Measuring the polarization of the light coming from celestial sources provides a powerful tool to investigate their internal structure, as well as to study the physics at work in the emission ...processes. However, due to the technical challenges it presents, X-ray polarimetry is still quite undeveloped compared to other branches of astronomy, with the only significant measurement ever performed dating back to more than 40 years ago. The Imaging X-ray Polarimetry Explorer (IXPE), the next mission of the NASA SMall EXplorer program, is aimed at curtailing such a gap. Scheduled for launch in 2021, it will observe for the first time the polarization in the soft X-ray band (2–10 keV) of dozens of astrophysical objects such as black holes, active galactic nuclei, supernova remnants, magnetars and pulsar wind nebulae. At the very heart of the concept of IXPE lays the latest generation of a technology developed by INFN over the last twenty years, the Gas Pixel Detector (GPD). It is an innovative gas detector based on the photo-electric effect, which is able to image with high resolution the short tracks produced by photo-electrons of few keV in order to extract the relevant observables for imaging, spectroscopy and polarimetry. I will describe the development and design of the Gas Pixel Detector and discuss its most critical capabilities in relation to the scientific program of the IXPE mission.
X-ray polarimetry based on gas pixel detectors (GPDs) has reached a high level of maturity thanks to the Imaging X-ray Polarimeter Explorer (IXPE) providing the first-ever spatially resolved ...polarimetric measurements. However, as this a new technique, a few unexpected effects have emerged in the course of in-flight operations. In particular, it was almost immediately found that, the unpolarized calibration sources on-board were showing radially polarized halos. The origin of these features was recognized in a correlation between the error in reconstructing the absorption point of the X-ray photon and the direction of its electric field vector. Here, we present and discuss this effect in detail, showing that it is possible to provide a simple and robust mathematical formalism to handle it. We further show its role and relevance for the recent IXPE measures as well as for the use of GPD-based techniques in general. We also illustrate how to model it in the context of studying extended sources.
The flux of positrons and electrons (e+ + e−) has been measured by the Fermi Large Area Telescope (LAT) in the energy range between 7 GeV and 2 TeV. We discuss a number of interpretations of Pass 8 ...Fermi-LAT e+ + e− spectrum, combining electron and positron emission from supernova remnants (SNRs) and pulsar wind nebulae (PWNe), or produced by the collision of cosmic rays (CRs) with the interstellar medium. We find that the Fermi-LAT spectrum is compatible with the sum of electrons from a smooth SNR population, positrons from cataloged PWNe, and a secondary component. If we include in our analysis constraints from the AMS-02 positron spectrum, we obtain a slightly worse fit to the e+ + e− Fermi-LAT spectrum, depending on the propagation model. As an additional scenario, we replace the smooth SNR component within 0.7 kpc with the individual sources found in Green's catalog of Galactic SNRs. We find that separate consideration of far and near sources helps to reproduce the e+ + e− Fermi-LAT spectrum. However, we show that the fit degrades when the radio constraints on the positron emission from Vela SNR (which is the main contributor at high energies) are taken into account. We find that a break in the power-law injection spectrum at about 100 GeV can also reproduce the measured e+ + e− spectrum and, among the CR propagation models that we consider, no reasonable break of the power-law dependence of the diffusion coefficient can modify the electron flux enough to reproduce the observed shape.
Due to be launched in late 2021, the imaging X-ray polarimetry explorer (IXPE) is a NASA Small Explorer mission designed to perform polarization measurements in the 2-8-keV band, complemented with ...imaging, spectroscopy, and timing capabilities. At the heart of the focal plane, there are a set of three polarization-sensitive gas pixel detectors, each based on a custom-integrated circuit acting as a charge-collecting anode, with their readout electronics. We designed an integrated test equipment based on the VMEbus protocol to characterize the performance of such detectors, debug the IXPE readout electronics, and perform stable data acquisitions to study the whole system. The test campaign was conducted to validate the readout processor against the mission requirements in more demanding conditions than the worst case foreseen for in-orbit operations. The test showed that the device under test could sustain an event rate with a 5.9-keV source, nearly twice the worst case of 300 Hz at 3 keV, with a dead time down to 1.1 ms. Additionally, we performed various scans in the readout electronics parameter space to optimize its performance in observing celestial sources. The success of the test campaign proved the robustness and reliability of the system in these conditions, legitimating its adoption for the IXPE mission.
