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Pulsars are small, dense, rotating stars that appear to flash as they sweep the sky with beams of radiation. This light originates from high energy particles following curved paths as they flow into interstellar space. Where these particles originate, and how they are accelerated to the necessary energies, is still an open question. The answer may change as the pulsar goes through its evolutionary process.Theorists have focused on four accelerating regions: the polar cap, slot gap, outer gap, and current sheet. Testing these models requires detecting pulsars and measuring their spectra and spin characteristics. It is to this task that I devote myself in this thesis, focusing in particular on the millisecond and soft gamma-ray pulsar populations.Millisecond pulsars are the oldest known pulsars, and are defined by their millisecond periods. I characterized the Fermi-LAT spectrum of PSR J0218+4232 as part of a larger work on searching for very high energy emission with MAGIC. While MAGIC was not able to detect the source, the Fermi-LAT spectrum was useful in providing insight as to why this was the case. Soft gamma-ray pulsars are brightest at MeV energies, and are challenging to study due to the lack of an instrument sensitive in this energy region. I characterize the spin characteristics of the soft pulsars PSR J1813-1749 and PSR J1846-0258 with NICER, and attempt to detect them with the Fermi-LAT to characterize a portion of their MeV spectra. While the spin characterization will aid in detecting these sources with future MeV telescopes, there was not a strong enough LAT detection to extract spectral information. Examining PSR J2022+3842 with NICER did lead to a detection of pulsations with Fermi-LAT, which improved upon the prior detection, as well as provided spectral information for emission modeling.

Studies of the non-thermal Galactic source population are essential to understand how and where the bulk of cosmic rays are being accelerated and to understand the mechanisms underlying very high energy (VHE, E>50 GeV) emitters [39, 51]. The plane of the Milky Way is rich with supernova remnants (SNRs) and pulsar wind nebulae (PWNe) which are efficient accelerators of cosmic rays (CRs) - whose interaction with the surrounding photon fields produces energetic γ-rays and neutrinos. SNRs and PWNe are some of the most powerful objects in our Galaxy and because they emit at very high energies (VHE, E>50 GeV), γ-rays represent an excellent probe of the non-thermal astrophysical processes in these objects. Relativistic electrons (i.e. leptons) can produce γ-rays by non-thermal bremsstrahlung or by inverse Compton scattering (IC) on ambient photon fields, whereas protons and heavier nuclei (i.e. hadrons) can generate γ-rays by the process of pion decay, produced in collisions between relativistic hadrons and ambient material. Understanding the particle population responsible for the observed γ-ray emission can provide clues to the potential of CR acceleration as most cosmic rays are made of protons or heavier nuclei (∼10% of all cosmic rays are leptons) so, if it can be established that the γ-ray emission is hadronic in origin, then we can better understand the likelihood for hadron CR acceleration in VHE objects. In this thesis, we report on the investigation of a very high energy (VHE), Galactic γ-ray source recently discovered at >50 GeV using the Large Area Telescope (LAT) on board Fermi. This object, 2FHL J0826.1−4500, displays one of the hardest >50 GeV spectra (Γγ ∼ 1.6) in the 2FHL sample, and a follow-up observation with XMM-Newton has uncovered diffuse, soft thermal emission at the position of the γ-ray source. A detailed analysis of the available multi-wavelength data shows that this source is located on the Western edge of the Vela supernova remnant: the observations and the spectral energy distribution modeling support a scenario where this γ-ray source is the byproduct of the interaction between the SNR shock and a neutral Hydrogen cloud. If confirmed, this shock-cloud interaction would make 2FHL J0826.1−4500 a promising candidate for efficient particle acceleration. This work has been recently published in the Astrophysical Journal [30]. In chapter 1, the objective of this thesis is introduced. In chapter 2, SNRs and PWNe are explained in detail with a focus on the Vela SNR - the closest composite SNR to Earth. In chapter 3, we discuss the main instruments used to obtain the γ-ray and X-ray data, namely XMM-Newton and the Fermi-LAT. Chapter 4 describes the data reduction process and spectral analysis and a multi- wavelength description of 2FHL J0826.1−4500 is presented in chapter 5. Chapter 6 tests the spectral energy distribution (SED) of the source, attempting to determine the dominant parent particle population to better understand its emission mechanisms. In chapter 7 we report our conclusions on 2FHL J0826.1−4500 and emphasize important properties that still need to be probed in order to best answer the underlying question: if 2FHL J0826.1−4500 is an efficient particle accelerator, can we safely establish if this is a site generating fresh CRs or does the energetic environment favor a scenario where pre-existing CRs are being re-accelerated here?

