Search and multi-frequency follow-up studies of radio transients: novel approaches and large campaigns

One of the most fascinating phenomena in modern radio astronomy is related to the Fast Radio Bursts (FRBs). FRBs are Jy-intense, ms-duration radio transients of extra-galactic origin, whose nature is not assessed yet. To date, FRBs have been observed only in the radio band. Despite their origin has not been disambiguated yet, observational facts point towards highly magnetised neutron stars, such as magnetars, as the putative sources behind at least some of them. In particular, on April 2020 the Galactic magnetar SGR J1935+2154 emitted a radio flash closely resembling the ones produced by the FRBs, with simultaneous detections in the X-rays. This result remarkably strengthened the FRB/magnetar link and strongly motivates panchromatic campaigns towards FRB sources in order to find, as in the case of SGR J1935+2154, their high-energy and also, possibly, optical/infrared counterparts. On one side, a multi-wavelength (MWL) detection of a FRB would lead to the confirmation of the aforementioned FRB/magnetar connection and/or to the evidence that other classes of astrophysical objects/events could be also responsible for a fraction of these radio transients. On the other hand, the observation of a MWL burst (outside the radio band) might discriminate between various proposed emission models, some of which predict MWL emission(s) simultaneous with the radio, while others prescribe that the MWL emission should occur well before or after the radio burst. However, any panchromatic detection of a FRB will mainly rely on the detection of a burst in radio. In order to detect as many radio bursts as possible, dedicated instruments and tailored search algorithms are a fundamental asset. MWL observations of FRBs are indeed the main driver of this PhD Thesis. On the other hand, MWL campaigns strongly depend on the capability to detect bursts in the radio band, which can happen with the deployment of proper instruments and with the use of dedicated search algorithms. In view of that, the PhD work presented in this Thesis, revolves around the following three main themes: (i) the analysis and possible improvement of the FRB detection algorithms; (ii) the development of radio facilities tailored for the FRB observations, and lastly (iii) a series of large MWL campaigns targeted to some of the FRB sources

Freezing of a Lennard-Jones fluid: from nucleation to spinodal regine

Using molecular dynamics, we investigate the crystal nucleation in a Lennard-Jones fluid as a function of the degree of supercooling. At moderate supercooling, a nucleation picture applies, while for deeper quenches, the phenomenon progressively acquires a spinodal character. We show that in the nucleation regime, the freezing is a two-step process. The formation of the critical nucleus is indeed preceded by the abrupt formation of a precritical crystallite from a density fluctuation in the fluid. In contrast, as the degree of supercooling is increased, crystallization proceeds in a more continuous and collective fashion and becomes more spatially diffuse, indicating that the liquid is unstable and crystallizes by a spinodal mechanism

Water in acid boralites: Hydration effects on framework B sites

Properties and behavior of protonated boron-containing zeolites at different hydration degree have been investigated by means of periodic DFT approaches. Geometry optimization and room-temperature Car-Parrinello molecular dynamics results, in line with experimental findings, indicate that the BO3-bound silanolic acid site typical of dry boralites should convert to a solvated H3O+ hydrogen bonded to tetrahedral BO4 at moderate water content. By increasing the water loading, the tetrahedral structure of the B site is stabilized and the physicochemical properties of the water molecules solvating the acid proton gradually approach the liquid-phase ones. A relevant role of structural and vibrational properties of the zeolite framework in the water-induced trigonal-to-tetrahedral transition at the B site is highlighted by simulation results

Il riutilizzo dei nuraghi tra Marmilla e Sarcidano in epoca romana

Riassunto: Il riutilizzo dei nuraghi in epoca romana appare un fenomeno numericamente rilevante e caratteristico delle dinamiche di occupazione delle aree rurali della Sardegna, che risulta ancora di difficile inquadramento. Sono attestate, infatti, diverse finalità di riutilizzo ma non è stato finora possibile tracciare un quadro ipotetico delle modalità e delle finalità complessive di tale riuso, che viene tradizionalmente legato a una continuità di frequentazione di comunità locali e a un processo di acculturazione e romanizzazione più o meno profonda degli etnici indigeni. Nel presente contributo si presentano i dati relativi alle attestazioni di riutilizzo in un’area compresa tra Marmilla e Sarcidano, particolarmente importante per la sua collocazione geografica e strategica, nella quale si evidenzia un interessante quadro diacronico delle dinamiche di riutilizzo e della loro matrice culturale.Abstract: Reuse of nuraghi in Roman age appears as a numerically significant phenomenon and  a characteristic of settlement dynamics in Sardinian rural areas, but  still difficult to classify. Different purposes of reuse are known but has not yet been possible to draw a picture of the mode of operation and of the objectives of such re-use, which is traditionally linked to a continuity of  use related to local communities and to a process of acculturation and Romanization of indigenous ethnic groups. In this paper we present the reuse exemples of an area between Marmilla and Sarcidano, particularly important for its geographical and strategic position, in which is possible to highlight an interesting diachronic picture of reuse dynamics and of the cultural matrix of the phenomenon

Vici, pagi, agglomérations secondaires. Insediamenti e abitati di epoca romana nella Sardegna centro-orientale

