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.     

High-energy gamma-ray sources of cosmological origin

The current generation of instruments in gamma-ray astrophysics launched a new era in the search for a dark matter signal in the high-energy sky. Such searches are said indirect, in the sense that the presence of a dark matter particle is inferred from the detection of products of its pair-annihilation or decay. They have recently started to probe the natural domain of existence for weakly interacting massive particles (WIMPs), the favorite dark matter candidates today. In this article, we review the basic framework for indirect searches and we present a status of current limits obtained with gamma-ray observations. We also devote a section to another possible class of cosmological gamma-ray sources, primordial black holes, also considered as a potential constituent of dark matter.     

Constraining dark matter properties with gamma-rays from the Galactic Center with Fermi-LAT

We study the capabilities of the Fermi-LAT instrument on board of the Fermi mission to constrain particle dark matter properties, as annihilation cross section, mass and branching ratio into dominant annihilation channels, with gamma-ray observations from the Galactic Center. Besides the prompt gamma-ray flux, we also take into account the contribution from the electrons/positrons produced in dark matter annihilations to the gamma-ray signal via inverse Compton scattering off the interstellar photon background, which turns out to be crucial in the case of dark matter annihilations into μ⁺μ⁻ and e⁺e⁻ pairs. We study the signal dependence on different parameters like the region of observation, the density profile, the assumptions for the dark matter model and the uncertainties in the propagation model. We also show the effect of the inclusion of a 20% systematic uncertainty in the gamma-ray background. If Fermi-LAT is able to distinguish a possible dark matter signal from the large gamma-ray background, we show that for dark matter masses below ∼200 GeV, Fermi-LAT will likely be able to determine dark matter properties with good accuracy.     

Nonlinear increase in the energy input into a medium at the fusion of regularized femtosecond filaments

We consider dark matter consisting of long-living particles with masses 10⁷ GeV ? M ?10¹⁶ GeV decaying through hadronic channel as a source of high-energy neutrino. Using recent data on high-energy neutrino from IceCube and Pierre Auger experiments, we derive the upper-limits on neutrino flux from dark matter decay and constraints on dark matter parameter space. For the dark matter masses of order 10⁸ GeV the constraints derived are slightly stronger than those obtained for the same dark matter model using the highenergy gamma-ray limits.     

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.     

Proper motion of gamma rays from microhalo sources

I discuss the prospects of detecting the smallest dark matter bound structures present in the Milky Way by searching for the proper motion of gamma-ray sources in the upcoming Gamma Ray Large Area Space Telescope all sky map. I show that for dark matter particle candidates that couple to photons the detection of at least one gamma-ray microhalo source with proper motion places a constraint on the couplings and mass of the dark matter particle.     

Experimental identification of nonpointlike dark-matter candidates

We show that direct dark-matter detection experiments can distinguish between pointlike and nonpointlike dark-matter candidates. The shape of the nuclear recoil-energy spectrum from pointlike dark-matter particles, e.g., neutralinos, is determined by the velocity distribution of dark matter in the galactic halo and by nuclear form factors. Typical cross sections of nonpointlike dark matter, for example, Q-balls, have a new form factor, which decreases rapidly with the recoil energy. A signal from nonpointlike dark matter is expected to peak near the experimental threshold and to fall off rapidly at higher energies.     

Significant gamma lines from inert Higgs dark matter

One way to unambiguously confirm the existence of particle dark matter and determine its mass would be to detect its annihilation into monochromatic gamma-rays in upcoming telescopes. One of the most minimal models for dark matter is the inert doublet model, obtained by adding another Higgs doublet with no direct coupling to fermions. For a mass between 40 and 80 GeV, the lightest of the new inert Higgs particles can give the correct cosmic abundance of cold dark matter in agreement with current observations. We show that for this scalar dark matter candidate, the annihilation signal of monochromatic gammagamma and Zgamma final states would be exceptionally strong. The energy range and rates for these gamma-ray line signals make them ideal to search for with the soon upcoming GLAST satellite.     

Dark minihalos with intermediate mass black holes

We argue that the Milky Way (MW) contains 10(3)-10(4) intermediate mass black holes (IMBHs) of mass approximately 10(2-3)M. Some IMBHs are naked, and some are enshrouded by dense dark minispikes and by minihalos of (10(6)-10(7))M circle. The IMBH is formed off-center by gas accretion in a minihalo. These dense minihalos (the nearest at about 2 kpc) survive mostly without tidal stripping by the MW, and are largely invisible except that their pointlike neutralino annihilation signals (with bolometric luminosities of 10(3-5)L circle 50 GeV / mChi within 0.01-0.1 pc of their centers) stand out well above the MW background and are more luminous than outer dwarf satellite galaxies.     

Model-independent approach for dark matter phenomenology: Signatures in linear colliders and cosmic positron experiments

We have studied the phenomenology of dark matter at the ILC and cosmic positron experiments based on model-independent approach. We have found a strong correlation between dark matter signatures at the ILC and those in the indirect detection experiments of dark matter. Once the dark matter is discovered in the positron experiments such as the PAMELA, its nature will be investigated in detail at the ILC.      

