Procedures of quality control and data analysis of multi-site ground-based observations for the absolute flux calibration of Gaia

The Gaia space mission is a major project for the European astronomical community. As challenging as it is, the processing and analysis of the huge data-flow incoming from Gaia is the subject of thorough study and preparatory work by the DPAC (Data Processing and Analysis Consortium), in charge of all aspects of the Gaia data reduction. This PhD Thesis was carried out in the framework of the DPAC, within the team based in Bologna. The task of the Bologna team is to define the calibration model and to build a grid of spectro-photometric standard stars (SPSS) suitable for the absolute flux calibration of the Gaia G-band photometry and the BP/RP spectrophotometry. Such a flux calibration can be performed by repeatedly observing each SPSS during the life-time of the Gaia mission and by comparing the observed Gaia spectra to the spectra obtained by our ground-based observations. Due to both the different observing sites involved and the huge amount of frames expected (≃100000), it is essential to maintain the maximum homogeneity in data quality, acquisition and treatment, and a particular care has to be used to test the capabilities of each telescope/instrument combination (through the “instrument familiarization plan”), to devise methods to keep under control, and eventually to correct for, the typical instrumental effects that can affect the high precision required for the Gaia SPSS grid (a few % with respect to Vega). I contributed to the ground-based survey of Gaia SPSS in many respects: with the observations, the instrument familiarization plan, the data reduction and analysis activities (both photometry and spectroscopy), and to the maintenance of the data archives. However, the field I was personally responsible for was photometry and in particular relative photometry for the production of short-term light curves. In this context I defined and tested a semi-automated pipeline which allows for the pre-reduction of imaging SPSS data and the production of aperture photometry catalogues ready to be used for further analysis. A series of semi-automated quality control criteria are included in the pipeline at various levels, from pre-reduction, to aperture photometry, to light curves production and analysis

Globular clusters with Gaia

The treatment of crowded fields in Gaia data will only be a reality in a few years from now. In particular, for globular clusters, only the end-of-mission data (public in 2022-2023) will have the necessary full crowding treatment and will reach sufficient quality for the faintest stars. As a consequence, the work on the deblending and decontamination pipelines is still ongoing. We describe the present status of the pipelines for different Gaia instruments, and we model the end-of-mission crowding errors on the basis of available information. We then apply the nominal post-launch Gaia performances, appropriately worsened by the estimated crowding errors, to a set of 18 simulated globular clusters with different concentration, distance and field contamination. We conclude that there will be 103-104 stars with astrometric performances virtually untouched by crowding (contaminated by -1; (II) internal kinematics will be of unprecedented quality, cluster masses will be determined to ≃10 per cent up to 15 kpc and beyond, and it will be possible to identify differences of a few km s-1 or less in the kinematics (if any) of cluster sub-populations up to 10 kpc and beyond; (III) the brightest stars (V ≃ 17 mag) will have space-quality, wide-field photometry (mmag errors), and all Gaia photometry will have 1-3 per cent errors on the absolute photometric calibration

The Gaia-ESO Survey: Calibration strategy

The Gaia-ESO survey (GES) is now in its fifth and last year of observations and has produced tens of thousands of high-quality spectra of stars in all Milky Way components. This paper presents the strategy behind the selection of astrophysical calibration targets, ensuring that all GES results on radial velocities, atmospheric parameters, and chemical abundance ratios will be both internally consistent and easily comparable with other literature results, especially from other large spectroscopic surveys and from Gaia. The calibration of GES is particularly delicate because of (i) the large space of parameters covered by its targets, ranging from dwarfs to giants, from O to M stars; these targets have a large wide of metallicities and also include fast rotators, emission line objects, and stars affected by veiling; (ii) the variety of observing setups, with different wavelength ranges and resolution; and (iii) the choice of analyzing the data with many different state-of-the-art methods, each stronger in a different region of the parameter space, which ensures a better understanding of systematic uncertainties. An overview of the GES calibration and homogenization strategy is also given, along with some examples of the usage and results of calibrators in GES iDR4, which is the fourth internal GES data release and will form the basis of the next GES public data release. The agreement between GES iDR4 recommended values and reference values for the calibrating objects are very satisfactory. The average offsets and spreads are generally compatible with the GES measurement errors, which in iDR4 data already meet the requirements set by the main GES scientific goals

The Gaia spectrophotometric standard stars survey - III. Short-term variability monitoring

