|
Abstract:
|
Studying cosmic rays is very important for better understanding of high energy physical processes, since particles accelerated in space can reach energies far above what we can produce in accelerators on Earth, at the moment. Processes that produce cosmic rays are still unexplained to some extent, and models that have been proposed are awaiting confirmation. Presence of cosmic rays can be measured by detecting different products of interactions of these high energy particles with the interstellar medium through which they propagate. This thesis deals with the accretion shock as sources of cosmic rays in clusters of galaxies, as well as cosmic rays accelerated in supernova remnants inside galaxies. No matter which of these mechanisms is being considered, cosmic rays will collide with atoms and ions in the interstellar medium, and produce, among other things, gamma rays, neutrinos, as well as light elements, of which we will discuss lithium.
In the thesis we primarily develop models that describe gamma rays produced by cosmic rays accelerated in shocks that can appear in different processes. We first examine accretion of new gas onto already virialized structures (for example in galaxy clusters). For the first time, we include the change of gamma-ray pro- duction with time, through the history of the universe, that reflects the evolution of accretion shocks which appear during large scale structure formation. Therefore, the models developed in this thesis describe the gamma rays from large scale struc- tures more realistically, compared to models which have previously been developed and which use single redshift approximation for the gamma-ray origin. Models are used to derive the gamma-ray flux of all unresolved galaxy clusters. These mode- led gamma rays are then compared to the isotropic diffuse gamma-ray background,
measured by telescope Fermi-LAT. This leads to the conclusion that these cosmic rays have non-negligible contribution to the isotropic diffuse gamma-ray background (depending on the normalization, they can even explain the whole isotropic diffuse gamma-ray background) and that this population of cosmic rays has to be taken into consideration in addition to other components that are thought to be major contributors, like for example, unresolved normal galaxies or blazars.
In the thesis, models of gamma-ray production in accretion shocks are also com- pared to observations of high-energy neutrinos detected by IceCube detector. Neu- trinos are used to normalize gamma-ray models, from which we conclude that if the accretion shocks are predominantly strong, neutrino background is more limiting to the possible gamma-ray emissivity of these objects, compared to the gamma-ray background we first used. Study of neutrinos as products of cosmic-ray interactions is very important, since neutrinos interact weakly with other particles, and therefore keep all of the information about the time they were produced and about cosmic rays that produced them.
One part of the thesis deals with the production of cosmic rays in supernova remnants, in particular, the case of the Small Magellanic Cloud, which was detected in gamma rays. In this galaxy we also have the first measurements of the lithium abundances in the interstellar gas outside of the Milky Way. Since gamma rays and lithium are produced through interactions of cosmic rays with the interstellar medium, their same origin can be used to estimate the production of lithium and gamma rays by any cosmic-ray population. We show that galactic cosmic rays, which are considered to be dominant population of cosmic rays in the Small Magellanic Cloud, can only explain a very small part of the observed abundance of lithium, if we assume that the entire present gamma-ray emissivity that we observe also originates from the interaction of galactic cosmic rays with gas within the galaxy. This conclusion is interesting, because it leads to the possible existence of other sources of lithium in the Small Magellanic Cloud. Also, using the fact that gamma rays and lithium share the same origin, we estimate how much can irregular dwarf galaxies contribute to the diffuse gamma-ray background.
Study of several different products of cosmic-ray interactions with the interstellar
medium (gamma rays, neutrinos and lithium) on smaller scales (within the galaxy), as well as on the largest scales (galaxy clusters), showed that in addition to the galactic cosmic rays accelerated in supernova remnants, other still hypothetical co- smic rays (produced for example during accretion of gas on largest scales, or tidal interactions of galaxies) can have a non-negligible contribution to the measurements. |