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We construct empirical models of the diffuse gamma-ray background toward the Galactic Center. Including all known point sources and a template of emission associated with interactions of cosmic rays with molecular gas, we show that the extended emission observed previously in the Fermi Large Area Telescope data toward the Galactic Center is detected at high significance for all permutations of the diffuse model components. However, we find that the fluxes and spectra of the sources in our model change significantly depending on the background model. In particular, the spectrum of the central Sgr A* source is less steep than in previous works and the recovered spectrum of the extended emission has large systematic uncertainties, especially at lower energies. If the extended emission is interpreted to be due to dark matter annihilation, we find annihilation into pure b-quark and τ-lepton channels to be statistically equivalent goodness of fits. In the case of the pure b-quark channel, we find a dark matter mass of 39.4(-2.9+3.7 stat)(±7.9 sys) GeV, while a pure τ+τ--channel case has an estimated dark matter mass of 9.43(0.52+0.63 stat)(±1.2 sys) GeV. Alternatively, if the extended emission is interpreted to be astrophysical in origin such as due to unresolved millisecond pulsars, we obtain strong bounds on dark matter annihilation, although systematic uncertainties due to the dependence on the background models are significant. [ABSTRACT FROM AUTHOR]

We show the existence of a statistically significant, robust detection of a gamma-ray source in the Milky Way Galactic Center that is consistent with a spatially extended signal using about 4 years of Fermi-LAT data. The gamma-ray flux is consistent with annihilation of dark matter particles with a thermal annihilation cross section if the spatial distribution of dark matter particles is similar to the predictions of dark matter only simulations. We find statistically significant detections of an extended source with gamma-ray spectrum that is consistent with dark matter particle masses of approximately 10 GeV to 1 TeV annihilating to bƃ quarks and masses approximately 10-30 GeV annihilating to ττ leptons. However, a part of the allowed region in this interpretation is in conflict with constraints from Fermi observations of the Milky Way satellites. The biggest improvement over the fit including just the point sources is obtained for a 30 GeV dark matter particle annihilating to bƃ quarks. The gamma-ray intensity and spectrum are also well fit with emission from a millisecond pulsar population following a density profile like that of low-mass x-ray binaries observed in M31. The greatest goodness of fit of the extended emission is with spectra consistent with known astrophysical sources like millisecond pulsars in globular clusters or cosmic-ray bremsstrahlung on molecular gas. Therefore, we conclude that the bulk of the emission is likely from an unresolved or spatially extended astrophysical source. However, the interesting possibility of all or part of the extended emission being from dark matter annihilation cannot be excluded at present. [ABSTRACT FROM AUTHOR]

We study the spectra of gamma ray signals that arise from dark matter annihilation in the Universe. We focus on the large class of theories where the photon spectrum includes both continuum spectrum of gamma rays that arise from annihilation into standard model states at tree level, as well as monochromatic gamma rays arising from annihilation directly into two photons at the one-loop level. In this class of theories we obtain lower bounds on the ratio of the strength of the gamma ray line relative to the gamma ray continuum as a function of the dark matter mass and spin. These limits are obtained from the unitarity relation between the tree-level amplitude of the primary annihilation channel and the imaginary part of the loop-level amplitude for annihilation directly into photons, with the primary decay products running in the loop. These results are exact in the limit that dark matter annihilation at tree level is exclusively to a single standard model species, occurs through the lowest partial wave and respects CP. Away from this limit the bounds are approximate. Our conclusions agree with known results in the literature for the cases of the minimal supersymmetric standard model, universal extra dimensions and the littlest Higgs with T parity. We use the Fermi-LAT observations to translate these limits into upper bounds on the dark matter annihilation cross section into any specific standard model state. [ABSTRACT FROM AUTHOR]

We perform a detailed examination of current constraints on annihilating and decaying dark matter models from both prompt and inverse-Compton emission photons, including both model-dependent and model-independent bounds. We also show that the observed isotropic diffuse gamma-ray background (DGRB). which provides one of the most conservative constraints on models of annihilating weak scale dark matter particles, may enhance its sensitivity by a factor of ∼2 to 3 (95% C.L.) as the Fermi-LAT experiment resolves DGRB contributing blazar sources with five years of observation. For our forecasts. we employ the results ol constraints to the luminosity-dependent density evolution plus blazar spectral energy distribution sequence model, which is constrained by the DGRB and blazar source count distribution function. [ABSTRACT FROM AUTHOR]

