The survival of the dark-matter interpretation of the Fermi-LAT Galactic center excess under consolidated gamma-ray, antiproton, and positron constraints

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The Fermi-LAT Galactic center gamma-ray excess (GCE) is fit by annihilating WIMP dark matter, and Di Mauro and Winkler (2021) showed that a joint Fermi-LAT dwarf-spheroidal plus AMS-02 antiproton and positron test excludes all hadronic and semi-hadronic GCE dark-matter candidates unless the cosmic-ray diffusion halo height is $L\lesssim 2$ kpc (a value 2-3 sigma below radioactive-cosmic-ray determinations), excludes channels with an $e^+e^-$ component under a pulsar-inclusive (optimistic) positron-background assumption, and leaves a single survivor: annihilation into $\mu^+\mu^-$ with mass near 60 GeV and a roughly thermal cross section. The open problem is the completion of that multimessenger test: determine whether the dark-matter interpretation of the GCE survives once the systematics it rests on -- the halo height $L$, the pulsar positron background, interstellar-emission-model and interstellar-radiation-field modeling in the inner Galaxy, antiproton production cross sections and AMS-02 error correlations, dwarf-spheroidal $J$-factors, and the treatment of cosmic-ray diffusion in the inverse-Compton calculation and near the Galactic center -- are determined or marginalized, consistently with the data that constrain them, rather than assumed. Later work (2022-2026) tightened individual bands: antiproton bounds on thermal $b\bar b$ are now robust under marginalized propagation, the halo-height determination is itself biased by Galactic gas modeling, concrete leptophilic models reproducing the GCE remain viable under the assumptions of the cited analyses, and the GCE morphology is disputed between dark-matter and stellar-bulge templates. No consolidated verdict exists; the problem remains open.

Background

The Galactic center gamma-ray excess (GCE) is a persistent excess of GeV gamma rays in data from the Large Area Telescope (LAT) aboard the Fermi Gamma-ray Space Telescope. First reported by Goodenough and Hooper (2009) and characterized by Daylan et al. (2016), it peaks at photon energies of 1-3 GeV, is approximately symmetric about the Galactic center, and its radial brightness profile resembles the square of a generalized Navarro-Frenk-White (NFW) dark-matter density profile, conventionally labeled by its inner logarithmic slope $\gamma$. Its spectrum and morphology are fit well by annihilation of a weakly interacting massive particle (WIMP), a weak-scale-mass dark-matter candidate, into Standard-Model final states with a velocity-weighted cross section $\langle\sigma v\rangle$ near the thermal-relic value $\sim 3\times 10^{-26}\,\mathrm{cm^3\,s^{-1}}$ expected for a particle freezing out with the observed cosmological abundance. The best-fit hadronic channel was $b\bar b$ with mass $m_\chi\sim 30$-60 GeV, although the recovered spectrum depends substantially on how the diffuse Galactic gamma-ray foreground is modeled (Calore et al. 2015).

A competing astrophysical explanation is a large population of unresolved millisecond pulsars in the Galactic bulge: photon-count statistics of the inner-Galaxy sky reveal a point-source-like component (Bartels, Krishnamurthy and Weniger 2016), and spatial templates tracing the boxy/peanut-shaped stellar bulge and the nuclear bulge can reproduce much of the excess morphology (Macias et al. 2018). Whether the GCE traces dark matter or old stellar populations is itself disputed; the multimessenger test below addresses the dark-matter reading of the excess, and the morphology dispute enters that test as an uncertainty in the GCE characterization (through the interstellar-emission-model and interstellar-radiation-field modeling of item 3 in the Problem Statement) rather than as an independent deliverable.

Because WIMP annihilation in the Galactic halo must also produce charged cosmic rays and other gamma-ray signals, the dark-matter interpretation can be tested across wavebands and messengers. Dwarf spheroidal galaxies (dSphs), dark-matter-dominated Milky Way satellites with negligible astrophysical gamma-ray backgrounds, provide the cleanest targets: their expected annihilation signal scales with the line-of-sight integral of the squared dark-matter density, the $J$-factor, derived from stellar-kinematic modeling of each galaxy. AMS-02, a precision cosmic-ray spectrometer on the International Space Station, measures the antiproton and positron fluxes. Antiprotons are produced copiously by hadronic annihilation channels ($b\bar b$, light quarks, or gauge-boson final states that shower into quarks); positrons directly probe leptonic channels ($e^+e^-$, $\mu^+\mu^-$, $\tau^+\tau^-$), whose injected electrons and positrons also generate inverse-Compton (ICS) gamma rays by upscattering photons of the interstellar radiation field (ISRF). Predicting any charged-particle flux requires a model of cosmic-ray propagation: cosmic rays diffuse on magnetic turbulence inside a roughly cylindrical diffusion halo of half-height $L$ before escaping, and the value of $L$ controls how efficiently antiprotons produced near the Galactic center reach Earth. Radioactive cosmic-ray clocks, such as the $^{10}$Be/$^{9}$Be ratio ($^{10}$Be decays with a 1.4 Myr half-life), together with diffuse radio synchrotron and diffuse gamma-ray data, constrain $L$ independently of any dark-matter assumption.

