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Particle acceleration at supernova remnant shocks and the origin of cosmic rays

Final Activity Report Summary - COSMIC RAY ORIGIN (Particle acceleration at supernova remnant shocks and the origin of cosmic rays)

Cosmic rays (CRs) are relativistic particles, mainly protons, which reach the Earth with the same intensity regardless from their direction of origin in the sky. They were discovered by Victor Hess in 1912, but we still do not know how and where they are accelerated. What we know from direct measurements is that the energy density of CRs in the galaxy is comparable with the energy density of both magnetic field and interstellar gas. This makes CRs an essential ingredient for studies on the galaxy dynamical balance. CRs also play a key role in the process of star formation, since they are the dominant source of ionisation in dense proto-stellar regions, where ionising radiation cannot penetrate. Finally, understanding the origin of CRs is extremely important in order to understand the mechanisms through which particles are accelerated up to ultra-relativistic energies. Since the presence of non-thermal particles is ubiquitous in astrophysical environments, these studies are of fundamental importance in order to understand the non-thermal universe.

In 1934, Baade and Zwicky were the first to propose that supernovae were the sources of galactic CRs. Such idea was based on the consideration that the CR population could be maintained at the observed level if a small fraction, such as a few percent, of the galactic supernovae kinetic energy was somehow converted into CRs. The argument was strengthened by the fact that it was commonly believed that CRs could be efficiently accelerated via the Fermi mechanism at shock waves that formed during the expansion of supernova remnants (SNRs) in the interstellar medium. The acceleration of CRs in SNRs should be accompanied by a copious emission of gamma-rays and neutrinos, because of the decay of pions produced in hadronic interactions between CRs and the interstellar medium.

The main idea of this research project was to search for a definite proof of the fact that supernova remnants were the sources of galactic cosmic rays. The rationale of the approach was to test the supernova remnant hypothesis by searching not only for gamma-rays and neutrinos coming from the supernova remnants themselves, but also for gamma-rays emitted by nearly located molecular clouds. In case the gamma-ray and the neutrino emission from a molecular cloud were due to cosmic ray interactions in the dense gas, a molecular cloud located in proximity of a cosmic ray source would have its gamma-ray emission enhanced.

The most important results of the research project were predictions of the expected spectra of molecular clouds located in proximity of SNRs. The peculiarities of these spectra, once detected by future observations in the gamma ray and neutrino domain, were anticipated to allow us to perform a decisive test of the fact that SNRs were indeed the sources of galactic CRs.