TeV Emission of Galactic Plane Sources with HAWC and H.E.S.S.

Abstract

The High Altitude Water Cherenkov (HAWC) observatory and the High Energy Stereoscopic System (H.E.S.S.) are two leading instruments in the ground-based very-high-energy γ-ray domain. HAWC employs the water Cherenkov detection (WCD) technique, while H.E.S.S. is an array of Imaging Atmospheric Cherenkov Telescopes (IACTs). The two facilities therefore differ in multiple aspects, including their observation strategy, the size of their field of view, and their angular resolution, leading to different analysis approaches. Until now, it has been unclear if the results of observations by both types of instruments are consistent: several of the recently discovered HAWC sources have been followed up by IACTs, resulting in a confirmed detection only in a minority of cases. With this paper, we go further and try to resolve the tensions between previous results by performing a new analysis of the H.E.S.S. Galactic plane survey data, applying an analysis technique comparable between H.E.S.S. and HAWC. Events above 1 TeV are selected for both data sets, the point-spread function of H.E.S.S. is broadened to approach that of HAWC, and a similar background estimation method is used. This is the first detailed comparison of the Galactic plane observed by both instruments. H.E.S.S. can confirm the γ-ray emission of four HAWC sources among seven previously undetected by IACTs, while the three others have measured fluxes below the sensitivity of the H.E.S.S. data set. Remaining differences in the overall γ-ray flux can be explained by the systematic uncertainties. Therefore, we confirm a consistent view of the γ-ray sky between WCD and IACT techniques.

HAWC Study of the Ultra-High-Energy Spectrum of MGRO J1908+06

“…HAWC measures a higher flux than IACT experiments. Differences in the field-of-view, angular resolution, and background estimation methods between IACTs and HAWC contribute to discrepancies in the measured flux (Abdalla et al 2021). HAWC uses a background estimation method called "direct integration" (Atkins et al 2003).…”

The gamma-ray emission from 3HWC J1928+178

“…It corresponds to events triggering more than 25% of the array, which gives an energy threshold of approximately 1 TeV. For this specific bin selection, the sources 3HWC J1928+178 is refered to as HAWC J1928+178 in [2]. The corresponding PSF of the instrument is ∼0.4°for bin 4 and decreases to less than 0.2°for bin 9.…”

“…This choice is motivated by the fact that 3HWC J1930+188 was detected as a point-like source by HAWC, H.E.S.S. [4] and VERITAS [5], and by previous studies of 3HWC J1928+178 that showed that it is likely extended [2] [10]. A simple power law dN/dE = F 0 (E/E 0 ) Γ is assumed as energy spectrum for all components.…”

“…The spectral energy distribution is plotted in Figure 3 for the component representing the source 3HWC J1928+178 for the two models. It is compared with the spectrum from the HAWC automatic source search for the same set of data [2]. The flux point from LHAASO [14] seems to be more compatible with the 4 components model, or imply a cut-off in the power low spectrum from the diffusion model.…”

“…No Imaging Atmospheric Cherenkov Telescope (IACT) has detected it before the confirmation by the H.E.S.S. collaboration [2], using an analysis method adapted to extended sources. The origin of the observed very high energy gamma-ray emission is still unclear.…”