Riassunto: Importanti indicazioni relative alle dinamiche di romanizzazione delle aree interne della Sardegna possono essere ricavate dai dati relativi all’effettiva consistenza numerica degli insediamenti e degli agglomerati rustici minori, frequentati o realizzati ex novo in epoca romana, alla loro natura e alla loro distribuzione territoriale. Nel contributo vengono presentati i dati ricavati dallo spoglio e dall’analisi delle notizie edite relative alle attestazioni di abitati o piccoli agglomerati rustici, indagati o individuati in un’area campione corrispondente al settore centro-orientale dell’isola, definita dalle fonti antiche come Barbaria. Particolarmente interessanti e inattesi per la loro consistenza risultano i valori quantitativi evidenziati dalla ricerca e riferibili a settori tradizionalmente considerati come non completamente stabilizzati e refrattari al processo di romanizzazione.Parole chiave: Sardegna, Barbaria, Romanizzazione, Insediamenti, CivitatesAbstract: Important informations on the dynamics of the Romanization of the inland areas of Sardinia can be derived from data relating to the numerical strength of the settlements and rural agglomerations, attended or made in Roman times, their nature and their spatial distribution. In this contribution are presented the data obtained from the analysis of published literature and news concerning excavated or identified settlements or small towns in a sample area corresponding to the central-eastern Sardinia, defined as Barbaria by the ancient sources. The quantitative values highlighted by the research are particularly interesting and unexpected for their consistency as they refer to sectors traditionally considered refractory to Romanization processes.Keywords: Sardinia, Barbaria, Romanization, Settlements, Civitate

Daedaleia, Nurac, Oikeseis katagheioi? Alcune note sul riutilizzo dei nuraghi nelle aree interne della Sardegna

L’elevato numero di nuraghi presenti in Sardegna caratterizza fortemente, oggi come in età antica, il paesaggio dell’isola; tuttavia poche fonti antiche fanno menzione di queste strutture e risultano vaghe e generiche, se non di dubbia attribuzione. I dati archeologici testimoniano un notevole riutilizzo in età repubblicana e soprattutto in età imperiale, ma non è ancora chiaro se il riu­tilizzo indichi una continuità di insediamento di comunità indigene o se sia dovuto ad un nuovo apporto di elementi allogeni roma­nizzati. Il presente contributo è una prima analisi delle attestazioni di riutilizzo in un’area campione corrispondente alla zona centro orientale della Sardegna, tradizionalmente indicata come sede delle Civitates Barbariae e refrattaria al processo di romanizzazione. The large number of Nuraghi is a landmark of Sardinian landscape today like in ancient times; notwithstanding they are mentioned by few literary sources, unfortunately vague and generic, and sometimes doubtful. The archaeological data shows a considerable reuse of these monuments in Republican period and mainly during the Imperial age, although is not clear if this trend is related to a settlement continuity of local communities or it has to be considered as an evidence of the superimposition of new external Romanized inhabitants. This article is a preliminary analysis of reuse attestations in east-central Sardinia, an area also known as Barbaria, traditionally related to Civitates Barbariae, and considered refractory to romanization process.

Silicone membrane contactor for selective volatile fatty acid and alcohol separation

The effect of pH and extraction temperature on flux, recovery, mass transfer coefficient and separation factor of volatile fatty acids (VFAs) and alcohols from synthetic solutions and cheese whey fermentate was investigated using a silicone membrane contactor with water as extractant. The silicone membrane allowed extraction of undissociated acids only, resulting in substantially higher recovery efficiencies at pH 3 than at pH 5. Furthermore, the non-porous silicone membrane favoured extraction of longer chain over shorter chain acids. Caproic acid was extracted with the highest flux of 1.30 (± 0.02) g m−2 h−1 in short time (32 h), with a 41.5 % recovery efficiency at pH 3 and 20 °C, indicating the feasibility of its selective separation from the VFA mixture. A similar trend was observed for alcohols, with butanol being extracted with a 39 % recovery efficiency at 40 °C, against 32 % and 19 % of propanol and ethanol, respectively, while the mass transfer coefficients were not affected by temperature. When applying the silicone membrane contactor to real cheese whey fermentate at pH 3, butyric and acetic acid were extracted with 21.5 % and 7% recovery efficiency, respectively, suggesting the feasibility of the contactor for VFA recovery from real fermentate

Characterisation of flow dynamics within and around an isolated forest, through measurements and numerical simulations

The case study of ‘Bosco Fontana’, a densely-vegetated forest located in the north of Italy, is analysed both experimentally and numerically to characterise the internal ventilation of a finite forest with a vertically non-homogeneous canopy. Measurements allow for the evaluation of the turbulent exchange across the forest canopy. The case study is then reproduced numerically via a two-dimensional RANS simulation, successfully validated against experimental data. The analysis of the internal ventilation leads to the identification of seven regions of motion along the predominate-wind direction, for whose definition a new in-canopy stability parameter was introduced. In the vertical direction, the non-homogeneity of the canopy leads to the separation of the canopy layer into an upper foliage layer and a lower bush layer, characterised respectively by an increasing streamwise velocity and turbulence intensity, and a weak backflow. The conclusions report an improved description of the dynamic layer and regions of motion presented in the literature