Abstract Background The Luminex Single-Antigen Beads (LSA) assay allows an accurate detection and characterization of preexisting donor-specific antibodies (DSA) in kidney transplant candidates. But ...the ability of LSA to detect quite low levels of antibodies makes it hard to correctly predict crossmatch results in donor selection. In this study we retrospectively analyzed the accuracy of our virtual crossmatch (v-XM) protocol, which was used for selection of potential kidney transplant recipients, in predicting the results of actual crossmatch (a-XM) in cadaver-donor renal transplantation. We also investigated correlation between negative a-XM results and strength/specificity of preformed DSA. Methods The correlation between negative v-XMs and a-XMs performed in 2007–2012 at the Regional Transplant Center of the Lazio Region, Italy, was analyzed. In carrying out v-XM, the donor HLA molecules against which patients showed LSA-detected DSA with normalized mean fluorescence intensity (MFI) ≥5,000 were considered to be “unacceptable DSA,” and LSA-DSA showing MFI <5,000 were defined as “acceptable DSA.” All cadaver donors had been typed for HLA-A, -B, -DR, and -DQB molecules by sequence-specific primer methods. On the basis of a negative v-XM, we performed 507 a-XMs between serum samples from 256 renal transplant candidates and T/B lymphocytes from 302 cadaver donors with the use of both complement-dependent cytotoxicity (CDC) and flow cytometry (FC) methods. Results The v-XM negative results showed good correlation with both CDC and FC a-XMs (97% and 90%, respectively). The sensitivity of v-XM was 100%; this high value was related to the lack of false-negative DSA results. The limited specificity with both techniques (CDC-XM, 74%; FC-XM, 79%) was due to the presence of “acceptable” and/or anti-DQA/DPB DSA in some patient sera used to perform the a-XMs. During the study period, 171 (67%) of the 256 sensitized patients received a kidney transplant: 30% of these had “acceptable DSA” and/or anti-DQA/DPB DSA. No antibody-mediated rejection due to preformed HLA-DSA was observed. Conclusions Our v-XM protocol showed high sensitivity in predicting donor-recipient immunologic compatibility. The results of this study also demonstrated the importance of evaluating DSA strength for implementing v-XM results in the selection of kidney transplant recipients. Moreover, the finding of anti-DQA/DPB DSA, especially in serum samples that gave positive results with the use of both CDC and FC a-XMs, highlights the importance of defining all of the donor HLA molecules to perform an accurate v-XM.
Art and Experience Manfreda, Luigi A
Sociétés (Paris),
12/2021, Letnik:
153, Številka:
3
Journal Article
Recenzirano
Notre réflexion a pour but d’analyser certains aspects de l’art du XX e siècle à partir des transformations que l’expérience du monde a éprouvées pendant l’époque contemporaine. Comme l’ont bien ...illustré dans leurs œuvres Simmel et Benjamin, l’idée même d’expérience a subi une crise profonde, conjointement à la crise du quotidien que l’Occident a connue en raison du progrès technologique. L’art du XX e siècle tente de répondre à cette crise en se tournant d’une part vers la recherche de l’ immédiat , d’autre part vers une réappropriation, à travers un travail artistique conscient, du faire au sens propre du terme.
Art and Experience [1] Manfreda, Luigi A
Sociétés (Paris),
2021, Letnik:
153, Številka:
3
Journal Article
Recenzirano
This text aims to analyze some aspects of twentieth-century art based on the transformations that the experience of the world has undergone in the contemporary age. As Simmel and Benjamin show in ...their works, the very idea of experience has undergone a profound crisis, which mirrors that of daily life in the West, marked by technological progress. Twentieth-century art tries to respond to this crisis by turning, on the one hand, to the search for the immediate and, on the other hand, to a reappropriation, through a conscious artistic work, of doing in the truest sense of the term.
We report a molecular modeling study to investigate the polymer–carbon black (CB) composite–analyte interactions in resistive sensors. These sensors comprise the JPL electronic nose (ENose) sensing ...array developed for monitoring breathing air in human habitats. The polymer in the composite is modeled based on its stereoisomerism and sequence isomerism, while the CB is modeled as uncharged naphthalene rings with no hydrogens. The Dreiding 2.21 force field is used for the polymer, solvent molecules and graphite parameters are assigned to the carbon black atoms. A combination of molecular mechanics (MM) and molecular dynamics (NPT-MD and NVT-MD) techniques are used to obtain the equilibrium composite structure by inserting naphthalene rings in the polymer matrix. Polymers considered for this work include poly(4-vinylphenol), polyethylene oxide, and ethyl cellulose. Analytes studied are representative of both inorganic and organic compounds. The results are analyzed for the composite microstructure by calculating the radial distribution profiles as well as for the sensor response by predicting the interaction energies of the analytes with the composites.
Ce texte propose quelques considérations sur l’idée de tragédie chez Walter Benjamin. Dans l’ Ursprung des deutschen Trauerspiel et dans quelques courts écrits précédant l’ouvrage de 1928, Benjamin ...s’attarde sur l’idée de tragédie dans le but principal de mettre en évidence celle de Trauerspiel. Néanmoins, ses pages sur la tragédie antique sont très pénétrantes et mettent en évidence certains de ses motifs essentiels. Le héros tragique pour Benjamin vit dans un temps accompli, tout compris dans l’horizon du mythe. Son attitude, qui ne connaît aucune médiation, finit par le faire entrer en collision avec l’ethos consolidé de la polis , et le condamne ainsi au silence. Pour l’auteur de l’essai, ce silence inaugure néanmoins un nouveau dialogue dans les profondeurs de l’individualité du héros.
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