Gamma rays from Kaluza-Klein dark matter

A TeV gamma-ray signal from the direction of the Galactic center (GC) has been detected by the HESS experiment. Here, we investigate whether Kaluza-Klein (KK) dark matter annihilations near the GC can be the explanation. Including the contributions from internal bremsstrahlung as well as subsequent decays of quarks and tau leptons, we find a very flat gamma-ray spectrum which drops abruptly at the dark matter particle mass. For a KK mass of about 1 TeV, this gives a good fit to the HESS data below 1 TeV. A similar model, with gauge coupling roughly 3 times as large and a particle mass of about 10 TeV, would give both the correct relic density and a photon spectrum that fits the complete range of data.     

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.     

511 keV photons from color superconducting dark matter

We discuss the possibility that the recent detection of 511 keV gamma rays from the galactic bulge, as observed by the International Gamma-Ray Astrophysics Laboratory, can be naturally explained by the supermassive very dense droplets (strangelets) of dark matter. These droplets are assumed to be made of ordinary light quarks (or antiquarks) condensed in a nonhadronic color superconducting phase. The droplets can carry electrons (or positrons) in the bulk or/and on the surface. The e(+)e(-) annihilation events take place due to the collisions of electrons from the visible matter with positrons from dark matter droplets which may result in the bright 511 keV gamma-ray line from the bulge of the Galaxy.     

Detactability of Dark Matter Subhalos by Means of the GAMMA-400 Telescope

The potential of the planned GAMMA-400 gamma-ray telescope for detecting subhalos of mass between 10⁶M_⊙ and 10⁹M_⊙ in the Milky Way Galaxy that consist of annihilating dark matter in the form of weakly interacting massive particles (WIMPs) is studied. The inner structure of dark matter subhalos and their distribution in the Milky Way Galaxy are obtained on the basis of respective theoretical models. Our present analysis shows that the expected gamma-ray flux from subhalos depends strongly on the WIMP mass and on the subhalo concentration, but that it depends less strongly on the subhalo mass. Optimistically, a flux of 10 to 100 ph per year in the energy range above 100 MeV can be expected from the closest and most massive subhalos, which can therefore be thought to be detectable sources for GAMMA-400. Because of the smallness of fluxes, however, only via a joint analysis of future GAMMA-400 data and data from other telescopes would it become possible to resolve the inner structure of the subhalos. Also, the recent subhalo candidates 3FGL J2212.5+0703 and J1924.8–1034 are considered within our model. Our conclusion is that these sources hardly belong to the subhalo population.     

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.     

Gamma-ray spectrum from gravitino dark matter decay

Gravitinos are very promising candidates for the cold dark matter of the Universe. Interestingly, to achieve a sufficiently long gravitino lifetime, R parity conservation is not required, thus preventing any dangerous cosmological influence of the next-to-lightest supersymmetric particle. When R parity is violated, gravitinos decay into photons and other particles with a lifetime much longer than the age of the Universe, producing a diffuse gamma-ray flux with a characteristic spectrum that could be measured in future experiments, such as GLAST or AMS-02. In this Letter we compute the energy spectrum of photons from gravitino decay and discuss its main qualitative features.     

Quark matter in astrophysics and cosmology

We discuss the role of quark matter in astrophysics and cosmology. The implications of the dynamics of the quark-hadron phase transition in the early universe for the element abundances from big gang nucleosynthesis and the composition of the dark matter in the universe are addressed. We discuss the possibility of deciding on an equation of state for high density matter by observing the cooling of a neutron star remnant of SN1987A. Quark matter models for the Centauros events, Cygnus X-3 cosmic ray events, high energy gamma-ray bursts and the solar neutrino problem are described.     

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.     

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.     

Small-scale clumps in the Galactic halo

A mass function of small-scale dark matter clumps is calculated. We take into account the tidal destruction of clumps at early stages of structure formation starting from a time of clump detachment from the Universe expansion. Only a small fraction of these clumps, ∼0.1%, in each logarithmic mass interval Δ log M ∼ 1 survives the stage of hierarchical clustering. We calculate the probability of surviving of the remnants of dark matter clumps in the Galaxy by modelling the tidal destruction of the small-scale clumps by disk and stars. It is demonstrated that a substantial fraction of clump remnants may survive through the tidal destruction during the lifetime of the Galaxy if a radius of core is rather small. The resulting mass spectrum of survived clumps is extended down to the mass of the core of the cosmologically produced clumps with a minimal mass. The survived dense remnants of tidally destructed clumps provides a large contribution to the annihilation signal in the Galaxy. We describe the anisotropy of dark matter clump distribution caused by tidal destruction of clumps in the Galactic disk. A corresponding annihilation of dark matter particles in small-scale clumps produces the anisotropic gamma-ray signal with respect to the Galactic disk.