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Register a new userDim but not entirely dark: Extracting the Galactic Center Excess source-count distribution with neural nets
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The two leading hypotheses for the Galactic Center Excess (GCE) in the $textit{Fermi}$ data are an unresolved population of faint millisecond pulsars (MSPs) and dark-matter (DM) annihilation. The dichotomy between these explanations is typically reflected by modeling them as two separate emission components. However, point-sources (PSs) such as MSPs become statistically degenerate with smooth Poisson emission in the ultra-faint limit (formally where each source is expected to contribute much less than one photon on average), leading to an ambiguity that can render questions such as whether the emission is PS-like or Poissonian in nature ill-defined. We present a conceptually new approach that describes the PS and Poisson emission in a unified manner and only afterwards derives constraints on the Poissonian component from the so obtained results. For the implementation of this approach, we leverage deep learning techniques, centered around a neural network-based method for histogram regression that expresses uncertainties in terms of quantiles. We demonstrate that our method is robust against a number of systematics that have plagued previous approaches, in particular DM / PS misattribution. In the $textit{Fermi}$ data, we find a faint GCE described by a median source-count distribution (SCD) peaked at a flux of $sim4 times 10^{-11} text{counts} text{cm}^{-2} text{s}^{-1}$ (corresponding to $sim3 - 4$ expected counts per PS), which would require $N sim mathcal{O}(10^4)$ sources to explain the entire excess (median value $N = text{29,300}$ across the sky). Although faint, this SCD allows us to derive the constraint $eta_P leq 66%$ for the Poissonian fraction of the GCE flux $eta_P$ at 95% confidence, suggesting that a substantial amount of the GCE flux is due to PSs.
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We re-examine evidence that the Galactic Center Excess (GCE) originates primarily from point sources (PSs). We show that in our region of interest, non-Poissonian template fitting (NPTF) evidence for GCE PSs is an artifact of unmodeled north-south asymmetry of the GCE. This asymmetry is strongly favored by the fit (although it is unclear if this is physical), and when it is allowed, the preference for PSs becomes insignificant. We reproduce this behavior in simulations, including detailed properties of the spurious PS population. We conclude that NTPF evidence for GCE PSs is highly susceptible to certain systematic errors, and should not at present be taken to robustly disfavor a dominantly smooth GCE.
The Fermi Large Area Telescope has observed an excess of ~GeV energy gamma rays from the center of the Milky Way, which may arise from near-thermal dark matter annihilation. Firmly establishing the dark matter origin for this excess is however complicated by challenges in modeling diffuse cosmic-ray foregrounds as well as unresolved astrophysical sources, such as millisecond pulsars. Non-Poissonian Template Fitting (NPTF) is one statistical technique that has previously been used to show that at least some fraction of the GeV excess is likely due to a population of dim point sources. These results were recently called into question by Leane and Slatyer (2019), who showed that a synthetic dark matter annihilation signal injected on top of the real Fermi data is not recovered by the NPTF procedure. In this work, we perform a dedicated study of the Fermi data and explicitly show that the central result of Leane and Slatyer (2019) is likely driven by the fact that their choice of model for the Galactic foreground emission does not provide a sufficiently good description of the data. We repeat the NPTF analyses using a state-of-the-art model for diffuse gamma-ray emission in the Milky Way and introduce a novel statistical procedure, based on spherical-harmonic marginalization, to provide an improved description of the Galactic diffuse emission in a data-driven fashion. With these improvements, we find that the NPTF results continue to robustly favor the interpretation that the Galactic Center excess is due, in part, to unresolved astrophysical point sources across the analysis variations that we have explored.
The Fermi Large Area Telescope observed an excess in gamma ray emission spectrum coming from the center of the Milky Way galaxy. This data reveals that a light Dark Matter (DM) candidate of mass in the range 31-40 GeV, dominantly decaying into $bbar b$ final state, can explain the presence of the observed bump in photon energy. We try to interpret this observed phenomena by sneutrino DM annihilation into pair of fermions in the Supersymmetric Inverse Seesaw Model (SISM). This model can also account for tiny non-zero neutrino masses satisfying existing neutrino oscillation data. We show that a Higgs portal DM in this model is in perfect agreement with this new interpretation besides satisfying all other existing collider, cosmological and low energy experimental constraints.
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.
The dark matter spike induced by the adiabatic growth of a massive black hole in a cuspy environment, may explain the thermal dark matter density required to fit the cut-off in the HESSJ1745-290 gamma-ray spectra as TeV dark matter signal with a background component. The spike extension appears comparable with the HESS angular resolution.
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