HEGS : Revisiting a decade of H.E.S.S. extragalactic observations

TL;DR

HEGS revisits a comprehensive, uniform re-analysis of H.E.S.S. extragalactic observations (2004–2012) to produce a 23-source catalog and high-level data products. By applying a consistent likelihood-based framework, EBL-corrected spectra, and robust background treatment, the work characterizes source properties, variability, and biases relative to Fermi-LAT, and assesses the aggregate blazar contribution to the Extragalactic Gamma-Ray Background. The study finds a predominance of BL Lacs, reveals spectral softening between HE and VHE ranges, and highlights a potential overestimation of HBL contributions to the EGB at the highest energies, while providing rich data products for the community. The released data and maps enable cross-mission comparisons, improved modeling of the EGB, and future constraints with upcoming facilities like H.E.S.S. phase II and CTAO.

Abstract

During its first phase, from 2004 up to the end of 2012, the H.E.S.S. (High Energy Stereoscopic System) experiment observed the extragalactic skies for more than 2700 hours. These data have been re-analysed in a single consistent framework, leading to the derivation of a catalog of 23 sources. In total, about 5.7% of the sky was observed, allowing for several additional studies to be conducted: source variability, extragalactic gamma-ray background light, and comparison with the Fermi-LAT catalogues. In this contribution, we discuss these results and present the high-level data (catalogs, maps) released to the astrophysical community.

Figures (4)

• Figure 1: Hammer-Aitoff sky map showing HEGS clusters (dark green). The grey band near the Galactic plane ($|b| < 10^{\circ}$) and other grey regions are excluded from the analysis. Blue areas indicate sky regions unobservable by H.E.S.S. due to zenith angles above $60^{\circ}$. This figure is taken from hegs_paper
• Figure 2: Spectra of detected HEGS sources. Red line: best-fit model; orange bands: 1$\sigma$ likelihood contours. No EBL correction is applied. The blue points indicate the flux points and the arrows indicate the 95% CL upper limits computed for energy bins with a significance below $2\sigma$. This figure is taken from hegs_paper
• Figure 3: EGB measured by Fermi-LAT (blue points). Red, grey, and green lines show contributions from BL Lac (with and without spectral break $\Delta \Gamma = 0.4$) and FSRQ, respectively. Shaded areas indicate uncertainties from luminosity function normalization. The dashed red line corresponds to the HBL luminosity function from 2014ApJ...780...73A. This figure is taken from hegs_paper
• Figure 4: Example maps available from the catalogue FITS files: significance, average live time, and average energy threshold (top to bottom) for two observation clusters (left/right). This figure is taken from hegs_paper.