Reconstruction of extensive air shower images of the first Large Size Telescope prototype of CTA using a novel likelihood technique

Ground-based gamma-ray astronomy aims at reconstructing the energy and direction of gamma rays from the extensive air showers they initiate in the atmosphere. Imaging Atmospheric Cherenkov Telescopes (IACT) collect the Cherenkov light induced by secondary charged particles in extensive air showers (EAS), creating an image of the shower in a camera positioned in the focal plane of optical systems. This image is used to evaluate the type, energy and arrival direction of the primary particle that initiated the shower. This contribution shows the results of a novel reconstruction method based on likelihood maximization. The novelty with respect to previous likelihood reconstruction methods lies in the definition of a likelihood per single camera pixel, accounting not only for the total measured charge, but also for its development over time. This leads to more precise reconstruction of shower images. The method is applied to observations of the Crab Nebula acquired with the Large Size Telescope prototype (LST-1) deployed at the northern site of the Cherenkov Telescope Array.Comment: Proceedings of the 37th International Cosmic Ray Conference (ICRC 2021), Berlin, Germany. https://pos.sissa.it/395/71

All-sky Medium Energy Gamma-ray Observatory: Exploring the Extreme Multimessenger Universe

The All-sky Medium Energy Gamma-ray Observatory (AMEGO) is a probe class mission concept that will provide essential contributions to multimessenger astrophysics in the late 2020s and beyond. AMEGO combines high sensitivity in the 200 keV to 10 GeV energy range with a wide field of view, good spectral resolution, and polarization sensitivity. Therefore, AMEGO is key in the study of multimessenger astrophysical objects that have unique signatures in the gamma-ray regime, such as neutron star mergers, supernovae, and flaring active galactic nuclei. The order-of-magnitude improvement compared to previous MeV missions also enables discoveries of a wide range of phenomena whose energy output peaks in the relatively unexplored medium-energy gamma-ray band

The extreme blazar phenomenon in a multi-messenger context

This Thesis illustrates the activities and the results that I have obtained during my three-year PhD course at the University of Padova. My work was mainly based on the study of the extreme blazars, or better “extreme high-energy peaked BL Lac objects” (generally referred to as EHBLs). EHBLs are an emerging class of blazars with exceptional spectral properties, presenting a spectral energy distribution peaking with the synchrotron emission in hard X-rays and a high-energy emission in TeV gamma rays. The hard-TeV spectrum of EHBLs is important also for the implications on the indirect estimations of the extragalactic background light and of the intergalactic magnetic fields. The emission processes producing such extremely-high photon energies are currently not completely understood, and may represent a challenge for the standard modelling of these sources. In fact, the standard models are generally able to fit the experimental spectral data, but the parameters they assume are unusual compared to the standard blazar modelling currently available in literature. More complex models, assuming – for example – multiple emission zones or hadronic contributions, may provide more relaxed parameter spaces. In particular, the presence of hadronic processes in the blazar emission might make the EHBLs producers of extragalactic high-energy neutrinos.The number of EHBL objects is currently very limited. Thanks to recent observations, new sources have been classified as candidate EHBLs. Their spectral properties reveal new features that enrich the properties of this class of objects. In this framework, my research work started with the search for new EHBL candidates on the basis of all-sky surveys in hard X-rays. This new method allowed me to select the most promising extreme blazars relying on their X-ray to high-energy gamma-ray flux ratio. The analysis of the spectral properties of this new sample revealed interesting spectral features and systematic differences that are emerging in the EHBL class, especially in TeV gamma rays. This suggests the possible emergence of a sub-classification, and a new population of EHBLs may be unveiled. One specific aspect of my analysis was to predict the feasibility of observations of these objects with the current and next generation of Cherenkov telescopes. In the MAGIC Collaboration, I had the opportunity to propose a list of EHBL candidates for observations. Among them, a source named PGC 2402248 was observed and finally detected in TeV gamma rays, and several others are now being observed with the MAGIC telescopes. These results have been published together with a detailed discussion on the comparison between several theoretical models proposing different interpretations of the spectral properties of these sources. During my PhD, I had the opportunity to contribute to several other projects that complemented my preparation in this field. In the MAGIC Collaboration, I was involved in projects related to the detection of new EHBLs in TeV gamma rays and in the study of interesting blazars like PG 1553+113. I was also involved in two analyses on the relation between extreme blazars and extragalactic background light, and on the selection of extreme blazar candidates from a sample of faint sources in the high-energy gamma-ray band. Finally, an important part of my PhD activity within the MAGIC Collaboration was devoted to a contribution to the discovery of the first association between a high-energy neutrino, the so-called IceCube EHE-170922A event, and the blazar TXS 0506+056. During the analysis of this event, registered on September 2017, I was main analyser of the MAGIC data. Considering the importance of the discovery – with implications about the origin of the very-high-energy cosmic rays – and the intense multi-collaboration work, it was really a unique opportunity for me to gain new experience in this field and to further motivate my interest in the multi-messenger astrophysics

Spettroscopia gamma con il telescopio LAT

Le misure di precisione nella banda HE (High Energy: raggi camma da 30 GeV fino a 300 GeV) stanno rilevando per la prima volta l'esistenza di linee della radiazione gamma emessa dalle sorgenti extragalattiche. Tali linee permetteranno di estrarre molte informazioni di notevole valore sulla Fisica dei Nuclei Galattici Attivi e sulla cosmologia. Si chiede allo studente di impratichirsi con gli strumenti software usati dalla collaborazione LAT e SED (Spectral Energy Distribution, o curva di intensità spettrale) di alta precisione su alcune sorgenti di particolare interesse

