annihilation processes of cold dark matter particles in the galactic halo may result in monochromatic gamma rays with an astrophysically significant rate. This paper summarizes the calculation of these rates and discusses the expected gamma ray line flux in comparison with the diffuse cosmic background.     

Possible signature of low-scale gravity in ultrahigh-energy cosmic rays

We show that the existence of low-scale gravity at the TeV scale could lead to a direct production of photons with energy above 10²² eV due to annihilation of ultrahigh-energy neutrinos on relic massive neutrinos of the galactic halo. Air showers initialized in the terrestrial atmosphere by these ultraenergetic photons could be collected in near future by the new generation of cosmic ray experiments.