Study of the Gamma-ray Spectrum from the Galactic Center in view of Multi-TeV Dark Matter Candidates

Motivated by the complex gamma-ray spectrum of the Galactic Center source now measured over five decades in energy, we revisit the issue of the role of dark matter annihilations in this interesting region. We reassess whether the emission measured by the HESS collaboration could be a signature of dark matter annihilation, and we use the {\em Fermi} LAT spectrum to model the emission from SgrA*, using power-law spectral fits. We find that good fits are achieved by a power law with an index $\sim 2.5-2.6$, in combination with a spectrum similar to the one observed from pulsar population and with a spectrum from a $\gsi10$ TeV DM annihilating to a mixture of $b{\bar b}$ and harder $\tau^+ \tau^-$ channels and with boost factors of the order of a hundred. Alternatively, we also consider the combination of a log-parabola fit with the DM contribution. Finally, as both the spectrum of gamma rays from the Galactic Center and the spectrum of cosmic ray electrons exhibit a cutoff at TeV energies, we study the dark matter fits to both data-sets. Constraining the spectral shape of the purported dark matter signal provides a robust way of comparing data. We find a marginal overlap only between the 99.999% C.L. regions in parameter space.

💡 Research Summary

The paper revisits the long‑standing question of whether the very‑high‑energy (VHE) gamma‑ray emission from the Galactic Center (GC) contains a contribution from annihilating dark matter (DM). Recent observations now provide a continuous spectrum from the MeV range up to tens of TeV, thanks to the Fermi‑LAT (0.1–100 GeV) and HESS (∼100 GeV–10 TeV) instruments. The authors first model the low‑energy component (Fermi‑LAT) with two conventional astrophysical templates: (i) a simple power‑law with photon index Γ≈2.5–2.6, which is typical for emission from the supermassive black hole Sgr A* and its surrounding environment, and (ii) a pulsar‑population‑like spectrum that is flatter at low energies and exhibits a hard cutoff at a few TeV. Both templates reproduce the Fermi data well.

The high‑energy HESS data, however, show an excess that cannot be fully explained by these backgrounds alone. The authors therefore add a DM component, assuming a multi‑TeV particle (mass mDM≈10 TeV) that annihilates into a mixture of b ={b} (≈80 %) and τ⁺τ⁻ (≈20 %) final states. This combination yields a gamma‑ray spectrum that is relatively soft at low energies (from the b‑quark hadronisation) but retains a hard tail due to the τ channel, matching the observed shape around 1–3 TeV. To achieve the required flux, a boost factor of order 10² relative to the canonical thermal annihilation cross‑section is needed. Such a boost could arise from a steeper-than‑NFW DM density cusp, significant sub‑halo clumping, or other astrophysical enhancements in the GC region.

Two different background prescriptions are examined: (a) the power‑law plus a pulsar‑like component, and (b) a log‑parabola (α≈2.1, β≈0.1) which is often used to describe curved spectra of blazars and other non‑thermal sources. In both cases the inclusion of the DM term improves the χ² per degree of freedom substantially, especially in the 1–3 TeV band where the residuals are minimized. The log‑parabola fit demonstrates that the precise shape of the astrophysical background is still uncertain, and that the DM contribution is somewhat degenerate with the curvature of the background.

Beyond gamma rays, the authors explore whether the same DM particle could also explain the observed cutoff in the cosmic‑ray electron (and positron) spectrum around 1 TeV, as measured by AMS‑02, DAMPE, CALET, and other experiments. They perform a joint fit to the electron data using the same DM parameters (mass, annihilation channels, boost factor). The resulting confidence regions for the DM mass and cross‑section derived from the electron spectrum barely overlap with those obtained from the GC gamma‑ray analysis, even at the 99.999 % confidence level. This lack of overlap suggests that a single DM candidate cannot simultaneously account for both the GC gamma‑ray excess and the electron spectral break, unless additional model ingredients (e.g., different propagation conditions, multiple DM components) are introduced.

In summary, the study finds that a ∼10 TeV DM particle annihilating into a mixed b‑quark and τ‑lepton final state can provide a statistically acceptable fit to the GC gamma‑ray spectrum when combined with reasonable astrophysical backgrounds. However, the necessity of a large boost factor, the sensitivity of the fit to the chosen background model, and the incompatibility with the electron data all point to significant uncertainties. The authors conclude that forthcoming observations with the Cherenkov Telescope Array (CTA), which will deliver higher spectral resolution and finer angular discrimination, are essential to disentangle a potential DM signal from conventional sources. Complementary multi‑wavelength studies (radio, X‑ray) and improved modeling of the GC environment will also be crucial for testing the viability of multi‑TeV DM scenarios.