Ebook: Foundations of Cosmic Ray Astrophysics

The International School of Physics “Enrico Fermi” has been for several decades now one of the most prominent cultural initiatives promoted by the Italian Physical Society (SIF). The School was initiated in 1953 by the President of SIF at that time, Prof. Giovanni Polvani. That first edition focused on the contribution of cosmic ray physics to our knowledge of the laws of elementary particle physics. It was still a time in which many remarkable discoveries in particle physics originated from observations of cosmic rays, but it was also a period that marked the beginning of what we now call cosmic ray astrophysics. The results of the pioneering work of Enrico Fermi on particle acceleration were published in two articles in 1949 and 1954, respectively: for the first time, rather than wondering what one could learn on particle physics from these very energetic particles hitting the atmosphere of the Earth, scientists were wondering how could Nature produce those particles. Almost seventy years after that first School dedicated to cosmic rays, we decided to organise a school focused on the foundations of cosmic ray astrophysics. In this perspective, we selected some topics that, in our opinion, are more foundational than others and made an effort in having dedicated mini-courses covering these topics, with lectures delivered by known experts in the field. The lectures range from particle acceleration to cosmic ray transport, from radiation processes to feedback of cosmic rays on galaxy formation.

Being the energy density of cosmic rays, magnetic fields and gas in the Milky Way roughly the same, it is clear that these three components strongly feed back onto each other. This is true in the Galaxy at large and even more so in the proximity of acceleration regions, as well as in the large scale structure of the universe. Hence it is of crucial importance to achieve a satisfactory understanding of the interplay between these components, with cosmic rays generating magnetic fields and at the same time being affected by the presence of magnetic fields, and gas being moved around by the presence of cosmic rays and magnetic fields. The implications of this feedback are far-reaching especially in terms of star formation and the evolution of large scale structures in the universe.

We have made a special effort in providing the students with a proper description of the physics of cosmic ray transport in turbulent magnetic fields, and of the production of magnetic perturbations in the presence of cosmic ray gradients. Both these phenomena are essential in describing particle acceleration at shocks and transport of cosmic rays in the Galaxy. These topics are discussed from multiple points of view and in a number of sources, from shocks in supernova remnants to pulsar wind nebulae, from star clusters to reconnection layers in plasmas and in all these cases the role of radiative losses has received special attention. The transport of cosmic rays is also discussed in different environments, from the Milky Way to clusters of galaxies, and the universe at large, especially important for ultra high energy cosmic rays that, we now know, are mainly of extragalactic origin.

Although the main focus of the School is of theoretical type, we made sure that proper contact with current observations is provided, through dedicated lectures on the most recent developments in the measurement of the physical properties of both charged and neutral (gamma rays, neutrinos) cosmic rays. In particular, the accuracy of recent gamma-ray measurements over more than six decades in energy, from ∼100 MeV to ∼100 TeV and beyond, has reached a level that allows us to achieve a careful modelling of the acceleration and radiation processes inside or in the close proximity of cosmic ray factories.

There is no doubt that this field of investigation has been experiencing an extraordinary boost in recent times, both due to observations carried out with unprecedented accuracy, and to the possibility of testing new ideas and complex non-linear scenarios directly thanks to a leap in the development of numerical simulations. Dedicated lectures have covered these numerical challenges and the perspectives for their use to model sources and cosmic ray transport.

If the planning and construction of new experimental/observational facilities can be used as an indicator of the field dynamical state and its potential for future growth, the field of high energy astrophysics can look forward to decades of excitement, a fertile soil for todays students. We hope that this School may provide the necessary knowledge for them to play an important role in such exciting times.

