On the Temporal Variability of the Galactic Multi-TeV Interstellar Emissions

TL;DR

This study models Galactic cosmic ray distributions and diffuse gamma-ray emissions considering discrete, finite lifetime sources, revealing significant variability and the importance of source discreteness in interpreting high-energy gamma-ray data.

Contribution

It introduces a time-dependent, discrete-source modeling approach for Galactic cosmic rays and gamma-ray emissions, highlighting the impact of source variability on observed diffuse emissions.

Findings

Models explain LHAASO gamma-ray data with up to 50% unresolved leptonic sources.

Diffuse gamma-ray emissions vary by about 25% due to source discreteness.

Source variability is crucial for accurate modeling of Galactic high-energy emissions.

Abstract

We use the GALPROP cosmic ray (CR) framework to model the Galactic CR distributions and associated non-thermal diffuse emissions up to PeV energies. We consider ensembles of discrete, finite lifetime CR sources, e.g.\ supernova remnants (SNRs), for a range of creation rates and lifetimes. We find that global properties of the CR sources are likely not directly recoverable from the current `snapshot' of the historic injection and propagation of CRs within the Galaxy that are provided by the data. We show that models for the diffuse $\gamma$ rays based on the discrete/time-dependent scenarios we consider are able to explain LHAASO very-/ultra-high energy (VHE/UHE) $\gamma$-ray data with up to 50\% contribution by unresolved leptonic sources at the highest energies. Over the models that we consider, variations in the diffuse VHE emissions can be $\sim$25\%, which is comparable to those for steady-state models that we investigated in earlier work. Such variations due to the discrete/finite nature of the CR sources are an important factor that are necessary to construct accurate physical models of the diffuse emissions from the Galaxy at VHE/UHEs.