1,721,500 research outputs found
The Fermi Large Area Telescope as a cosmic-ray electron detector
No full textSuccessfully launched into a low-Earth orbit on June 11, 2008, the Fermi Large Area Telescope (LAT) is a pair conversion telescope specifically designed to survey the gamma-ray sky with unprecedented sensitivity and resolution. The broad list of exciting scientific results that the observatory has achieved in the first year of operation, though, is not limited to photon science, as it includes the first high-statistics measurement of the cosmic-ray electron spectrum for energies above 20 GeV. Here we thoroughly discuss the main aspects of the data analysis underlying this measurement and present the ongoing study aimed at lowering the energy threshold of the primary spectrum (measured by the LAT) as well as exploring the properties of the electron albedo population
An observation-simulation and analysis framework for the Imaging X-ray Polarimetry Explorer (IXPE)
No full textWe present a new simulation framework, based on the Python programming language and specifically developed for the Imaging X-ray Polarimetry Explorer (IXPE) mission. Starting from an arbitrary source model (including morphological, temporal, spectral and polarimetric information), this framework uses the instrument response functions to produce fast and realistic observation-simulations. The generated event lists can be directly fed into the standard X-ray visualization and analysis tools, including XSPEC—which make this framework a useful tool not only for simulating observations of astronomical sources, but also to develop and test end-to-end analysis chains. We will give an overview of the basic architecture of the software and we will present a few physically interesting case studies in the context of the IXPE mission
Exploring the accretion-ejection geometry of GRS 1915+105 in the obscured state with future X-ray spectro-polarimetry
Full text linkContext. GRS 1915+105 has been in a bright flux state for more than two decades, but in 2018 a significant drop in flux was observed, partly due to changes in the central engine along with increased X-ray absorption. Aims. The aim of this work is to explore how X-ray spectro-polarimetry can be used to derive the basic geometrical properties of the absorbing and reflecting matter. In particular, the expected polarisation of the radiation reflected off the disc and the putative outflow is calculated. Methods. We used NuSTAR data collected after the flux drop to derive the parameters of the system from hard X-ray spectroscopy. The spectroscopic parameters were then used to derive the expected polarimetric signal, using results from a Monte Carlo radiative transfer code, both in the case of neutral and fully ionised matter. Results. From the spectral analysis, we find that the continuum emission becomes softer with increasing flux, and that in all flux levels the obscuring matter is highly ionised. This analysis, on the other hand, confirms that spectroscopy alone is unable to put constraints on the geometry of the reflectors. Simulations show that X-ray polarimetric observations, such as those that will be provided soon by the Imaging X-ray Polarimetry Explorer (IXPE), will help to determine the geometrical parameters which are left unconstrained by the spectroscopic analysis
XIMPOL: A new X-ray polarimetry observation-simulation and analysis framework
Full text linkWe present a new simulation framework, based on the python programming language and specifically developed for X-ray polarimetric applications, called XIMPOL. Starting from an arbitrary source model (including morphological, temporal, spectral and polarimetric informations), XIMPOL uses the response functions of the detector under study to produce fast and realistic observationsimulations. A Chandra-to-XIMPOL converter is also available within the framework, designed to convert a real Chandra observation into a XIMPOL simulation. The generated output files can be directly fed into the standard visualization and analysis tools, including XSPEC, which make XIMPOL a useful tool not only for simulating observations of astronomical sources, but also to develop and test end-to-end analysis chains
Prospects for a polarimetric mapping of the Sgr A molecular cloud complex with IXPE
Full text linkContext. The X-ray polarization degree of the molecular clouds surrounding Sgr A∗ is expected to be greatly reduced due to the mixing of the polarized reflection emission with the unpolarized thermal emission that pervades the Galactic center region. This makes this type of observation a challenging experiment for the upcoming Imaging X-ray Polarimeter Explorer (IXPE), whose launch is expected in 2021. Aims. We aim to determine the detectability of four molecular clouds of the Sgr A complex (MC2, Bridge B2, Bridge E, and G0.11-0.11) in a realistic IXPE pointing of the Sgr A field of view. We assess the minimum increase in the detectable polarization when a molecular cloud is off-axis. We provide two different strategies for reconstructing the intrinsic cloud polarization once the data is available. Methods. We used the Monte Carlo tool ixpeobssim to simulate IXPE observations of the Sgr A molecular cloud complex. We used Chandra maps and spectra to model the diffuse emission in the Galactic center region along with a realistic model of the instrumental and diffuse background. We created synthetic polarization products of the unpolarized emission and combined them with a test data set obtained from a simulation of a 2 Ms long IXPE observation to retrieve the intrinsic polarization degree of the molecular clouds. Results. We find that for the molecular clouds considered here, the minimum detectable polarization (MDP) increases by ∼1-15% with respect to the case in which a cloud is observed on-axis. We successfully retrieve the intrinsic polarization degree in the 4.0-8.0 keV band and line-of-sight distance of one of them taken as an example, namely, G0.11-0.11, by correcting the observed (i.e., for a 2 Ms-long simulation) polarization degree map using either a synthetic dilution map or a Stokes intensity map of the unpolarized emission. With both methods, the position of the cloud along the line-of-sight is derived from the reconstructed polarization degree with an uncertainty of 7 and 4 pc, respectively. Conclusions. We confirm the results of previous studies, finding that G0.11-0.11 is the most promising target. For the Sgr A molecular complex region, we propose an observation strategy that may permit detection up to three clouds in the 4.0-8.0 keV band, depending on their true line-of-sight position. We demonstrate that by using simulated data products of the unpolarized components, it is possible to clean up the observed polarization maps from the environmental contamination. The methods we present here are potentially useful for the analysis of X-ray polarimetric data of any extended source that is affected by environmental dilution of the polarized signal. To accurately measure (i.e, with uncertainties of the order of a few parsec) the distance of the cloud along the line-of-sight, a high-quality spectrum and image of the clouds quasi simultaneous to the IXPE pointing are needed
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Gas Pixel Detectors
No full textWith the Gas Pixel Detector (GPD), the class of micro-pattern gas detectors has reached a complete integration between the gas amplification structure and the read-out electronics. To obtain this goal, three generations of application-specific integrated circuit of increased complexity and improved functionality has been designed and fabricated in deep sub-micron CMOS technology. This implementation has allowed manufacturing a monolithic device, which realizes, at the same time, the pixelized charge-collecting electrode and the amplifying, shaping and charge measuring front-end electronics of a GPD. A big step forward in terms of size and performances has been obtained in the last version of the 0.18 mu m CMOS analog chip, where over a large active area of 15 x 15 mm(2) a very high channel density (470 pixels/mm(2)) has been reached. On the top metal layer of the chip, 105,600 hexagonal pixels at 50 mu m pitch have been patterned. The chip has customable self-trigger capability and includes a signal pre-processing function for the automatic localization of the event coordinates. In this way, by limiting the output signal to only those pixels belonging to the region of interest, it is possible to reduce significantly the read-out time and data volume. In-depth tests performed on a GPD built up by coupling this device to a fine pitch (50 mu m) gas electron multiplier are reported. Matching of the gas amplification and read-out pitch has let to obtain optimal results. A possible application of this detector for X-ray polarimetry of astronomical sources is discussed. (c) 2006 Elsevier B.V. All rights reserved
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
- …