The extreme blazar phenomenon in a multi-messenger context

This Thesis illustrates the activities and the results that I have obtained during my three-year PhD course at the University of Padova. My work was mainly based on the study of the extreme blazars, or better “extreme high-energy peaked BL Lac objects” (generally referred to as EHBLs). EHBLs are an emerging class of blazars with exceptional spectral properties, presenting a spectral energy distribution peaking with the synchrotron emission in hard X-rays and a high-energy emission in TeV gamma rays. The hard-TeV spectrum of EHBLs is important also for the implications on the indirect estimations of the extragalactic background light and of the intergalactic magnetic fields. The emission processes producing such extremely-high photon energies are currently not completely understood, and may represent a challenge for the standard modelling of these sources. In fact, the standard models are generally able to fit the experimental spectral data, but the parameters they assume are unusual compared to the standard blazar modelling currently available in literature. More complex models, assuming – for example – multiple emission zones or hadronic contributions, may provide more relaxed parameter spaces. In particular, the presence of hadronic processes in the blazar emission might make the EHBLs producers of extragalactic high-energy neutrinos.The number of EHBL objects is currently very limited. Thanks to recent observations, new sources have been classified as candidate EHBLs. Their spectral properties reveal new features that enrich the properties of this class of objects. In this framework, my research work started with the search for new EHBL candidates on the basis of all-sky surveys in hard X-rays. This new method allowed me to select the most promising extreme blazars relying on their X-ray to high-energy gamma-ray flux ratio. The analysis of the spectral properties of this new sample revealed interesting spectral features and systematic differences that are emerging in the EHBL class, especially in TeV gamma rays. This suggests the possible emergence of a sub-classification, and a new population of EHBLs may be unveiled. One specific aspect of my analysis was to predict the feasibility of observations of these objects with the current and next generation of Cherenkov telescopes. In the MAGIC Collaboration, I had the opportunity to propose a list of EHBL candidates for observations. Among them, a source named PGC 2402248 was observed and finally detected in TeV gamma rays, and several others are now being observed with the MAGIC telescopes. These results have been published together with a detailed discussion on the comparison between several theoretical models proposing different interpretations of the spectral properties of these sources. During my PhD, I had the opportunity to contribute to several other projects that complemented my preparation in this field. In the MAGIC Collaboration, I was involved in projects related to the detection of new EHBLs in TeV gamma rays and in the study of interesting blazars like PG 1553+113. I was also involved in two analyses on the relation between extreme blazars and extragalactic background light, and on the selection of extreme blazar candidates from a sample of faint sources in the high-energy gamma-ray band. Finally, an important part of my PhD activity within the MAGIC Collaboration was devoted to a contribution to the discovery of the first association between a high-energy neutrino, the so-called IceCube EHE-170922A event, and the blazar TXS 0506+056. During the analysis of this event, registered on September 2017, I was main analyser of the MAGIC data. Considering the importance of the discovery – with implications about the origin of the very-high-energy cosmic rays – and the intense multi-collaboration work, it was really a unique opportunity for me to gain new experience in this field and to further motivate my interest in the multi-messenger astrophysics

Neutrinos from interactions between the relativistic jet and large-scale structures of BL Lac objects investigated through their gamma-ray spectrum

International audienceAbsorption and emission lines in the optical spectrum are typically used to investigate the presence of large-scale environments in active galactic nuclei (AGNs). BL Lac objects - which are a category of AGNs with the relativistic jet pointing directly to the observer - are supposed to represent a late evolution stage of AGNs. Their large-scale structures are probably poorer of material, which is distributed with lower densities throughout the circumnuclear environment. Their accretion disk is weak and weakly reprocessed, making the non-thermal continuum of the relativistic jet dominate their optical spectrum and preventing us from identifying the thermal emission of the photon fields produced by such large-scale structures. However, these photon fields may still exist and eventually interact with the gamma rays traveling in the blazar jet via gamma-gamma pair production, producing observable effects such as absorption features in their spectral energy distribution.Interestingly, the same photon field might also lead to the production of high-energy neutrinos, acting as targets for proton-photon interactions. In this contribution, we present the results of a set of simulations over a wide parameter space describing both the blazar jet and the photon field properties. We discuss the most effective conditions that may produce fluxes of neutrinos compatible with the sensitivities of the current and the next generation of neutrino detectors. We will also discuss how the possible neutrino flux would be related to the properties of the large-scale structures investigated indirectly through the analysis of the gamma-ray spectrum of the BL Lac object

A new hard X-ray selected sample of extreme high-energy peaked BL Lac objects and their TeV gamma-ray properties

Extreme high-energy peaked BL Lac objects (EHBLs) are an emerging class of blazars with exceptional spectral properties. The non-thermal emission of the relativistic jet peaks in the spectral energy distribution (SED) plot with the synchrotron emission in X-rays and with the gamma-ray emission in the TeV range or above. These high photon energies may represent a challenge for the standard modeling of these sources. They are important for the implications on the indirect measurements of the extragalactic background light, the intergalactic magnetic field estimate, and the possible origin of extragalactic high-energy neutrinos. In this paper, we perform a comparative study of the multi-wavelength spectra of 32 EHBL objects detected by the Swift-BAT telescope in the hard X-ray band and by the Fermi-LAT telescope in the high-energy gamma-ray band. The source sample presents uniform spectral properties in the broad-band SEDs, except for the TeV gamma-ray band where an interesting bimodality seems to emerge. This suggests that the EHBL class is not homogeneous, and a possible sub-classification of the EHBLs may be unveiled. Furthermore, in order to increase the number of EHBLs and settle their statistics, we discuss the potential detectability of the 14 currently TeV gamma-ray undetected sources in our sample by the Cherenkov telescopes