The extended excess toward the Galactic Center (GC) in gamma rays inferred from Fermi-LAT observations has been interpreted as being due to dark matter (DM) annihilation. Here, we perform new likelihood analyses of the GC and show that, when including templates for the stellar galactic and nuclear bulges, the GC shows no significant detection of a DM annihilation template, even after generous variations in the Galactic diffuse emission models and a wide range of DM halo profiles. We include Galactic diffuse emission models with combinations of three-dimensional inverse Compton maps, variations of interstellar gas maps, and a central source of electrons. For the DM profile, we include both spherical and ellipsoidal DM morphologies and a range of radial profiles from steep cusps to kiloparsec-sized cores, motivated in part by hydrodynamical simulations. Our derived upper limits on the dark matter annihilation flux place strong constraints on DM properties. In the case of the pure b-quark annihilation channel, our limits on the annihilation cross section are more stringent than those from the Milky Way dwarfs up to DM masses of approximately TeV and rule out the thermal relic cross section up to approximately 300 GeV. Better understanding of the DM profile, as well as the Fermi-LAT data at its highest energies, would further improve the sensitivity to DM properties. [ABSTRACT FROM AUTHOR]

We discuss numerous mechanisms for production of sterile neutrinos, which can account for all or a fraction of dark matter and which can range from warm to effectively cold dark matter, depending on the cosmological scenario. We investigate production by Higgs boson decay, (B-L) gauge boson production at high temperature, as well as production via resonant and nonresonant neutrino oscillations. We calculate the effects on structure formation in these models, some for the first time. If two populations of sterile neutrinos, one warm and one cold, were produced by different mechanisms or if sterile neutrinos account for only a fraction of dark matter, while the remainder is some other cold dark matter particle, the resulting multicomponent dark matter may alleviate some problems in galaxy formation. We examine the x-ray constraints and the candidate signal at 3.5 keV. Finally, we also show that the σ8 problem can be a signature of fractional dark matter in the form of sterile neutrinos in several mechanisms. [ABSTRACT FROM AUTHOR]

We assess galactic dark matter (DM) sensitivities to photons from annihilation and decay using the spatial and kinematic information determined by state-of-the-art simulations in the Latte suite of feedback in realistic environments (FIRE-2). For kinematic information, we study the energy shift pattern of DM narrow emission lines predicted in FIRE-2 and discuss its potential as a DM-signal diagnosis, showing for the first time the power of symmetric observations around l=0°. We find that the exposures needed to resolve the line separation of DM to gas by XRISM at 5σ to be large, ≳4 Ms, while exposures are smaller for Athena (≲50 ks) and Lynx (≲100 ks). We find that large field-of-view exposures remain the most sensitive methods for the detection of DM annihilation or decay by the DM-emission signals in the field of view dominating the velocity information. The ~4 sr view of the Galactic Center region by the wide field monitor (WFM) aboard the eXTP mission will be highly sensitive to DM signals, with a prospect of ~105 to 106 events from the 3.5 keV line in a 100 ks exposure, with the range dependent on the photon acceptance in WFM's field of view. We also investigate detailed full-sky emission maps for both DM annihilation and decay signals--evaluating the signal-to-noise for a DM detection with realistic x-ray and gamma-ray backgrounds--as a guideline for what could be a forthcoming era of DM astronomy. [ABSTRACT FROM AUTHOR]

We perform a detailed study of the weak interactions of standard model neutrinos with the primordial plasma and their effect on the resonant production of sterile neutrino dark matter. Motivated by issues in cosmological structure formation on small scales, and reported x-ray signals that could be due to sterile neutrino decay, we consider 7 keV-scale sterile neutrinos. Oscillation-driven production of such sterile neutrinos occurs at temperatures T≳100 MeV, where we study two significant effects of weakly charged species in the primordial plasma: (1) the redistribution of an input lepton asymmetry; (2) the opacity for active neutrinos. We calculate the redistribution analytically above and below the quark-hadron transition, and match with lattice QCD calculations through the transition. We estimate opacities due to tree-level processes involving leptons and quarks above the quark-hadron transition, and the most important mesons below the transition. We report final sterile neutrino dark matter phase space densities that are significantly influenced by these effects, and yet relatively robust to remaining uncertainties in the nature of the quark-hadron transition. We also provide transfer functions for cosmological density fluctuations with cutoffs at k≃10h Mpc-1, that are relevant to galactic structure formation. [ABSTRACT FROM AUTHOR]

We show that the canonical oscillation-based (nonresonant) production of sterile neutrino dark matter is inconsistent at > 99% confidence with observations of galaxies in the Local Group. We set lower limits on the nonresonant sterile neutrino mass of 2.5 keV (equivalent to 0.7 keV thermal mass) using phase-space densities derived for dwarf satellite galaxies of the Milky Way as well as limits of 8.8 keV (equivalent to 1.8 keV thermal mass) based on subhalo counts of N-body simulations of M 31 analogs. Combined with improved upper mass limits derived from significantly deeper x-ray data of M 31 with full consideration for background variations, we show that there remains little room for nonresonant production if sterile neutrinos are to explain 100% of the dark matter abundance. Resonant and nonoscillation sterile neutrino production remain viable mechanisms for generating sufficient dark matter sterile neutrinos. [ABSTRACT FROM AUTHOR]