Cosmic rays: constraints from future MeV detectors

Abstract

Cosmic rays are charged energetic particles that permeate the interstellar medium. Their sizeable energy share and penetration power makes them essential players in the dynamical and chemical processes that rule Galactic evolution, such as the launching of outflows and the formation of star and planets. For these processes low-energy (MeV-GeV) CRs are particularly important, both because they are the most abundant and because they have the largest cross-section for ionization. The study of cosmic rays naturally connects with gamma-ray astronomy, as high-energy photons are the principal products of their interaction with the interstellar plasma. In this article, after reviewing our current understanding of Galactic cosmic rays as derived from direct measurements, we present the state of the art regarding Galactic cosmic rays covering their direct observables, their acceleration processes and models for their propagation in the Galactic Disk. We present then an excursus on the current state of gamma-ray observations, and propose new prospects for investigating the physical properties of Galactic cosmic rays, exploiting the observational capability of future MeV missions.

Figures (19)

• Figure 1: CR spectral energy distribution up to the knee as a function of kinetic energy for different species: on the left the spectra of nuclei are reported; on the right the spectra of leptons are plotted together with the spectrum of CR hydrogen and helium nuclei. The data were recorded by several experiments (as listed in the Figure legend) and refer to the collection of the Cosmic-Ray Database (CRDB; Maurin2014).
• Figure 2: Abundance of elements present in the local interstellar medium (yellow line) and in the measured CRs (blue line). The CR data refer to Voyager measurements at 80 MeV 2016ApJ...831...18C.
• Figure 3: Left panel: Proton (blue and cyan dots) and electron (magenta and purple squares) fluxes as a function of the kinetic energy. The measurements are by AMS-02(AMS02-2016-protons-antiprotonsAMS02-2021PhR...894....1A) and Voyager (Voyager1-2016Voyager2-2019), as specified in the figure inset.Right panel: Boron-over-Carbon ratio as a function of the kinetic energy per nucleon. Measurements are by Voyager (black squares Voyager1-2016) and AMS-02 (magenta dots AMS02-2016-B-over-C).
• Figure 4: Typical CR loss timescales in the interstellar medium as a function of the particle kinetic energy Ravikularaman-2025-ionization-GC. Protons (left panel): ionization (red solid curve), proton-proton interactions (blue solid curve) and total (black solid curve) for a reference hydrogen density of $n_H =\, \rm 1~cm^{-3}$. Electrons (right panel): ionization (red solid curve) and Bremsstrahlung (green solid curve) for a reference hydrogen density of $n_H =\, \rm 1~cm^{-3}$; inverse Compton scattering in the interstellar radiation field (blue solid curve); synchrotron in a $B_0 = \rm 3~\mu G$ magnetic field; total (black solid curve).
• Figure 5: Typical diffusion-loss length of CR protons (blue curve) and electrons (red curve) corresponding to the loss timescales shown in Fig. \ref{['fig:loss-time-p-e']}, and with a diffusion coefficient in the form of Eq. \ref{['eq:D-parallel-beta-gamma']} with $D(10\, {\rm GeV}) = 5\times 10^{28}\, \rm cm^2/s$ and $\delta = 0.63$Phan-2023-stochasticity.