We present the results of the short-term constancy monitoring of candidate Gaia Spectrophotometric Standard Stars (SPSS). We obtained time series of typically 1.24 h - with sampling periods from 1-3 min to a few hours, depending on the case - to monitor the constancy of our candidate SPSS down to 10 mmag, as required for the calibration of Gaia photometric data. We monitored 162 out of a total of 212 SPSS candidates. The observing campaign started in 2006 and finished in 2015, using 143 observing nights on nine different instruments covering both hemispheres. Using differential photometry techniques, we built light curves with a typical precision of 4 mmag, depending on the data quality. As a result of our constancy assessment, 150 SPSS candidates were validated against short-term variability, and only 12 were rejected because of variability including some widely used flux standards such as BD+174708, SA 105-448, 1740346, and HD 37725

GRAWITA: VLT Survey Telescope observations of the gravitational wave sources GW150914 and GW151226

We report the results of deep optical follow-up surveys of the first two gravitational-wave sources, GW150914 and GW151226, done by the GRAvitational Wave Inaf TeAm Collaboration (GRAWITA). The VLT Survey Telescope (VST) responded promptly to the gravitational wave alerts sent by the LIGO and Virgo Collaborations, monitoring a region of 90 and 72 deg2 for GW150914 and GW151226, respectively, and repeated the observations over nearly two months. Both surveys reached an average limiting magnitude of about 21 in the r band. The paper describes the VST observational strategy and two independent procedures developed to search for transient counterpart candidates in multi-epoch VST images. Several transients have been discovered but no candidates are recognized to be related to the gravitational wave events. Interestingly, among many contaminant supernovae, we find a possible correlation between the supernova VSTJ57.77559-59.13990 and GRB 150827A detected by Fermi-GBM. The detection efficiency of VST observations for different types of electromagnetic counterparts of gravitational wave events is evaluated for the present and future follow-up surveys

Supplement: "Localization and broadband follow-up of the gravitational-wave transient GW150914" (2016, ApJL, 826, L13)

This Supplement provides supporting material for Abbott et al. (2016a). We briefly summarize past electromagnetic (EM) follow-up efforts as well as the organization and policy of the current EM follow-up program. We compare the four probability sky maps produced for the gravitational-wave transient GW150914, and provide additional details of the EM follow-up observations that were performed in the different bands

Gaia Early Data Release 3: Structure and properties of the Magellanic Clouds

We compare the Gaia DR2 and Gaia EDR3 performances in the study of the Magellanic Clouds and show the clear improvements in precision and accuracy in the new release. We also show that the systematics still present in the data make the determination of the 3D geometry of the LMC a difficult endeavour; this is at the very limit of the usefulness of the Gaia EDR3 astrometry, but it may become feasible with the use of additional external data. We derive radial and tangential velocity maps and global profiles for the LMC for the several subsamples we defined. To our knowledge, this is the first time that the two planar components of the ordered and random motions are derived for multiple stellar evolutionary phases in a galactic disc outside the Milky Way, showing the differences between younger and older phases. We also analyse the spatial structure and motions in the central region, the bar, and the disc, providing new insights into features and kinematics. Finally, we show that the Gaia EDR3 data allows clearly resolving the Magellanic Bridge, and we trace the density and velocity flow of the stars from the SMC towards the LMC not only globally, but also separately for young and evolved populations. This allows us to confirm an evolved population in the Bridge that is slightly shift from the younger population. Additionally, we were able to study the outskirts of both Magellanic Clouds, in which we detected some well-known features and indications of new ones

The Gaia mission

Gaia is a cornerstone mission in the science programme of the EuropeanSpace Agency (ESA). The spacecraft construction was approved in 2006, following a study in which the original interferometric concept was changed to a direct-imaging approach. Both the spacecraft and the payload were built by European industry. The involvement of the scientific community focusses on data processing for which the international Gaia Data Processing and Analysis Consortium (DPAC) was selected in 2007. Gaia was launched on 19 December 2013 and arrived at its operating point, the second Lagrange point of the Sun-Earth-Moon system, a few weeks later. The commissioning of the spacecraft and payload was completed on 19 July 2014. The nominal five-year mission started with four weeks of special, ecliptic-pole scanning and subsequently transferred into full-sky scanning mode. We recall the scientific goals of Gaia and give a description of the as-built spacecraft that is currently (mid-2016) being operated to achieve these goals. We pay special attention to the payload module, the performance of which is closely related to the scientific performance of the mission. We provide a summary of the commissioning activities and findings, followed by a description of the routine operational mode. We summarise scientific performance estimates on the basis of in-orbit operations. Several intermediate Gaia data releases are planned and the data can be retrieved from the Gaia Archive, which is available through the Gaia home page. http://www.cosmos.esa.int/gai

IL “BENEFICIARIO EFFETTIVO” NELLA FISCALITA' INTERNAZIONALE ED EUROPEA. L'EVOLUZIONE DELLA CLAUSOLA TRA SOGGETTIVITA', IMPUTAZIONE DEL REDDITO E ABUSO DEL DIRITTO