Felix A. Aharonian, Elena Amato, Pasquale Blasi and Carmelo Evoli

The main goal of the present lectures is to outline the key particle interactions and energy loss mechanisms in the Galactic medium that high-energy particles are subject to. These interactions are an important ingredient entering the cosmic ray propagation equation, contributing to shape cosmic ray spectra. They also source the so-called secondary species, like gamma rays, neutrinos, “fragile” nuclei not synthesised in stars, and antiparticles, all routinely used as diagnostic tools in a multi-messenger context. These lectures are complementary to Denise Boncioli’s ones, focusing instead on processes happening at ultra-high energies in the extragalactic environment. They include propædeutic material to Felix Aharonian’s and, to some extent, Stefano Gabici’s and Carmelo Evoli’s lectures.

The theory of transport of charged particles in cosmic magnetic fields is at the very center of the investigation of non-thermal phenomena in the universe, ranging from our local neighborhood to supernovae, clusters of galaxies or distant active galaxies. It is crucial to understand how particles get energized to non-thermal energies as well as to describe their motion from the sources to an observer or to another location in the universe. Here I summarize some essential, basic aspects of the theory and discuss some topics in the theoretical framework that are currently being developed. I will also discuss some simple applications of the theory of transport to particle acceleration and propagation in the Galaxy.

These lectures address the effects of cosmic rays over macroinstabilities which develop in the interstellar medium and the microinstabilities the particles are able to trigger themselves. The lectures are centred on the derivation of linear growth rates but also discuss some numerical simulations addressing the issue of magnetic field saturation. A particular emphasis is made on the streaming instability, an instability driven by anisotropic cosmic-ray distributions.

These notes present the fundamentals of Fermi acceleration at shocks, with a special attention to the role that supernova remnants have in producing Galactic cosmic rays. Then, the recent discoveries in the theory of diffusive shock acceleration (DSA) that stem from first-principle kinetic plasma simulations are discussed. When ion acceleration is efficient, the back-reaction of non-thermal particles and self-generated magnetic fields becomes prominent and leads to both enhanced shock compression and particle spectra significantly softer than those predicted by the standard test-particle DSA theory. These results are discussed in the context of the non-thermal phenomenology of astrophysical shocks, with a special focus on the remnant of SN1006.

These notes summarise the contents of the lectures I delivered at the International School of Physics “Enrico Fermi” on “Foundations of Cosmic Ray Astrophysics”. The lectures were dealing with the physics of Pulsars and Pulsar Wind Nebulae (PWNe) in the Cosmic Ray (CR) perspective. It has become now clear that the processes taking place in the environment of fast rotating, highly magnetized neutron stars, often detected as pulsars, play a crucial role in the formation of the CR spectrum detected at the Earth. These lectures discuss the main aspects of this connection. Pulsars are likely contributors of the CR lepton flux at the Earth thanks to their nature of electron-positron factories. Pulsars and their nebulae are the best potential leptonic PeVatron in the Galaxy, and the Crab Nebula, the prototype of the Pulsar Wind Nebula class is the only established PeVatron in the Galaxy. Pulsars are however also potential sources of high energy hadrons, up to the energies relevant for UHECRs. Pulsars and their nebulae are the best potential leptonic PeVatrons in the Galaxy, and the Crab Nebula, the prototype of the Pulsar Wind Nebula class, is the only established PeVatron in the Galaxy. Finally, regions of suppressed particle diffusion have been observed around evolved pulsars, the so-called TeV halos, which could have an impact on galactic CR transport. These lectures discuss the physics of pulsars and PWNe, summarising what we know about these systems and what pieces of information are still missing to fully assess their role in all the above mentioned Cosmic Ray connected aspects.

Massive stars blow powerful winds and eventually explode as supernovae. By doing so, they inject energy and momentum in the circumstellar medium, which is pushed away from the star and piles up to form a dense and expanding shell of gas. The effect is larger when many massive stars are grouped together in bound clusters or associations. Large cavities form around clusters as a result of the stellar feedback on the ambient medium. They are called superbubbles and are characterised by the presence of turbulent and supersonic gas motions. This makes star clusters ideal environments for particle acceleration, and potential contributors to the observed Galactic cosmic ray intensity.

