Do the LHAASO Galactic diffuse emission data require a contribution from unresolved sources?

¹ Norwegian University of Science and Technology, Department of Physics, NO-7491 Trondheim, Norway
² INAF Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50125, Firenze, Italy
³ Astrofísica de Andalucía, CSIC, 18080 Granada, Spain
⁴ INFN, Laboratori Nazionali del Gran Sasso, 67100 Assergi (AQ), Italy
⁵ University of L’Aquila, Physics and Chemistry Department, 67100 L’Aquila, Italy

^* e-mail: vittoria.vecchiotti@ntnu.no

Published online: 6 March 2025

Abstract

The Large High-Altitude Air Shower Observatory (LHAASO) collaboration has recently measured the ultra-high energy gamma-ray diffuse emission (10 − 10³ TeV) after masking the contribution of known sources. The observed signal appears to be 2 − 3 times higher than expected from the hadronic interactions of diffuse cosmic rays with the interstellar medium, potentially suggesting a contribution from unresolved sources. However, estimates of the diffuse emission are affected by large uncertainties. In this work, we calculate the hadronic gamma-ray diffuse emission, accounting for uncertainties in the gas content of the Galactic disk, the energy and spatial distribution of cosmic rays, and the hadronic interaction cross-section. We show that the LHAASO data above ∼ 30 TeV are consistent with this model, not requiring (nor probing) the existence of any further contribution due to unresolved sources or cosmic ray spectral variations in the inner Galaxy.