Nel corso del tempo il diritto tributario internazionale, inteso come disciplina delle fattispecie con elementi di estraneità all'ordinamento nazionale, è stato protagonista di una continua evoluzione, giustificata e stimolata dal diverso atteggiarsi del commercio mondiale e dalla globalizzazione dei mercati. La legislazione odierna risulta molto più complessa rispetto al passato, per effetto della molteplicità degli scambi su base transnazionale e della impalpabilità dei criteri di collegamento tra questi e i vari ordinamenti coinvolti, cui si affianca la pluralità delle fonti, nazionali e sovranazionali, che si intersecano ed incidono sull'ordinamento di ogni singolo Stato. Le logiche sottese a quest'ultimo mutano col tempo e con esse vengono rimessi in discussione i principi condivisi, che avevano precedentemente ispirato l'adozione di specifiche regole. Ciò pone evidenti problemi di coordinamento tra giurisdizioni fiscali diverse, in base all'allocazione di ricchezza nei vari territori. L'intersecarsi delle fonti normative, unita alla celerità delle transazioni e alla loro complessità, ha determinato poi una sorta di fluidità soggettiva, con conseguente difficoltà di identificazione immediata dei soggetti del rapporto: il che ha minato alla base il criterio classico di imputazione dei redditi, rendendo evidente che il mero “possesso” di una componente reddituale non assolve più, di per sé, la funzione di individuare la persona in capo alla quale far gravare l'onere fiscale. In questo articolato panorama si è imposto agli studiosi di ricercare altrove l'elemento di collegamento tra capacità reddituale e soggettività. La clausola del “beneficiario effettivo”, introdotta a partire dal 1977 nel Modello di Convenzione OCSE, ha consentito di superare talune criticità concernenti la soggettività tributaria e l'imputazione del reddito, contemperando l'esigenza di identificare il responsabile fiscale con quella (solo apparentemente contrapposta) di evitare doppie imposizioni. Lo studio del percorso evolutivo del beneficial owner evidenzia che la riferibilità del reddito a un contribuente, sia esso persona fisica o giuridica, potrebbe avvenire non sulla base del possesso di un determinato indice di ricchezza, ma in virtù del controllo che quel soggetto esercita su quel reddito o su chi lo possiede: in questa prospettiva, il potere incondizionato di disporre del bene, prescindendo dal mero possesso materiale, rappresenterebbe un presupposto impositivo “contemporaneo”, idoneo ad intercettare, anticipandola, l'effettiva capacità di ciascun soggetto passivo. L'evoluzione della clausola del beneficiario effettivo mette in luce come, nel tempo, siano stati sempre più enfatizzati gli effettivi poteri di chi dispone e gode del reddito, disancorando da requisiti astrattamente previsti la modalità di accertamento della soggettività passiva di imposta, imponendo l'abbandono di un approccio fondato su una individuazione formale dei soggetti passivi, in favore di una valutazione che guarda alla sostanza del rapporto.International tax law, understood as the discipline of cases with elements extraneous to the national system, has been the protagonist of a continuous evolution, justified and stimulated by the different attitudes of world trade and the globalization of markets. Today's legislation is much more complex than in the past, due to the multiplicity of exchanges on a transnational basis and the intangibility of the connecting criteria between these and the various legal systems involved, which is flanked by the plurality of sources, national and extranational, which intersect and affect the organization of each individual state. The logics underlying the latter change over time and with them the shared principles that had previously inspired the adoption of specific rules are called into question. This lead to coordination problems between different tax jurisdictions, based on the allocation of wealth in the various territories. The intersection of the legal provisions, combined with the speed of transactions and their complexity, has determined a sort of subjective fluidity, with consequent difficulty in immediately identifying the subjects of the relationship: which undermined the classic criterion of imputation of income, making it clear that the mere "possession" of an income component no longer fulfills, in itself, the function of identifying the person to whom the tax burden is to be charged. In this complex panorama, scholars have been forced to look elsewhere for the link between income capacity and subjectivity. The "beneficial owner" clause, introduced in 1977 in the OECD Model Convention, has made it possible to overcome certain critical issues concerning tax subjectivity and the imputation of income, balancing the need to identify the tax officer with that (only apparently opposed) to avoid double taxation. The evolution of the “beneficial owner” shows that the traceability of income to a taxpayer, be it a natural or legal person, could take place not on the basis of the possession of a specific wealth index, but by virtue of the control that that subject exercises over that income or on who owns it: in this perspective, the unconditional power to dispose of the property, regardless of mere material possession, would represent a "contemporary" tax assumption, suitable for intercepting, anticipating it, the effective capacity of each taxable person. The evolution of the beneficial owner clause highlights how, over time, the effective powers of those who have and enjoy the income have been increasingly valued, abandoning the formal approach in favor of an assessment that looks at the substance of the relationship