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

TL;DR

Fifteen years of Fermi-LAT data reveal a halo-like gamma-ray excess in the Milky Way halo with a spectral peak near $\sim$20 GeV, spatially consistent with a spherically symmetric DM halo. The analysis employs a comprehensive template fit including point sources, GALPROP diffuse emission, Loop I, and Fermi bubbles, with a separately constructed halo template based on NFW morphologies; the excess persists across systematic checks and GIEM-based zero-point tests. WIMP interpretations favor $m_χ$ in the $0.4-0.8$ TeV range with $\langle \sigma v \rangle \sim (5-9)\times10^{-25}$ cm$^3$ s$^{-1}$ for $b\bar{b}$ and $W^+W^-$ channels under an $NFW$-ρ$^2$ model, though these values exceed some dwarf-galaxy limits and rely on MW-halo density uncertainties. The study discusses the compatibility with the GC GeV excess, compares to prior constraints, and highlights the need for corroboration via other DM searches, line emission, and next-generation instruments to substantiate the DM interpretation.

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 $m_χ\sim$ 0.5-0.8 TeV and cross section $\langle σ\upsilon \rangle \sim$ (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.

Figures (16)

• Figure 1: The image of the Fermi bubbles (the flat bubble template plus fit residuals, after Gaussian smoothing with $\sigma = 1^\circ$) in the two photon energy bins (1.5 and 4.3 GeV). The boundaries of the final flat template are indicated by gray lines. The gray circular regions are excluded from the analysis due to the catalogued extended sources. The large regions at $(l, b) \sim (-50^\circ, 20^\circ)$ and $(-60^\circ, -45^\circ)$ are Cen A lobes and the SMC, respectively.
• Figure 2: The best-fit spectra of model map templates, for the fit including the flat template of the Fermi bubbles (FB) and the GC GeV excess (GCE). The error bars are statistical 1$\sigma$ estimated by MCMC. The fit is performed including the Galactic disk region ($|b| < 10^\circ$), and the mean background flux within the ROI, including the disk, is shown. To prevent symbols and error bars from overlapping, some symbols are slightly shifted to the left or right of the true energy bin center.
• Figure 3: The same as figure \ref{['fig:specall_FB_b0']}, but showing the flux excluding the Galactic disk region ($|b| < 10^\circ$), while the fitting is the same as figure \ref{['fig:specall_FB_b0']} (including the disk).
• Figure 4: The best-fit spectra of the model map templates. Compared to figure \ref{['fig:specall_FB']}, the Galactic disk region ($|b| < 10^\circ$) is removed in the fit, the flat template of the Fermi bubbles is replaced by the two (positive and negative) residual maps, and the GC excess is not included. The sign of the negative template is inverted so that the flux becomes positive in this plot.
• Figure 5: The same as figure \ref{['fig:specall_noDM']}, but including the GC GeV excess (GCE) template (NFW-$\rho^{2.5}$).