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arXiv · 2610.32845

The Galactic Center Gamma-Ray Excess and the Look-Elsewhere Effect

Isabel S. Sands·Philip F. Hopkins
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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.

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Isabel S. Sands, Philip F. Hopkins. 2026-10-23. The Galactic Center Gamma-Ray Excess and the Look-Elsewhere Effect. https://arxiv.org/abs/2610.32845

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