An excess gamma-ray signal coming from the direction of the Galactic center (GC) has been detected by the Fermi Large Area Telescope (Fermi-LAT). Possible explanations for the excess include inaccuracies in the foreground/background model, an unresolved population of millisecond pulsars, and/or dark matter (DM) annihilation. However, the GC is a complicated region, and the true nature of the excess currently remains uncertain.M31 is the closest spiral galaxy to us and has been the subject of numerous studies. Because it is both massive and relatively nearby, M31 is an ideal target for studying galaxies, and historically it has played a major role in the development of our understanding of the Universe. Gamma-ray radiation coming from the inner region of M31 was first detected in 2010 by Fermi-LAT. Since the initial detection, all studies to date have focused on the innermost region, where the galactic disk can be observed. However, M31’s galactic disk only amounts to roughly 10% of the galaxy’s total mass, with the other 90% being in the form of DM, which extends well beyond the inner region of the galaxy.In this dissertation, a detailed study of the gamma-ray emission towards M31's outer halo is made. Using the cosmic ray propagation code GALPROP, specialized interstellar emission models are constructed to characterize the foreground emission from the Milky Way (MW), including a self-consistent determination of the isotropic component, and an in-depth analysis of the systematic uncertainties related to the observations. Evidence is found for an extended excess that appears to be distinct from the conventional MW foreground, having a total radial extension upwards of ~120-200 kpc from the center of M31. A DM interpretation is found to provide a good description of the observed emission and is consistent with the GC excess DM interpretation. However, uncertainties in the MW foreground, and in particular, modeling of the H I-related components, have not been fully explored and may impact the results.

This PhD thesis includes theoretical and observational studies of γ-ray emission from radio-quiet accreting black holes. The theoretical motivation for the search of γ-ray emission from such sources concerns the considerable hadronic production of γ-rays predicted by models of hot flows, which most likely power these sources at low luminosities. I thoroughly investigated this model prediction and I found that the luminosity at either hundreds of MeV or in the GeV range, depending on proton distribution, can reach ∼ 10−5LEdd for the X-ray luminosities between ∼ 10−4LEdd and 10−3LEdd. These levels of γ-ray luminosities can be probed in some Seyfert galaxies. Comparing the model predictions with Fermi/LAT upper limits for NGC 4258, NGC 7213 and NGC 4151 I found interesting constraints on the acceleration efficiency of protons, plasma magnetization and black hole spins. I found an interesting hint for a γ-ray signal in the LAT data from NGC 4151, which is only slightly below the formal detection threshold of 5σ. I also found hints for the correlation between the X-ray and γ-ray emission in the nearby galaxy NGC 4945, which harbors both an active galactic nucleus and a nuclear starburst region. I have divided the Fermi/LAT observations of NGC 4945 into two datasets, comprising events detected during the low and high level of X-ray emission from the active nucleus of this galaxy, determined using the Swift/BAT light curve. I found a ∼ 5σ difference between spectral parameters fitted to these datasets, and a similar significance of the reversal of the γ-ray signal in significance maps for low and high γ-ray energies. This X/γ-ray correlation indicates that the γ-ray production is dominated by the active nucleus rather than by cosmic rays interacting with the interstellar medium. I compared NGC 4945 with other starburst galaxies detected by LAT and I note similarities between those with active nuclei, e.g. unlikely high efficiencies of γ-ray production in starburst scenario, which argues for a significant contribution of their active nuclei to the γ-ray emission. Finally, I took into account AGILE observations of Cyg X-1. I found that it has not reached yet a sensitivity level needed for a detection of this source.