Gamma-ray and neutrino signatures of Galactic cosmic-ray accelerators

Supernova remnants are believed to be the major contributors to the observed Galactic cosmic-ray flux, though indisputable observational pieces of evidence of such statement are still missing. A crucial aspect of the supernova remnant paradigm for the origin of Galactic comic rays is that particle acceleration, as due to diffusive shock acceleration, requires effective confinement of particles in the shock region to let them achieve energies up to the so-called knee, around ∼ 10^15-10^16 eV. However, the current theoretical description of cosmic-ray acceleration and propagation within and around supernova remnants suffers from certain limitations, which also affect the predictions on the shape of the energy spectra of secondary gamma rays and neutrinos. In particular, in this thesis, two relevant aspects are investigated: the particle acceleration at shocks propagating in clumpy non-homogeneous environments and the particle escaping process from the acceleration site. The standard diffusive shock acceleration model usually assumes that shocks expand into ideally uniform environments, while a more realistic picture should consider a inhomogeneous gas distribution where supernova remnants expand. In this work, I conducted a detailed study on the particle acceleration and propagation through non-homogenous structures and on its effect on the resulting secondary radiation. Regarding the particle escape from the acceleration site, I developed a phenomenological model to investigate this process and its impact on the gamma-ray emission from middle-aged supernova remnants, where particle escape is expected to be effective. I will show that spectroscopic and morphological studies of the gamma rays coming from both inside and immediately outside of Supernova remnants are believed to be the major contributors to the observed Galactic cosmic-ray flux, though indisputable observational pieces of evidence of such statement are still missing. A crucial aspect of the supernova remnant paradigm for the origin of Galactic comic rays is that particle acceleration, as due to diffusive shock acceleration, requires effective confinement of particles in the shock region to let them achieve energies up to the so-called knee, around ∼ 10^15-10^16 eV. However, the current theoretical description of cosmic-ray acceleration and propagation within and around supernova remnants suffers from certain limitations, which also affect the predictions on the shape of the energy spectra of secondary gamma rays and neutrinos. In particular, in this thesis, two relevant aspects are investigated: the particle acceleration at shocks propagating in clumpy non-homogeneous environments and the particle escaping process from the acceleration site. The standard diffusive shock acceleration model usually assumes that shocks expand into ideally uniform environments, while a more realistic picture should consider a inhomogeneous gas distribution where supernova remnants expand. In this work, I conducted a detailed study on the particle acceleration and propagation through non-homogenous structures and on its effect on the resulting secondary radiation. Regarding the particle escape from the acceleration site, I developed a phenomenological model to investigate this process and its impact on the gamma-ray emission from middle-aged supernova remnants, where particle escape is expected to be effective. I will show that spectroscopic and morphological studies of the gamma rays coming from both inside and immediately outside of those remnants can provide insight into the escaping process in general, and in particular will shed light on their ability to act as cosmic-ray PeVatrons. So far, the only hint of the presence of a PeVatron has been found in the Galactic Center region, whose nature is however unclear. Under the assumption that the observed gamma-ray flux originates from hadronic interactions, I calculated the expected flux of multi-TeV neutrinos in order to investigate its detectability with future km3-scale neutrino telescopes. Finally, a comparative analysis of the performances of the two major upcoming detectors, namely CTA and KM3NeT, is presented in the context of future studies on the origin of Galactic cosmic rays through respectively gamma-ray and neutrino observations. The thesis is organized as follows: • In Chapter 1, the supernova remnant paradigm for the origin of cosmic rays is introduced, followed by a discussion concerning possible Galactic PeV accelerators. As gamma rays and neutrinos constitute observational signatures of particle acceleration and propagation, a review of their properties and detection technique is provided. • In Chapter 2, the propagation of accelerated particles within supernova remnants is investigated for the first time in the presence of strong shocks evolving through nonhomogeneous media. These conditions represent realistic situations for the environments where sources as supernova remnants usually expand. Since dense molecular clumps constitute ideal targets for accelerated protons, enhanced gamma-ray and neutrino emissions are expected. The model is shown to provide an adequate description of the broadband gamma-ray emission of the Galactic supernova remnant RX J1713.7-3946 both in terms of total flux and spectral shape. • In Chapter 3, a phenomenological description of particle escape from middle-aged supernova remnants is presented, which represents the first attempt of studying this process within the context of extended sources. A proper description of this phenomenon is extremely relevant for the correct interpretation of the radiation spectrum observed in these sources, which reflects not only the acceleration mechanism and the interaction processes, but also the escape from the acceleration site. The model is applied to three interesting middle-aged Galactic supernova remnants, namely IC 443, W 51C and W 28N. A major implication of the presence of particle escape is represented by the possible production of high-energy radiation also outside of the supernova shock, characterized by a very peculiar bump-like energy spectrum. This feature is interesting from the point of view of both gamma-ray and neutrino emissions, being experimentally connected to potentially background-free regions. Moreover, the escaping process is particularly relevant for a correct understanding of the cosmic-ray spectrum observed at Earth and to disentangle the propagation effects through the Galaxy. • In Chapter 4, a candidate source of PeV cosmic rays located at the center of the Galaxy is discussed. The Galactic Center, as recently observed in multi-TeV gamma rays, shows a central emission with a spectral cut-off energy in correspondence of 10 TeV. Nonetheless, a diffuse emission surrounding the central source shows no visible cut-off up to the energies currently probed by H.E.S.S.: the possibility of an intense infrared radiation field absorbing gamma rays from the central source is investigated for the first time. The detection of very-high-energy neutrinos in angular correlation with the electromagnetic radiation would confirm the hadronic hypothesis for the origin of gamma rays. Hence, expectations from current and next generation neutrino instruments are provided, indicating the relevance of a Northern Hemisphere detector for the observation of this region with a clean event sample. • In Chapter 5, the performances of the next-generation gamma-ray and neutrino detectors are investigated and differential sensitivities of CTA and KM3NeT for extended sources are derived. This study represents one of the first attempts towards the understanding of instrumental performances for extended sources related to spectroscopic detection of gamma rays through the imaging technique and capabilities of neutrino telescopes. Sensitivity analyses are hence applied to some interesting PeV cosmic-ray candidate sources, as the Galactic Center Ridge and the aforementioned supernova remnant RX J1713.7-3946. • The main results of the work are summarized and discussed in Chapter 6. The thesis contains three appendices, specifically separated from the text in order to facilitate its reading. In Appendix A, an overview of the equations regulating the magnetohydrodynamical properties of astrophysical plasma is presented, together with an insight into the numerical code adopted for the system solution. In Appendix B, a detailed description of the numerical methods adopted for the solution of the transport equation in the presence of molecular clumps is provided. It is a technical appendix, intended to support the interested reader in reproducing the physical results discussed in Chapter 2. Its content represents an original work developed by the author. Finally, Appendix C provides the mathematical framework developed by the author in order to derive the analytical solution of the diffusive transport equation, as satisfied by the escaping particle density function and presented in Chapter 3.