The X-ray and gamma-ray sky - Temporal, spectral and triangulation techniques for the characterization and localization of sources in High Energy Astrophysics

The X-ray and gamma-ray sky hides some of the most energetic events we can observe, and with them the struggle to shed some light onto the most extreme conditions of matter and the fabric of space-time. Low-Mass X-ray Binaries (LMXBs) are binary systems in which a compact object accretes matter from a companion star of mass less than 1 solar mass. The two companions interact with exchanges of matter that flows from the companion star towards the compact object, through Roche lobe overflow, where it is accreted with consequent emission of energy, visible mostly in the X-ray band. XTE J1810-189 is a LMXB hosting a neutron star (NS). I analyzed data of the NICER observatory from its 2020 outburst. The analysis revealed typical characteristics of Very-Faint X-ray Transients, systems characterized by peak X-ray luminosities of around 1036 erg/s. I could detect a disc component and a partially obscuring corona, while no NS direct component could be found. The source showed very little spectral evolution. MAXI J1834-021 is a new X-ray transient that shows features of a LMXB but with no certainty about the nature of the central accretor. The source remains in a low luminosity state, possibly performing a Failed-Transition outburst. From the spectral features, I could identify an accretion disc component and derive a lower limit of 6.5 gravitational radii on its truncation radius. The timing analysis shows features, such as Quasi-Periodic Oscillations, at around 2 Hz, compatible with a Black Hole Transient in a hard state. Accreting Millisecond X-ray Pulsars (AMXPs) are a subclass of NS LMXBs in which the NS shows X-ray pulsations at periods shorter than 10 milliseconds. In these systems, the X-ray emission comes from accretion of matter at the magnetic poles, modulated at the NS spin period. They are thought to be the progenitors of isolated, rotation-powered millisecond pulsars (MSPs). IGR J17591-2342 is an AMXP that went in outburst in 2018. The source underwent a canonical AMXP outburst: a low-luminosity state, with an identifiable NS emission and a region of Inverse Compton Scattering processes, often obscuring the direct NS emission. By taking into consideration the hydrogen column density, I could estimate the distance of the source, at around 7.2 kpc, which is consistent with the one previously estimated with the temporally-resolved spectral analysis of a type-I X-ray burst. Transitional Millisecond Pulsars (tMSPs) are defined as MSPs in binary systems that are observed to perform at least one transition between a rotation-powered state and an accretion-powered state. tMSPs in their sub-luminous state showed a gamma-ray emission several times higher than in the rotation-powered state. Therefore, establishing a connection between a known X-ray source and an unidentified gamma-ray source is one step towards the classification of the source as a tMSP. CXOU J1109 is a candidate tMSP. I studied the Fermi/LAT gamma-ray counterpart of J1109, in order to search for a compatible localization and study the spectral characteristics of the gamma-ray source. The background model resulted into around 180 sources. The new localization, calculated with 15 years of Fermi/LAT data, is compatible at the 95% confidence level with the X-ray source, with a detection significance of around 5 sigma, and improves the current cataloged position. The gamma-ray band also shows the most energetic events ever detected: Gamma-Ray Bursts (GRBs). As member of the HERMES collaboration, I worked on the development of a software to simulate the capabilities of the mission, based on a constellation of nano-satellites with the aim of accurately localizing gamma-ray bursts. The HERMES mission aims to study GRBs and provide quick and accurate localization through triangulation techniques. From the simulated data, we can test the spectral properties and the algorithm for the determination of the position of the GRB in the sky.