143 research outputs found
The extreme blazar phenomenon in a multi-messenger context
No full textThis 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
A new hard X-ray selected sample of extreme high-energy peaked BL Lac objects and their TeV gamma-ray properties
No full textHunting For Extreme Blazars In The Tev Band
No full textBlazars are a particular class of active galactic nuclei with their relativistic jets pointing close to the line of sight of the observer. Their spectral energy distributions are dominated by non-thermal emission from the jet, consisting of two main bumps. For the so-called extreme blazars, these components each peak in the X-ray and GeV-TeV bands.
Recent observations have revealed that in a few of these objects, the energy of the second peak exceeds several TeV (e.g. 1ES 0229+200). These intriguing objects have been suggested as sources of hadronic gamma-ray emission as well as high-energy neutrinos. Their hard TeV spectra are also valuable for indirectly probing the extragalactic background light and the intergalactic magnetic field.
In this contribution, we present the results of our observing campaign on ten promising targets performed in very-high-energy gamma rays with the MAGIC telescopes. Modelling of their broadband spectra is also discussed. Furthermore, we propose new targets for current and future TeV gamma-ray telescopes, selected on the basis of hard X-ray and GeV gamma-ray emission.</p
The variability patterns of PG 1553+113: a MAGIC perspective
No full textPG1553+113 is one of the few blazars with a convincing quasi-periodic emission in the gamma-ray
band detected by the Fermi-LAT satellite. The source is also a very high-energy (>100 GeV) gammaray emitter. The MAGIC collaboration started a multi-year, multi-wavelength monitoring campaign
of PG 1553+113 in 2015 involving several instruments in the radio, infra-red, optical photometry and
polarimetry, UV, and soft X-ray bands. The purpose of the campaign is to characterise the properties
of its broadband emission, in particular the variability at different timescales and energies, with the
ultimate goal of pinpointing the physical processes at work in the jet driving the emission variability.
In this contribution the main results of the campaign will be presented with a particular emphasis
on the multi-year light curve from MAGIC and its connection to the periodicity seen in gamma rays
by Fermi-LAT and, possibly, in the optical waveband, too
Cosmological perturbations in mimetic gravity models
No full textThe so-called LambdaCDM model of cosmology has been very successful at explaining all cosmological observations. Various attempts are ongoing in explaining the effect of a cosmological constant Lambda in terms of a modification of gravity w.r.t to Einstein General Relativity. Recently, an interesting scenario has been proposed, the so called mimetic gravity. In this scenario the dark matter components of the universe (dark matter and dark energy contributions) can be mimicked purely with a modification of gravity. In general this is achieved starting from a given initial theory and applying to it a non-invertible disformal transformation (the original paper by Chamsedinne and Mukhanov started from the Einstein-Hilbert action to provide a new theory where a new degree of freedom could mimick a cold dark matter componentâ ''mimetic dark matter''). The status of these scenarios is mature enough to study their cosmological perturbations with the goal of extracting some obervational predictions of particular interest for forthcoming Large Scale Structure surveys, like the one the forthcoming Euclid satellite mission will provide. The goal of this thesis is to make a detailed computation of the observational predictions for mimetic gravity models.ope
Spettroscopia gamma con il telescopio LAT
No full textLe 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
Cosmological perturbations in mimetic gravity models
No full textThe so-called LambdaCDM model of cosmology has been very successful at explaining all cosmological observations. Various attempts are ongoing in explaining the effect of a cosmological constant Lambda in terms of a modification of gravity w.r.t to Einstein General Relativity. Recently, an interesting scenario has been proposed, the so called mimetic gravity. In this scenario the dark matter components of the universe (dark matter and dark energy contributions) can be mimicked purely with a modification of gravity. In general this is achieved starting from a given initial theory and applying to it a non-invertible disformal transformation (the original paper by Chamsedinne and Mukhanov started from the Einstein-Hilbert action to provide a new theory where a new degree of freedom could mimick a cold dark matter componentâ ''mimetic dark matter''). The status of these scenarios is mature enough to study their cosmological perturbations with the goal of extracting some obervational predictions of particular interest for forthcoming Large Scale Structure surveys, like the one the forthcoming Euclid satellite mission will provide. The goal of this thesis is to make a detailed computation of the observational predictions for mimetic gravity models
Spettroscopia gamma con il telescopio LAT
No full textLe 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.reserve
REAL Reinforcement Learning: Planning with adversarial models
No full textModel-Based Reinforcement Learning (MBRL) algorithms solve sequential decision-making problems, usually formalised as Markov Decision Processes, using a model of the environment dynamics to compute the optimal policy. When dealing with complex environments, the environment dynamics are frequently approximated with function approximators (such as Neural Netoworks) that are not guaranteed to converge to an optimal solution. As a consequence, the planning process using samples generated by an imperfect model is also not guaranteed to converge to the optimal policy. In fact, the mismatch between source and target dynamics distribution can result in compounding errors, leading to poor algorithm performance during testing. To mitigate this, we combine the Robust Markov Decision Processes (RMDPs) framework and an ensemble of models to take into account the uncertainty in the approximation of the dynamics. With RMDPs, we can study the uncertainty problem as a two-player stochastic game where Player 1 aims to maximize the expected return and Player 2 wants to minimize it. Using an ensemble of models, Player 2 can choose the worst model to carry out the transitions when performing rollout for the policy improvement. We experimentally show that finding a maximin strategy for this game results in a policy robust to model errors leading to better performance when compared to assuming the learned dynamics to be correct.Computer Science | Data Science and Technolog
MAGIC and MWL monitoring of the blazar TXS 0506+056 in the 2018/2019 season
No full textThe gamma-ray blazar TXS 0506+056, was discovered in VHE gamma-rays by the
MAGIC telescopes in 2017 in a follow-up campaign of a high energy neutrino
event IceCube-170922A (IC+Fermi+MAGIC++, Science 361, eaat1378 (2018)).
Subsequent multivawelenght (MWL) observations and theoretical modeling in a
frame of hadro-leptonic emission confirmed that this source could be a
potential cosmic ray and neutrino emitter (MAGIC Collaboration, Ansoldi et al.,
(2018)). This is, by far, the most significant association between a
high-energy neutrino and an astrophysical source emitting gamma rays and
X-rays. TXS 0506+056 is a key object to help the astrophysics community to
establish connections between high-energy neutrinos and astrophysical sources.
Accurate and contemporaneous MWL spectral measurements are essential
ingredients to achieve this goal. In the conference, we present the
measurements from the MAGIC and MWL monitoring of this source, spanning the
time period from November 2017 till February 2019. These include the lowest VHE
gamma-ray emission state measured from this source so far as well as a flaring
episode in December 2018
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