Exploring the Galactic Plane: A Comparative Study of Fermi-LAT Sources and HESS's Non-Detection at TeV Energies

TL;DR

The paper tests whether high-energy LAT spectra, extrapolated to TeV energies, remain compatible with H.E.S.S. Galactic Plane Survey flux upper limits. It combines HGPS UL maps with 4FGL-DR4 LAT spectra, defines robust regions around the Galactic plane, and compares ULs at 1 TeV and 10 TeV to LAT fluxes, identifying 13 constrained sources. The results show that several LAT-detected sources have hard spectra that may require spectral breaks or cutoff features to reconcile with H.E.S.S. non-detections, with four notable cases tied to supernova remnants. These findings constrain particle acceleration scenarios in the Galactic plane and motivate future observations with the Cherenkov Telescope Array Observatory (CTAO) to resolve spectral and morphological details.

Abstract

The HESS Galactic Plane Survey (HGPS), published in 2018, presented a decade of very-high-energy (VHE) gamma-ray observations along the Galactic plane. This study was accompanied by the release of several maps in FITS format, offering a detailed view of the region. The flux upper-limits from these HGPS maps can be compared to the high-energy (HE) spectra of sources catalogued by the Fermi-LAT in the same region. For some sources, extrapolating the Fermi-LAT flux into the VHE range predicts flux values exceeding the upper-limits set by HESS. In this work, we present the results of this comparison and highlight the sources that are of particular interest for future VHE observations.

Figures (5)

• Figure 1: The map shows the region around the Galactic Center. The black regions corresponds to regions where no source is detected in the HGPS, where the HESS significance is different from 0 and where the HGPS flux UL value is different from 0. The green points indicate the 4FGL sources that are selected for the HGPS-4FGL spectral comparison, while the red one are those excluded for this comparison.
• Figure 2: SED of Fermi-LAT sources identified as constrained by the HGPS data. The green butterfly displays the best-fit Power-Law spectrum from the Fermi-LAT between $50$ MeV and $1$ TeV and the grey one shows an extrapolation of this measurement up to $10$ TeV. The red triangles show the H.E.S.S. flux ULs at 95% CL at 1 and 10 TeV, scaled by a factor $1.25$. The blue points are the Fermi-LAT spectral points derived in pre-defined energy bins. A point is displayed on the figure only if the Sqrt_TS_band parameter in the 4FGL-DR4 catalog is above 2. The value of this parameter -- which is indicative of the significance of the point -- is indicated next to the point.
• Figure 3: Continuation of Fig. \ref{['fig_specs1']}.
• Figure 4: Radio map from the Molonglo Observatory Synthesis Telescope most at 843 MHz towards the supernova remnant G354.8-00.8. The cross indicates the position of the Fermi-LAT source 4FGL J1735.9-3342 and the ellipse represents the position uncertainty at 95% Confidence Level. The dashed line indicates the boundary of the SNR as listed in greencat.
• Figure 5: Radio map of the SNR G16.2-02.7 obtained by MeerKAT at 1335.3 MHz meerkat. The crosses and ellipses shows the best-fit position and the 95% CL position uncertainty of the Fermi-LAT sources associated with this SNR.