| Περίληψη |
Cosmic rays are observed up to the astounding energies of more than 1020 eV, making them the most energetic particles
known in the Universe. For this reason, ever since their discovery in 1912, [Hess, 2018], they were of great help for
particle physics as they provide a source of high energy particles at no cost. By using specialized detectors, made
for observing all kind of different interesting phenomena resulting from the interaction of CRs with our atmosphere,
physicists were able to discover multiple new particles in the period 1930 − 1950. When accelerators were able to reach
energies beyond 1 GeV, particle physics shifted their focus on lab experiments. On the other hand, research on cosmic
rays was still ongoing, but their primary focus shifted to understanding their origin, the mechanisms responsible for
their acceleration and how they propagate from their source up to the Earth.
But what exactly are cosmic rays? Cosmic rays (CRs) are primarily atomic nuclei (a small fraction of them are
leptons) that travel through space with speeds that reach almost the speed of light. For most CRs, their energies
range from 107 − 1010 eV. However, there is a special class of CRs, known as ultra-high-energy cosmic rays (UHECRs),
whose energies range to over 1018 eV =1 EeV, with the most energetic one being the OMG particle, with an energy of
∼ 3.2 × 1020 eV or ∼ 51 J, [Bird et al., 1993]. This energy is extraordinarily large as it is equivalent to the energy of a
baseball that has a mass of ∼ 142 g going 100 km/h concentrated within a radius of a few Fermi (1 Fermi = 10−15 m).
The existence of UHECRs has been known for nearly 60 years and yet their origin and composition remain a mystery
up to this day. This is due to the fact that unlike neutral messengers (like photons), CRs are electrically charged and
thus get deflected by both extragalactic and galactic magnetic fields (EGMF and GMF respectively) as they traverse
through space, thereby making it difficult to reconstruct their paths and ultimately pinpoint their sources.
At the regime of UHECRs, that is, at energies above a few tens of EeV, the deflections could be small enough
for CRs to retain some directional information that would allow us to pinpoint the position of their source (at least
for nuclei with a sufficiently small charge). However, both the EGMF and the GMF are difficult to study and their
modeling is far from being complete. Focusing on the GMF, where knowledge of the 3D structure of the magnetic field
can be obtained from dust cloud measurements [Tritsis et al., 2019], we can reconstruct the path of individual CRs
through the Galaxy, recovering in this way their original direction before entering our Galaxy. These kind of studies
could shed light on the origin of the observed hot-spots and provide electromagnetic constraints on their composition,
independent of particle physics.
The nearby starburst galaxy M82 has been suggested by several authors (e.g. [Mollerach and Roulet, 2022],
[Telescope Array Collaboration et al., 2020]) to be the source of a cosmic ray hotspot detected by the Telescope Array
Collaboration in the northern Sky. This thesis has two aims. First, to test if one of existing GMF models confirms
this hypothesis. In other words, to answer the question: if we backtrack real cosmic ray events detected by Telescope
Array using one of these models, will they concentrate around M82? We did this using the two most recently updated
and most widely used GMF models, and we found that neither of them depropagates the real Telescope Array cosmic
rays onto M82. If one of these GMF models is accurate, therefore, then M82 is not the source of the Telescope Array
hotspot.
The second goal of this thesis is to find how sparse of a sampling for the magnetic field (i.e. how far apart can the
clouds be) we need in order for us to be able to reconstruct the path of a CR though the Galaxy accurately (in other
words to be able to depropagate CRs in our Galaxy accurately). To do this, we used a simulated "reality", where the
GMF is accurately represented by a known function (we used one of the two models studied, and multiples of that
to simulate the the location of M82 (within uncertainties). Simulating the sparsity parsity of the clouds through a
cubic grid, where in each cube we assumed a constant magnetic field, corresponding to a single possible measurement
in a cloud within that cube, we depropagated our fake CRs through this cubic grid. To quantify the accuracy of the
backtracking (or depropagation) of the CRs we plotted the angular distances of depropagated cosmic rays from the
source in each case as a function of the length of the side of the cube (or the distance between "clouds").
This work was based on the procedure created by Magos and Pavlidou, [Magkos and Pavlidou, 2019], but a new
code was developed from scratch for this project. This procedure is described in the next section.
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Cosmic rays: backtracking through the galactic magnetic field
