Galactic diffuse gamma rays meet the PeV frontier

¹ Stockholm University and The Oskar Klein Centre for Cosmoparticle Physics, Alba Nova, 10691 Stockholm, Sweden
e-mail: pedro.delatorreluque@fysik.su.se
² INFN Sezione di Pisa, Polo Fibonacci, Largo B. Pontecorvo 3, 56127 Pisa, Italy
³ Dipartimento di Fisica, Università di Pisa, Polo Fibonacci, Largo B. Pontecorvo 3, Pisa, Italy
⁴ Instituto de Física Corpuscular, Universidad de Valencia and CSIC, Edificio Institutos de Investigacíon, Calle Catedrático José Beltrán 2, 46980 Paterna, Spain
⁵ Gran Sasso Science Institute (GSSI), Viale Francesco Crispi 7, 67100 L’Aquila, Italy
⁶ INFN – Laboratori Nazionali del Gran Sasso (LNGS), Via G. Acitelli 22, 67100 Assergi, AQ, Italy
⁷ Institut für Physik und Astronomie, Universität Potsdam, Karl-Liebknecht-Strasse 24/25, 14476 Potsdam, Germany

Received: 5 April 2022
Accepted: 1 January 2023

Abstract

Context. The Tibet ASγ and LHAASO collaborations recently reported the observation of a γ-ray diffuse emission with energy up to the PeV level from the Galactic plane.

Aims. We discuss the relevance of non-uniform cosmic-ray transport scenarios and the implications of these results for cosmic-ray physics.

Methods. We used the DRAGON and HERMES codes to build high-resolution maps and spectral distributions of that emission for several representative models under the condition that they reproduce a wide set of local cosmic-ray data up to 100 PeV.

Results. We show that the energy spectra measured by Tibet ASγ, LHAASO, ARGO-YBJ, and Fermi-LAT in several regions of interest in the sky can all be reasonably described in terms of the emission arising by the Galactic cosmic-ray “sea”. We also show that all our models are compatible with IceTop γ-ray upper limits.

Conclusions. We compare the predictions of conventional and space-dependent transport models with those data sets. Although the Fermi-LAT, ARGO-YBJ, and LHAASO preliminary data slightly favor this scenario, due to the still large experimental errors, the poorly known source spectral shape at the highest energies, the potential role of spatial fluctuations in the leptonic component, and a possible larger-than-expected contamination due to unresolved sources, a solid confirmation requires further investigations. We discuss which measurements will be most relevant in order to resolve the remaining degeneracy.