- The paper reveals that the GeV excess persists across various cosmic-ray propagation models, challenging a straightforward dark matter interpretation.
- It employs advanced GALPROP simulations and spherical harmonics to isolate gamma-ray contributions, accounting for pulsars and interstellar gas uncertainties.
- The study sets competitive dark matter annihilation cross-section limits, underscoring the need for multi-wavelength and improved CR data analyses.
The Fermi Galactic Center GeV Excess and Implications for Dark Matter
The paper presents an extensive analysis of the gamma-ray excess observed in the vicinity of the Galactic Center (GC), detected using data from the Fermi Large Area Telescope (LAT). The authors scrutinize this excess emission, which peaks at energies of a few GeV, in the context of multiple hypotheses, including potential signals from dark matter (DM) annihilation. Their work meticulously evaluates the uncertainties associated with cosmic-ray (CR) propagation models, potential contributions from local sources such as pulsars, and possible miscalculations in interstellar gas distribution.
Analysis Methodology
The core of the analysis utilizes a comprehensive modeling approach where different components of gamma-ray sources are simulated and matched against the observed data. The researchers use variations of GALPROP models, which numerically solve CR transport equations to predict gamma-ray emissions from CR interactions with interstellar gas or radiative fields. Key parameters include different distributions of CR sources and the height of the CR confinement zone used to adjust the model's predictions of interstellar emissions.
In addition, spherical harmonics are employed to model and remove large-scale structured residuals, which helps isolate the excess emissions associated with GC or other localized phenomena, such as the Fermi bubbles—two large gamma-ray lobes extending from the GC.
Findings on the GeV Excess
The paper's results suggest that the GeV excess persists across a variety of models and analysis configurations, though its exact amplitude varies significantly. One possibility is that this excess arises from annihilations of weakly interacting massive particles (WIMPs), a popular DM candidate. Yet, the analysis introduces several alternative astrophysical interpretations, including emissions from a subset of yet-undefined point sources or a population of millisecond pulsars.
Intriguingly, the Fermi bubbles—which are massive structures extending over 10 kpc from the GC—could mask or revise some interpretations of the excess due to their contribution to the overall emissions in the region.
Implications for Dark Matter Searches
The authors perform DM modeling by fitting spectra of possible DM annihilation channels to the observed excess. Noting the challenges posed by substantial systematic uncertainties in modeling interstellar emissions, the analysis does not support a robust claim of detecting DM from the GC. Instead, they derive stringent upper limits on the annihilation cross-section for various parameterizations of DM profiles, such as the generalized Navarro-Frenk-White (gNFW) profile with varying inner slope indices.
Impressively, these limits are competitive with those obtained from other astrophysical searches, including dwarf spheroidal galaxies, and in some scenarios extend constraints to higher DM masses or fainter signatures.
Future Directions and Challenges
This work highlights the importance of accounting for local astrophysical phenomena in interpreting potential DM signals. The persistence of the GC excess across model variations suggests a need for a deeper understanding of astrophysical emissions at the GC. Continued accumulation of Fermi-LAT data, improved models of CR physics around the GC, and parallel multi-wavelength studies could enhance the interpretability of such data.
Potential developments in the analytical techniques could refine DM searches by providing better subtraction of the foreground and background. Cross-collaboration with radio and X-ray data to identify point sources more definitively could further isolate potential WIMP signatures. These insights underline the necessity of a holistic approach that balances astrophysical phenomena against potential new physics in the search for dark matter.