20 GeV halo-like excess of the Galactic diffuse emission and implications for dark matter annihilation

TL;DR

Analysis of 15 years of Fermi LAT data reveals a significant 20 GeV gamma-ray excess in the Milky Way halo, consistent with dark matter annihilation, suggesting a potential indirect detection signal with implications for dark matter properties.

Contribution

First detection of a halo-like gamma-ray excess at 20 GeV with spectral and spatial characteristics compatible with dark matter annihilation, using extensive Fermi LAT data and detailed modeling.

Findings

A statistically significant 20 GeV gamma-ray excess in the Galactic halo.

The excess fits a dark matter annihilation model with particle mass 0.5-0.8 TeV.

The cross section inferred exceeds canonical thermal relic values but remains plausible considering uncertainties.

Abstract

Fifteen years of the Fermi Large Area Telescope (LAT) data in the halo region of the Milky Way (MW) are analyzed to search for gamma rays from dark matter annihilation. Gamma-ray maps within the region of interest ($|l| \le 60^\circ$, $10^\circ \le |b| \le 60^\circ$) are modeled using point sources, the GALPROP models of cosmic-ray interactions, isotropic background, and templates of Loop I and the Fermi bubbles, and then the presence of a halo-like component is further examined. A statistically significant halo-like excess is found with a spectral peak around 20 GeV, while its flux is consistent with zero below 2 GeV and above 200 GeV. Examination of the fit residual maps indicates that a spherically symmetric halo component fits the map data well. The radial profile agrees with annihilation by the smooth NFW density profile, and may be slightly shallower than this, especially in the central region. Various systematic uncertainties are investigated, but the 20 GeV peak remains significant. In particular, the halo excess with a similar spectrum is detected even relative to the LAT standard background model, which contains non-template patches adjusted to match the observed map. The halo excess spectrum can be fitted by annihilation with a particle mass $m_\chi \sim$ 0.5-0.8 TeV and cross section $\langle \sigma \upsilon \rangle \sim$ (5-8)$\times 10^{-25} \ \rm cm^3 \, s^{-1}$ for the $b\bar{b}$ channel. This cross section is larger than the upper limits from dwarf galaxies and the canonical thermal relic value, but considering various uncertainties, especially the density profile of the MW halo, the dark matter interpretation of the 20 GeV ``Fermi halo'' remains feasible. The prospects for verification through future observations are briefly discussed.