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Register a new userAstrophysical and Dark Matter Interpretations of Extended Gamma-Ray Emission from the Galactic Center
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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$^ast$ 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 $tau$-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 $39.4left(^{+3.7}_{-2.9}rm stat.right)left(pm 7.9rm sys.right)rm GeV$, while a pure $tau^{+} tau^{-}$-channel case has an estimated dark matter mass of $9.43left(^{+0.63}_{-0.52}rm stat.right)(pm 1.2rm sys.) GeV$. 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.
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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 b/b-bar quarks, and masses approximately 10 GeV to 30 GeV annihilating to tau+ tau- 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 b/b-bar quarks. The gamma-ray intensity and spectrum are also well fit with emission from a millisecond pulsar (MSP) 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 MSPs 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.
Indirect searches for dark matter through Standard Model products of its annihilation generally assume a cross-section which is dominated by a term independent of velocity ($s$-wave annihilation). However, in many DM models an $s$-wave annihilation cross-section is absent or helicity suppressed. To reproduce the correct DM relic density in these models, the leading term in the cross section is proportional to the DM velocity squared ($p$-wave annihilation). Indirect detection of such $p$-wave DM is difficult because the average velocities of DM in galaxies today are orders of magnitude slower than the DM velocity at the time of decoupling from the primordial thermal plasma, suppressing the annihilation cross-section today by some five orders of magnitude relative to its value at freeze out. Thus $p$-wave DM is out of reach of traditional searches for DM annihilations in the Galactic halo. Near the region of influence of a central supermassive black hole, such as Sgr A$^*$, however, DM can form a localized over-density known as a `spike. In such spikes the DM is predicted to be both concentrated in space and accelerated to higher velocities, allowing the $gamma$-ray signature from its annihilation to potentially be detectable above the background. We use the $Fermi$ Large Area Telescope to search for the $gamma$-ray signature of $p$-wave annihilating DM from a spike around Sgr A$^*$ in the energy range 10 GeV-600 GeV. Such a signal would appear as a point source and would have a sharp line or box-like spectral features difficult to mimic with standard astrophysical processes, indicating a DM origin. We find no significant excess of $gamma$ rays in this range, and we place upper limits on the flux in $gamma$-ray boxes originating from the Galactic Center. This result, the first of its kind, is interpreted in the context of different models of the DM density near Sgr A$^*$.
We explore two possible scenarios to explain the observed gamma-ray emission associated with the atypical globular cluster Omega-Centauri: emission from millisecond pulsars (MSP) and dark matter (DM) annihilation. In the first case the total number of MSPs needed to produce the gamma-ray flux is compatible with the known (but not confirmed) MSP candidates observed in X-rays. A DM interpretation is motivated by the possibility of Omega-Centauri being the remnant core of an ancient dwarf galaxy hosting a surviving DM component. At least two annihilation channels, light quarks and muons, can plausibly produce the observed gama-ray spectrum. We outline constraints on the parameter space of DM mass versus the product of the pair-annihilation cross section and integrated squared DM density (the so-called J-factor). We translate upper limits on the dark matter content of Omega-Centauri into lower limits on the annihilation cross section. This shows s-wave annihilation into muons to be inconsistent with CMB observations, while a small window for annihilation into light quarks is allowed. Further analysis of Omega-Centauris internal kinematics, and/or additional information on the resident MSP population will yield much stronger constraints and shed light about the origin of this otherwise mysterious gamma-ray source.
Multiwavelength observations suggest that clusters are reservoirs of vast amounts relativistic electrons and positrons that are either injected into and accelerated directly in the intra-cluster medium, or produced as secondary pairs by cosmic ray ions scattering on ambient protons. In these possible scenarios gamma rays are produced either through electrons upscattering low-energy photons or by decay of neutral pions produced by hadronic interactions. In addition, the high mass-to-light ratios in clusters in combination with considerable Dark Matter (DM) overdensities makes them interesting targets for indirect DM searches with gamma rays. The resulting signals are different from known point sources or from diffuse emission and could possibly be detected with the Fermi-LAT. Both WIMP annihilation/decay spectra and cosmic ray induced emission are determined by universal parameters, which make a combined statistical likelihood analysis feasible. We present initial results of this analysis leading to limits on the DM annihilation cross section or decay time and on the hadron injection efficiency.
We present a study of the Galactic Center region as a possible source of both secondary gamma-ray and neutrino fluxes from annihilating dark matter. We have studied the gamma-ray flux observed by the High Energy Stereoscopic System (HESS) from the J1745-290 Galactic Center source. The data are well fitted as annihilating dark matter in combination with an astrophysical background. The analysis was performed by means of simulated gamma spectra produced by Monte Carlo event generators packages. We analyze the differences in the spectra obtained by the various Monte Carlo codes developed so far in particle physics. We show that, within some uncertainty, the HESS data can be fitted as a signal from a heavy dark matter density distribution peaked at the Galactic Center, with a power-law for the background with a spectral index which is compatible with the Fermi-Large Area Telescope (LAT) data from the same region. If this kind of dark matter distribution generates the gamma-ray flux observed by HESS, we also expect to observe a neutrino flux. We show prospective results for the observation of secondary neutrinos with the Astronomy with a Neutrino Telescope and Abyss environmental RESearch project (ANTARES), Ice Cube Neutrino Observatory (Ice Cube) and the Cubic Kilometer Neutrino Telescope (KM3NeT). Prospects solely depend on the device resolution angle when its effective area and the minimum energy threshold are fixed.
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