Di Mauro and Winkler (2021) combined these bands. Using 48 dSphs with state-of-the-art $J$-factors, AMS-02 antiproton data with uncertainties propagated from cosmic-ray propagation, antiproton production cross sections, and AMS-02 error correlations, and AMS-02 positron data under both a conservative (secondaries-only) and a pulsar-inclusive background assumption, they found: (i) the dSph limits are compatible with the GCE-fitting candidates; (ii) antiproton data exclude all hadronic and semi-hadronic GCE candidates unless $L\lesssim 2\,\mathrm{kpc}$ (for pure $b\bar b$, $L\le 1.7\,\mathrm{kpc}$), values 2-3 sigma below the $L=5^{+3}_{-2}\,\mathrm{kpc}$ preferred by AMS-02 beryllium data (Weinrich et al. 2020) and in tension with radio and diffuse gamma-ray analyses; (iii) under the pulsar-inclusive (optimistic) background assumption, positron data exclude channels with an $e^+e^-$ component, while under the conservative secondaries-only assumption they provide no further constraint; and (iv) the sole surviving candidate annihilates purely into $\mu^+\mu^-$ with $m_\chi\simeq 60$ GeV and roughly thermal cross section, pure leptonic channels being immune to the antiproton limits regardless of $L$ and of the assumed dark-matter density profile. The authors themselves enumerate the systematics on which this verdict rests: the halo height $L$; the pulsar (pulsar-wind-nebula) contribution to the positron background, including pulsar catalog incompleteness and the uncertain efficiency for converting spin-down power into electron-positron pairs; the interstellar emission model (IEM) and ISRF in the inner Galaxy, whose renormalization shifted the GCE spectrum and the ICS-versus-prompt split; antiproton production cross sections and AMS-02 systematic covariances; dSph $J$-factors; and the neglect of diffusion inside the region of interest in the ICS calculation, a controlled approximation affecting the result at the $\lesssim 25\%$ level. These six items are not six separate questions: each is a term in the error budget of one joint gamma-ray/antiproton/positron likelihood test, whose present output is the surviving $\mu^+\mu^-$ candidate.

Problem Statement

Complete the multimessenger test of the dark-matter interpretation of the Fermi-LAT Galactic center excess. Specifically: determine whether any dark-matter candidate that reproduces the measured GCE gamma-ray spectrum and morphology -- as characterized by Fermi-LAT analyses under the interstellar-emission-model and interstellar-radiation-field treatment of item (3) below, and in particular the pure $\mu^+\mu^-$ channel with $m_\chi\simeq 60$ GeV and approximately thermal $\langle\sigma v\rangle$ identified by Di Mauro and Winkler (2021) as the sole survivor of their analysis, or any model realizing it -- remains simultaneously compatible with (i) Fermi-LAT dwarf-spheroidal limits, (ii) AMS-02 antiproton data, and (iii) AMS-02 positron data, once each systematic entering the joint test is determined or marginalized rather than assumed, with any marginalization prior consistent with the data that constrain the systematic in question: (1) the vertical diffusion-halo height $L$, constrained consistently with radioactive cosmic-ray clocks, diffuse gamma-ray and radio synchrotron data, or marginalized over a prior consistent with those data; (2) the Galactic pulsar / pulsar-wind-nebula contribution to the positron background, including catalog incompleteness and the pair-production efficiency; (3) the interstellar emission model and interstellar radiation field in the inner Galaxy, including their effect on the GCE spectral-energy-distribution normalization and on the relative inverse-Compton versus prompt emission for leptonic channels; (4) antiproton production cross sections and the correlated systematic uncertainties of AMS-02; (5) dwarf-spheroidal $J$-factors from stellar kinematics; and (6) the effect on the predicted inverse-Compton emission of the neglect of cosmic-ray diffusion inside the region of interest -- a controlled approximation at the $\lesssim 25\%$ level in the source analysis -- and of any spatial variation of diffusion in the Galactic-center region. The dispute over whether the GCE morphology is better described by a dark-matter or by a stellar-bulge template (see Background) is an input uncertainty to this characterization, not a substitute for the joint test: the requested verdict must follow from compatibility with the gamma-ray, antiproton, positron and dwarf-spheroidal bands, not from morphology discrimination alone. A complete answer is a verdict on the single joint test: either a specific dark-matter candidate or model class that fits the GCE and passes all bands under systematics treated as above, at stated confidence, or an exclusion of the dark-matter interpretation of the GCE at stated confidence under the same treatment of systematics. The two outcomes are alternative resolutions of one question, not separate deliverables.

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