The Galactic Center Gamma-Ray Excess and the Look-Elsewhere Effect
Abstract
The origin of the excess of GeV gamma-rays in the Milky Way's galactic center observed by Fermi-LAT has been an open question for nearly two decades. The two leading interpretations of this signal are that it is produced either by annihilating dark matter (DM), or arises from unresolved point sources, such as a novel population of millisecond pulsars. In both scenarios, uncertainties and limitations in the modeling of galactic diffuse gamma-ray emission, such as the steady-state approximation of cosmic ray (CR) propagation and assumption of an axisymmetric distribution of CRs, complicate the interpretation of the galactic center excess (GCE). Simulations with full magnetohydrodynamics (MHD) and CR transport can provide a new avenue for understanding the astrophysical phenomena that drive galactic diffuse gamma-ray emission. We utilize MHD simulations of Milky Way analogs with time-dependent, spectrally-resolved CR transport to reconstruct diffuse gamma-ray emission. Comparing instantaneous emission to a time-averaged baseline, we find residuals up to several times larger than the Fermi GCE, with a wide range of spectral shapes and spatial morphologies. At some simulation snapshots, the spectrum and morphology of the residuals resemble the Fermi GCE. Our results suggest that the Fermi GCE may be a manifestation of the look-elsewhere effect- a transient fluctuation in diffuse emission from stellar sources, observed at an inopportune moment- and motivate the development of new Fermi-LAT analysis templates that account for time-dependent CR transport.
Related
- Dark Matter Interpretations of the Galactic Center Gamma-Ray Excess with 18 Years of Fermi-LAT Data and the FL16Y Source Catalog
- Inelastic Dark Photon Dark Matter for the LUX-ZEPLIN High-Recoil Event and the Galactic Halo Gamma-Ray Excess
- Comment on "Fermi-LAT Galactic Center Excess Morphology of Dark Matter in Simulations of the Milky Way Galaxy"
- Foreground dependence of the 20 GeV Galactic halo gamma-ray excess and its proposed connection to the LUX-ZEPLIN event
- A Simple Dark Matter Model to Explain the LZ Event and Galactic Center Excess
BibTeX
@misc{sands2026galactic,
title = {{The Galactic Center Gamma-Ray Excess and the Look-Elsewhere Effect}},
author = {Isabel S. Sands and Philip F. Hopkins},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.32845},
eprint = {2610.32845},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.32845},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - The Galactic Center Gamma-Ray Excess and the Look-Elsewhere Effect
AU - Isabel S. Sands
AU - Philip F. Hopkins
PY - 2026
DA - 2026/10/23
PB - arXiv
DO - 10.48550/arxiv.2610.32845
UR - https://arxiv.org/abs/2610.32845
AB - The origin of the excess of GeV gamma-rays in the Milky Way's galactic center observed by Fermi-LAT has been an open question for nearly two decades. The two leading interpretations of this signal are that it is produced either by annihilating dark matter (DM), or arises from unresolved point sources, such as a novel population of millisecond pulsars. In both scenarios, uncertainties and limitations in the modeling of galactic diffuse gamma-ray emission, such as the steady-state approximation of cosmic ray (CR) propagation and assumption of an axisymmetric distribution of CRs, complicate the interpretation of the galactic center excess (GCE). Simulations with full magnetohydrodynamics (MHD) and CR transport can provide a new avenue for understanding the astrophysical phenomena that drive galactic diffuse gamma-ray emission. We utilize MHD simulations of Milky Way analogs with time-dependent, spectrally-resolved CR transport to reconstruct diffuse gamma-ray emission. Comparing instantaneous emission to a time-averaged baseline, we find residuals up to several times larger than the Fermi GCE, with a wide range of spectral shapes and spatial morphologies. At some simulation snapshots, the spectrum and morphology of the residuals resemble the Fermi GCE. Our results suggest that the Fermi GCE may be a manifestation of the look-elsewhere effect- a transient fluctuation in diffuse emission from stellar sources, observed at an inopportune moment- and motivate the development of new Fermi-LAT analysis templates that account for time-dependent CR transport.
ER -