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Register a new userMultimessenger constraints on the dark matter interpretation of the Fermi-LAT Galactic center excess
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An excess of $gamma$ rays in the data measured by the Fermi Large Area Telescope in the direction of the Galactic center has been reported in several publications. This excess, labeled as the Galactic center excess (GCE), is detected analyzing the data with different interstellar emission models, point source catalogs and analysis techniques. The characteristics of the GCE, recently measured with unprecedented precision, are all compatible with dark matter particles (DM) annihilating in the main halo of our Galaxy, even if other interpretations are still not excluded. We investigate the DM candidates that fit the observed GCE spectrum and spatial morphology. We assume a simple scenario with DM annihilating into a single channel but we inspect also more complicated models with two and three channels. We perform a search for a $gamma$-ray flux from a list of 48 Milky Way dwarf spheroidal galaxies (dSphs) using state-of-the-art estimation of the DM density in these objects. Since we do not find any significant signal from the dSphs, we put upper limits on the annihilation cross section that result to be compatible with the DM candidate that fits the GCE. However, we find that the GCE DM signal is excluded by the AMS-02 $bar{p}$ flux data for all hadronic and semi-hadronic annihilation channels unless the vertical size of the diffusion halo is smaller than 2 kpc -- which is in tension with radioactive cosmic ray fluxes and radio data. Furthermore, AMS-02 $e^+$ data rule out pure or mixed channels with a component of $e^+ e^-$. The only DM candidate that fits the GCE spectrum and is compatible with constraints obtained with the combined dSphs analysis and the AMS-02 $bar{p}$ and $e^+$ data annihilates purely into $mu^+mu^-$, has a mass of 60 GeV and roughly a thermal cross section.
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We show that the Galactic Center Excess (GCE) emission, as recently updated by the Fermi-LAT Collaboration, could be explained by the sum of Fermi-bubbles-like emission plus dark matter (DM) annihilation, in the context of a scalar-singlet Higgs portal scenario (SHP). In fact, the standard SHP, where the DM particle, $S$, only has renormalizable interactions with the Higgs, is non-operational due to strong constraints, specially from DM direct detection limits. Thus we consider a most economical extension, called ESHP (for extended SHP), which simply consists in the addition of a second (heavier) scalar singlet in the dark sector. The second scalar can be integrated-out, leaving a standard SHP plus a dimension-6 operator. Essentially, this model has only two relevant parameters (the DM mass and the coupling of the dim-6 operator). DM annihilation occurs mainly into two Higgs bosons, $SSrightarrow hh$. We demonstrate that, despite its economy, the ESHP model provides excellent fits to the GCE (with p-value $sim 0.6-0.7$) for very reasonable values of the parameters, in particular $m_S simeq 130$ GeV. This is achieved without conflicting with other observables and keeping the $S-$particle relic density at the accepted value for the DM content in the universe.
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.
We have performed an analysis of the diffuse gamma-ray emission with the Fermi Large Area Telescope in the Milky Way Halo region searching for a signal from dark matter annihilation or decay. In the absence of a robust dark matter signal, constraints are presented. We consider both gamma rays produced directly in the dark matter annihilation/decay and produced by inverse Compton scattering of the e+e- produced in the annihilation/decay. Conservative limits are derived requiring that the dark matter signal does not exceed the observed diffuse gamma-ray emission. A second set of more stringent limits is derived based on modeling the foreground astrophysical diffuse emission using the GALPROP code. Uncertainties in the height of the diffusive cosmic-ray halo, the distribution of the cosmic-ray sources in the Galaxy, the index of the injection cosmic-ray electron spectrum and the column density of the interstellar gas are taken into account using a profile likelihood formalism, while the parameters governing the cosmic-ray propagation have been derived from fits to local cosmic-ray data. The resulting limits impact the range of particle masses over which dark matter thermal production in the early Universe is possible, and challenge the interpretation of the PAMELA/Fermi-LAT cosmic ray anomalies as annihilation of dark matter.
We present a novel interpretation of the $gamma$-ray diffuse emission measured by Fermi-LAT and H.E.S.S. in the Galactic center (GC) region and the Galactic ridge (GR). In the first part we perform a data-driven analysis based on PASS8 Fermi-LAT data: we extend down to few GeV the spectra measured by H.E.S.S. and infer the primary cosmic-ray (CR) radial distribution between 0.1 and 3 TeV. In the second part we adopt a CR transport model based on a position-dependent diffusion coefficient. Such behavior reproduces the radial dependence of the CR spectral index recently inferred from the Fermi-LAT observations. We find that the bulk of the GR emission can be naturally explained by the interaction of the diffuse steady-state Galactic CR sea with the gas present in the Central Molecular Zone. Although our results leave room for a residual radial-dependent emission associated with a central source, the relevance of the large-scale background prevents from a solid evidence of a GC Pevatron.
The excess of $gamma$ rays in the data measured by Fermi-LAT from the Galactic center region is one of the most intriguing mysteries in Astroparticle Physics. This Galactic center excess (GCE), has been measured with respect to different interstellar emission models (IEMs), source catalogs, data selections and techniques. Although several proposed interpretations have appeared in the literature, there are not firm conclusions as to its origin. The main difficulty in solving this puzzle lies in modeling a region of such complexity and thus precisely measuring the characteristics of the GCE. In this paper, we use 11 years of Fermi-LAT data, state of the art IEMs, and the newest 4FGL source catalog to provide precise measurements of the energy spectrum, spatial morphology, position, and sphericity of the GCE. We find that the GCE has a spectrum which is peaked at a few GeV and is well fit with a log-parabola. The normalization of the spectrum changes by roughly $60%$ when using different IEMs, data selections and analysis techniques. The spatial distribution of the GCE is compatible with a dark matter (DM) template produced with a generalized NFW density profile with slope $gamma = 1.2-1.3$. No energy evolution is measured for the GCE morphology between $0.6-30$ GeV at a level larger than $10%$ of the $gamma$ average value, which is 1.25. The analysis of the GCE modeled with a DM template divided into quadrants shows that the spectrum and spatial morphology of the GCE is similar in different regions around the Galactic center. Finally, the GCE centroid is compatible with the Galactic center, with best-fit position between $l=[-0.3^{circ},0.0^{circ}],b=[-0.1^{circ},0.0^{circ}]$, and it is compatible with a spherical symmetric morphology. In particular, fitting the DM spatial profile with an ellipsoid gives a major-to-minor axis ratio between 0.8-1.2.
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