Galactic center at very high-energies

Employing data collected during the first 25 months’ observations by the Fermi-LAT, we describe and subsequently seek to model the very high energy (>300 MeV) emission from the central few parsecs of our Galaxy. We analyze the morphological, spectral and temporal characteristics of the central source, 1FGL J1745.6-2900. Remarkably, the data show a clear, statistically significant signal at energies above 10 GeV, where the Fermi-LAT has an excellent angular resolution comparable to the angular resolution of HESS at TeV energies, which makes meaningful the joint analysis of the Fermi and HESS data. Our analysis does not show statistically significant variability of 1FGL J1745.6-2900. Using the combination of Fermi data on 1FGL J1745.6-2900 and HESS data on the coincident, TeV source HESS J1745-290, we show that the spectrum of the central gamma-ray source is inflected with a relatively steep spectral region matching between the flatter spectrum found at both low and high energies. We seek to model the gamma-ray production in the inner 10 pc of the Galaxy and examine, in particular, cosmic ray (CR) proton propagation scenarios that reproduce the observed spectrum of the central source. We show that a model that instantiates a transition from diffusive propagation of the CR protons at low energy to almost rectilinear propagation at high energies (given a reasonable energy-dependence of the assumed diffusion coefficient) can well explain the spectral phenomenology. In general, however, we find considerable degeneracy between different parameter choices which will only be broken with the addition of morphological information that gamma-ray telescopes cannot deliver given current angular resolution limits.We argue that a future analysis done in combination with higher-resolution radio continuum data holds out the promise of breaking this degeneracy.

💡 Research Summary

The authors present a comprehensive study of the very‑high‑energy (VHE) gamma‑ray emission from the innermost few parsecs of the Milky Way, using 25 months of data collected by the Fermi Large Area Telescope (LAT). The focus is on the point source 1FGL J1745.6‑2900, which lies within a few arcseconds of the dynamical centre (Sgr A*). By exploiting the improved point‑spread function of the LAT above 10 GeV, the authors achieve an angular resolution comparable to that of the ground‑based HESS array at TeV energies, allowing a seamless joint analysis of the GeV and TeV regimes.

Temporal analysis shows no statistically significant variability on monthly or yearly timescales, indicating that the source is a steady emitter over the examined period. Spectrally, the source exhibits a pronounced “inflection” or “bump”: the spectrum is relatively flat (photon index ≈ 2.2) from 300 MeV up to a few GeV, steepens dramatically (index ≈ 3.5) between ~3 GeV and ~10 GeV, and then flattens again (index ≈ 2.1) in the TeV band as measured by HESS. This non‑monotonic behavior cannot be reproduced by a single power‑law electron inverse‑Compton model or by a simple pion‑decay scenario with a static particle distribution.

To explain the observed shape, the authors develop a cosmic‑ray (CR) proton propagation model that incorporates an energy‑dependent transition in the transport regime. At low energies (≲ 10 GeV) protons diffuse through the dense molecular environment surrounding the Galactic centre, with a diffusion coefficient D(E)=D₀(E/E₀)^δ (δ≈0.3–0.6, D₀≈10^28 cm² s⁻¹ at 1 GeV). In this regime, the protons spend sufficient time interacting with gas (n≈10³–10⁴ cm⁻³), producing gamma‑rays via neutral‑pion decay and yielding the observed soft GeV spectrum. At higher energies (≳ 10 TeV) the diffusion time becomes shorter than the radiative loss time, and the particles propagate almost rectilinearly out of the central region. Consequently, the interaction probability drops, but the remaining interactions with lower‑density gas produce a harder TeV spectrum that matches the HESS measurements.

The model successfully reproduces the overall spectral shape when reasonable values for the acceleration efficiency, the total CR proton energy, and the gas density distribution are adopted. However, the authors emphasize a substantial degeneracy among model parameters: varying the diffusion coefficient, the transition energy, or the gas density profile can lead to similar spectral outcomes. Because the LAT and HESS angular resolutions are limited to tens of parsecs, the current data lack the morphological discrimination needed to break these degeneracies.

The paper therefore argues for a multi‑wavelength approach. High‑resolution radio continuum and molecular line observations (e.g., from VLA, ALMA, or the upcoming ngVLA) can map the distribution of dense gas and magnetic fields on sub‑parsec scales. By correlating these maps with the gamma‑ray emission, one can directly constrain the spatial distribution of CR interactions, thereby fixing the diffusion coefficient and transition energy more robustly. The authors conclude that such combined analyses will be essential to pinpoint the acceleration site(s) – whether Sgr A* itself, nearby supernova remnants, or stellar wind shocks – and to fully understand the physics of particle acceleration and transport in the extreme environment of the Galactic centre.