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Register a new userEGRET Excess of diffuse Galactic Gamma Rays interpreted as a Signal of Dark Matter Annihilation
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W. de Boer
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Elsaesser and Mannheim fit a contribution of Dark Matter Annihilation (DMA) to the extragalactic contribution of the galactic diffuse gamma ray flux, as deduced from the EGRET data by Strong, Moskalenko and Reimer.They find a WIMP mass of 515{+110}{-75} GeV and quote a systematic error of 30%. However, they do not include large systematic uncertainties from the fact that the determination of the extragalactic flux (EGF) requires a model for the subtraction of the Galactic flux from the data.The data used were obtained with a model without Galactic DM, so one expects additional uncertainty in the region where DMA contributes. Including a Galactic DMA contribution reduces the significance and the WIMP mass. The latter then becomes compatible with the Galactic excess of diffuse gamma rays, which posseses all the properties of DMA with a much higher significance than the extragalactic excess.
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The diffuse galactic EGRET gamma ray data show a clear excess for energies above 1 GeV in comparison with the expectations from conventional galactic models. The excess is seen with the same spectrum in all sky directions, as expected for Dark Matter (DM) annihilation. This hypothesis is investigated in detail. The energy spectrum of the excess is used to limit the WIMP mass to the 50-100 GeV range, while the sky maps are used to determine the halo structure, which is consistent with a triaxial isothermal halo with additional enhancement of Dark Matter in the disc. The latter is strongly correlated with the ring of stars around our galaxy at a distance of 14 kpc, thought to originate from the tidal disruption of a dwarf galaxy. It is shown that this ring of DM with a mass of $approx 2cdot 10^{11} M_odot$ causes the mysterious change of slope in the rotation curve at $R=1.1R_0$ and the large local surface density of the disc. The total mass of the halo is determined to be $3cdot 10^{12} M_odot$. A cuspy profile is definitely excluded to describe the gamma ray data. These signals of Dark Matter Annihilation are compatible with Supersymmetry for boost factors of 20 upwards and have a statistical significance of more than $10sigma$ in comparison with the conventional galactic model. The latter combined with all features mentioned above provides an intriguing hint that the EGRET excess is indeed a signal from Dark Matter Annihilation.
The public data from the EGRET space telescope on diffuse Galactic gamma rays in the energy range from 0.1 to 10 GeV are reanalyzed with the purpose of searching for signals of Dark Matter annihilation (DMA). The analysis confirms the previously observed excess for energies above 1 GeV in comparison with the expectations from conventional Galactic models. In addition, the excess was found to show all the key features of a signal from Dark Matter Annihilation (DMA): a) the excess is observable in all sky directions and has the same shape everywhere, thus pointing to a common source; b) the shape corresponds to the expected spectrum of the annihilation of non-relativistic massive particles into - among others - neutral $pi^0$ mesons, which decay into photons. From the energy spectrum of the excess we deduce a WIMP mass between 50 and 100 GeV, while from the intensity of the excess in all sky directions the shape of the halo could be reconstructed. The DM halo is consistent with an almost spherical isothermal profile with substructure in the Galactic plane in the form of toroidal rings at 4 and 14 kpc from the center. These rings lead to a peculiar shape of the rotation curve, in agreement with the data, which proves that the EGRET excess traces the Dark Matter.
Recently it was shown that the excess of diffuse Galactic gamma rays above 1 GeV traces the Dark Matter halo, as proven by reconstructing the peculiar shape of the rotation curve of our Galaxy from the gamma ray excess. This can be interpreted as a Dark Matter annihilation signal. In this paper we investigate if this interpretation is consistent with Supersymmetry. It is found that the EGRET excess combined with all electroweak constraints is fully consistent with the minimal mSUGRA model for scalars in the TeV range and gauginos below 500 GeV.
In this paper, we will discuss a specific case that the dark matter particles annihilate into right-handed neutrinos. We calculate the predicted gamma-ray excess from the galactic center and compare our results with the data from the Fermi-LAT. An approximately 10-60 GeV right-handed neutrino with heavier dark matter particle can perfectly explain the observed spectrum. The annihilation cross section $langle sigma v rangle$ falls within the range $0.5$-$4 times 10^{-26} text{ cm}^3/text{s}$, which is roughly compatible with the WIMP annihilation cross section.
Using gamma-ray data from observations of the Milky Way, Andromeda (M31), and the cosmic background, we calculate conservative upper limits on the dark matter self-annihilation cross section to monoenergetic gamma rays, <sigma_A v>_{gamma gamma}, over a wide range of dark matter masses. (In fact, over most of this range, our results are unchanged if one considers just the branching ratio to gamma rays with energies within a factor of a few of the endpoint at the dark matter mass.) If the final-state branching ratio to gamma rays, Br(gamma gamma), were known, then <sigma_A v>_{gamma gamma} / Br(gamma gamma) would define an upper limit on the total cross section; we conservatively assume Br(gamma gamma) > 10^{-4}. An upper limit on the total cross section can also be derived by considering the appearance rates of any Standard Model particles; in practice, this limit is defined by neutrinos, which are the least detectable. For intermediate dark matter masses, gamma-ray-based and neutrino-based upper limits on the total cross section are comparable, while the gamma-ray limit is stronger for small masses and the neutrino limit is stronger for large masses. We comment on how these results depend on the assumptions about astrophysical inputs and annihilation final states, and how GLAST and other gamma-ray experiments can improve upon them.
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