Progettazione ed implementazione dell'infrastruttura server per un sistema distribuito di scansione e mappatura della rete Internet

L'obiettivo della tesi è la realizzazione di un infrastruttura Server all'interno di un sistema distribuito che permetta la raccolta di informazioni sulla rete e di generarne una mappa completa. Fra le mansioni del Server, le principali sono la scelta dei Job da assegnare ad ognuno dei Client, in modo da assicurare una copertura efficiente della rete, e la raccolta dei dati generati dai Client; inoltre il Server si occupa dell'integrazione dei dati, dell'isolamento dei casi anomali e della risoluzione delle ambiguità che si possono riscontrare durante le analisi. La scelta dei Job viene effettuata in base a condizioni opportunistiche come la posizione geografica del Client o la rete a cui esso è connesso. The thesis main goal is to realize the Server infrastructure for a distributed system with the aim to collect information about the Internet and to generate a detailed map of it. Amongst the duties of the Server, the main ones are the correct choosing of a Job to assign to each Client, so to ensure an efficient covering of the Internet, and the collecting of the data generated from the Clients; the Server's due to merge these data, to identify anomalities within it and to solve ambigous cases which can be found during the analysis. Each Job is chosen considering opportunistic conditions like the Client's geographic location or the network he's connected to

Frontier knowledge and scientific production: evidence from the collapse of international science

We show that WWI and the subsequent boycott against Central scientists severely interrupted international scientific cooperation. After 1914, citations to recent research from abroad decreased and paper titles became less similar (evaluated by Latent Semantic Analysis), suggesting a reduction in international knowledge flows. Reduced international scientific cooperation led to a decline in the production of basic science and its application in new technology. Specifically, we compare productivity changes for scientists who relied on frontier research from abroad, to changes for scientists who relied on frontier research from home. After 1914, scientists who relied on frontier research from abroad published fewer papers in top scientific journals, produced less Nobel Prize-nominated research, introduced fewer novel scientific words, and introduced fewer novel words that appeared in the text of subsequent patent grants. The productivity of scientists who relied on top 1% research declined twice as much as the productivity of scientists who relied on top 3% research. Furthermore, highly prolific scientists experienced the starkest absolute productivity declines. This suggests that access to the very best research is key for scientific and technological progres

A Survey of Preference Estimation with Unobserved Choice Set Heterogeneity

We provide an introduction to the estimation of discrete choice models when choice sets are heterogeneous and unobserved to the econometrician. We survey the two most popular approaches: “integrating over” and “differencing out” unobserved choice sets. Inspired by Chamberlain (1980)’s original idea of constructing sufficient statistics from observed choices, we introduce the term “sufficient set” to refer to any combination of observed choices that lies within the true but unobserved choice set. The concept of sufficient set helps to unify notation and organize our thinking, to map econometric assumptions onto economic models, and to implement both methods in practice

Discovery of hard phase lags in the pulsed emission of GRO J1744-28

We report on the discovery and energy dependence of hard phase lags in the 2.14 Hz pulsed profiles of GRO J1744-28. We used data from XMM-Newton and NuSTAR. We were able to well constrain the lag spectrum with respect to the softest (0.3--2.3 keV) band: the delay shows increasing lag values reaching a maximum delay of $\sim$ 12 ms, between 6 and 6.4 keV. After this maximum, the value of the hard lag drops to 7 ms, followed by a recovery to a plateau at 9 ms for energies above 8 keV. NuSTAR data confirm this trend up to 30 keV, but the measurements are statistically poorer, and therefore, less constraining. The lag-energy pattern up to the discontinuity is well described by a logarithmic function. Assuming this is due to a Compton reverberation mechanism, we derive a size for the Compton cloud $R_{\rm{cc}}$ $\sim$ 120 $R_{\rm g}$, consistent with previous estimates on the magnetospheric radius. In this scenario, the sharp discontinuity at $\sim$ 6.5 keV appears difficult to interpret and suggests the possible influence of the reflected component in this energy range. We therefore propose the possible coexistence of both Compton and disk reverberation to explain the scale of the lags and its energy dependence.Comment: Accepted for publication in MNRAS Letters on 2016 June 0

Real analytic discrete choice models of demand : theory and implications

We demonstrate that a large class of discrete choice models of demand can be approximated by real analytic demand models. We obtain this result by combining (i) a novel real analytic property of the mixed logit and the mixed probit models with any distribution of random coefficients and (ii) an approximation property of finite mixtures of Gumbel and Gaussian distributions. To illustrate some of the implications of this result, we discuss how real analyticity facilitates nonparametric and semi-nonparametric identification, extrapolation to hypothetical counterfactuals, numerical implementation of demand inverses, and numerical implementation of the Maximum Likelihood Estimator

Study of the reflection spectrum of the accreting neutron star GX 3+1 using XMM-Newton and INTEGRAL

Broad emission features of abundant chemical elements, such as Iron, are commonly seen in the X-ray spectra of accreting compact objects and their studies can provide useful information about the geometry of the accretion processes. In this work, we focus our attention on GX 3+1, a bright, persistent accreting low mass X-ray binary, classified as an atoll source. Its spectrum is well described by an accretion disc plus a stable comptonizing, optically thick corona which dominates the X-ray emission in the 0.3-20 keV energy band. In addition, four broad emission lines are found and we associate them with reflection of hard photons from the inner regions of the accretion disc where doppler and relativistic effects are important. We used self-consistent reflection models to fit the spectra of the 2010 XMM-Newton observation and the stacking of the whole datasets of 2010 INTEGRAL observations. We conclude that the spectra are consistent with reflection produced at ~10 gravitational radii by an accretion disc with an ionization parameter of xi~600 erg cm/s and viewed under an inclination angle of the system of ~35{\deg}. Furthermore, we detected for the first time for GX 3+1, the presence of a powerlaw component dominant at energies higher than 20 keV, possibly associated with an optically thin component of non-thermal electrons.Comment: Accepted to appear on MNRAS, 9 pages, 5 figur

Broad-band spectral analysis of the accreting millisecond X-ray pulsar SAX J1748.9-2021

We analyzed a 115 ks XMM-Newton observation and the stacking of 8 days of INTEGRAL observations, taken during the raise of the 2015 outburst of the accreting millisecond X-ray pulsar SAX J1748.9-2021. The source showed numerous type-I burst episodes during the XMM-Newton observation, and for this reason we studied separately the persistent and burst epochs. We described the persistent emission with a combination of two soft thermal components, a cold thermal Comptonization component (~2 keV) and an additional hard X-ray emission described by a power-law (photon index ~2.3). The continuum components can be associated with an accretion disc, the neutron star (NS) surface and a thermal Comptonization emission coming out of an optically thick plasma region, while the origin of the high energy tail is still under debate. In addition, a number of broad (~0.1-0.4 keV) emission features likely associated to reflection processes have been observed in the XMM-Newton data. The estimated 1.0-50 keV unabsorbed luminosity of the source is ~5x10^37 erg/s, about 25% of the Eddington limit assuming a 1.4 solar mass NS. We suggest that the spectral properties of SAX J1748.9-2021 are consistent with a soft state, differently from many other accreting X-ray millisecond pulsars which are usually found in the hard state. Moreover, none of the observed type-I burst reached the Eddington luminosity. Assuming that the burst ignition and emission are produced above the whole NS surface, we estimate a neutron star radius of ~7-8 km, consistent with previous results.Comment: Accepted for publication in MNRAS; 12 pages, 9 figures, 2 table