When examining the abundance of elements in the placid interstellar medium, a deep hollow between helium and carbon becomes apparent. Notably, the fragile light nuclei lithium, beryllium, and boron (collectively known as LiBeB) are not formed, with the exception of Li7, during the process of Big Bang nucleosynthesis, nor do they arise as byproducts of stellar lifecycles. In contrast to the majority of elements, these species owe their existence to the most energetic particles in the Universe. Cosmic rays, originating in the most powerful Milky Way’s particle accelerators, reach the Earth after traversing tangled and lengthy paths spanning millions of years. During their journey, these primary particles undergo transformations through collisions with interstellar matter. This process, known as spallation, alters their composition and introduces secondary light elements in the cosmic-ray beam. In light of this, the relatively large abundance of LiBeB in the cosmic radiation provides remarkable insights into the mechanisms of particle acceleration, as well as the micro-physics of confinement within Galactic magnetic fields. These lecture notes are intended to equip readers with basic knowledge necessary for examining the chemical and isotopic composition, as well as the energy spectra, of cosmic rays, finally fostering a more profound comprehension of the complex high-energy astrophysical processes occurring within our Galaxy.

These notes summarize the lectures about “Cosmic-ray propagation in extragalactic space and secondary messengers”, focusing in particular on the interactions of cosmic-ray particles with the background photons in the Universe, including nuclear species heavier than hydrogen, and on the analytical computation of the expected cosmic-ray fluxes at Earth. The lectures were held at the Course 208 of the International School of Physics “Enrico Fermi” on Foundations of Cosmic-Ray Astrophysics, in Varenna (Como, Italy) from June 23rd to June 29th, 2022. These notes are complementary to the content of the lectures held by Pasquale Dario Serpico at the same school.

Cosmic rays exchange energy and momentum with their environments through kinetic scale plasma waves, affecting the dynamics, energy balance, and stability of the host systems on global scales. One of the most important consequences of cosmic ray – thermal gas coupling is its effect on the rate of star formation in galaxies, which is known as star formation feedback. Cosmic rays may also be implicated in regulating the activity of supermassive black holes in galactic nuclei, or AGN feedback. This article discusses the basic mechanisms by which cosmic rays feed back on their environments and places them in their astrophysical context.

Extraterrestrial γ-rays tell us about the most energetic and violent phenomena in the Universe. Three features characterise them as unique messengers: i) copious production in both hadronic and electromagnetic interactions; ii) (almost) free propagation over the substantial fraction of the Universe; iii) effective detection by space-borne and ground-based instruments. The respective research area —Gamma Ray Astronomy— being a part of Astroparticle Physics, is a discipline in its own right. It addresses a broad range of high-energy processes of particle acceleration, propagation, and radiation on all astronomical scales: from compact neutron stars and black holes to large-scale cosmological structures. γ-ray studies cover a wide range of areas linked to the physics and astrophysics of SNRs, Star Formation, Magnetospheres of Pulsars and Black Holes, physics of relativistic outflows —Pulsar Winds, AGN jets, GRBs, etc. The major objectives of Gamma Ray Astronomy are connected, in one way or another, to the “centuries-old mystery” of Origin of Cosmic Rays. Therefore, identifying the main CR contributors with different astronomical source populations is considered one of the top priorities of high-energy astrophysics. The search for the sites of Cosmic Ray production (“Cosmic Ray Factories”) can be done best with space-borne and ground-based γ-ray detectors covering unprecedented energy range over ten energy decades, from MeV to PeV energies. Studies of the physics of CR factories, especially the so-called extreme accelerators responsible for the production of particles up to ≥ 10¹⁵ eV in our Galaxy (the so-called PeVatrons) and ≥ 10²⁰ eV in extragalactic objects (the so-called ZeVatrons) are of particular interest. In this lecture, I briefly describe the status of γ-ray studies, emphasising the recent exciting results obtained in the Very-High-Energy (VHE) and Ultra-High-Energy (UHE) bands in the context of Origin of Cosmic Rays.