1,721,349 research outputs found
The Fermi gamma ray sky: Summary of recent observations
Full text linkThe Fermi Gamma-ray Space Telescope was launched more than 13 years ago and since then it has dramatically changed our knowledge of the gammaray sky. With more than three billions photons from the whole sky, collected in the
energy range between 20 MeV and more than 300 GeV, and beyond 6,000 detected sources, LAT observations have been crucial to improving our understanding of particle acceleration and gamma-ray production in astrophysical sources. In this review, I will review recent science highlights from the LAT. I will focus on the recent source catalog release, as well as on the main transient phenomena seen with the LAT with multi-wavelength and multi-messenger connection
Point source detection and flux determination with PGWave
No full textOne of the largest uncertainties in the Point Source (PS) studies, at Fermi-LAT energies [1], is the uncertainty in the diffuse background. In general there are two approaches for PS analysis: background-dependent methods, that include modeling of the diffuse background, and background-independent methods. In this work we study PGWave [2], which is one of the background-independent methods, based on wavelet filtering to find significant clusters of gamma rays. PGWave is already used in the Fermi-LAT catalog pipeline for finding candidate sources. We test PGWave, not only for source detection, but especially to estimate the flux without the need of a background model. We use Monte Carlo (MC) simulation to study the accuracy of PS detection and estimation of the flux. We present preliminary results of these MC studies
A search for new sources below 100 MeV in the Fermi-LAT data
No full textThe Fermi Large Area Telescope (LAT) has discovered more than 3000 sources between 100 MeV and 300 GeV (3FGL), including over 200 γ -ray pulsars. The majority of these pulsars have their spectral energy peaks (in ν Fν ) above 1 GeV. There exists another population of very energetic, young rotation-powered pulsars having non-thermal hard X-ray emission with spectral energy distributions (SEDs) peaking around 1 MeV, but having no detected emission by Fermi LAT above 100 MeV. Previous analyses of point sources in the gamma ray range were done only below 30 MeV (COMPTEL) or above 100 MeV (Fermi-LAT). Below 30 MeV, the imaging Compton telescope (COMPTEL) detected 26 steady sources in the energy range from 0.75 to 30 MeV. Since the Fermi LAT detects gamma rays down to 20 MeV, we create a list of sources detected in the energy range between 30 MeV and 100 MeV, using PGWave, a background-independent tool that makes use of a wavelet transform. This closes a gap of point source analysis between the COMPTEL catalog and the Fermi-LAT catalog and provides information and constraint for this missing MeV pulsar population
The first catalog of Fermi-LAT sources below 100 MeV
No full textWe present the first Fermi Large Area Telescope (LAT) low energy catalog (1FLE) of sources detected in the energy range 30 - 100 MeV. The imaging Compton telescope (COMPTEL) onboard NASA's Compton Gamma-Ray Observatory detected sources below 30 MeV, while catalogs of point sources released by the Fermi-LAT and EGRET collaborations use energies above 100 MeV. Because the Fermi-LAT detects gamma rays with energies as low as 20 MeV, we create a list of sources detected in the energy range between 30 and 100 MeV, which closes a gap of point source analysis between the COMPTEL catalog and the Fermi-LAT catalogs. One of the main challenges in the analysis of point sources is the construction of the background diffuse emission model. In our analysis, we use a background-independent method to search for point-like sources based on a wavelet transform implemented in the PGWave code. The 1FLE contains 198 sources detected above 3σ significance with eight years and nine months of the Fermi-LAT data. For 187 sources in the 1FLE catalog we have found an association in the Fermi-LAT 3FGL catalog: 148 are extragalactic, 22 are Galactic, and 17 are unclassified in the 3FGL. The ratio of the number of flat spectrum radio quasars (FSRQ) to BL Lacertae (BL Lacs) in 1FLE is three to one, which can be compared with an approximately 1:1 ratio for the 3FGL or a 1:6 ratio for 3FHL. The higher ratio of the FSRQs in the 1FLE is expected due to generally softer spectra of FSRQs relative to BL Lacs. Most BL Lacs in 1FLE are of low-synchrotron peaked blazar type (18 out of 31), which have softer spectra and higher redshifts than BL Lacs on average. Correspondingly, we find that the average redshift of the BL Lacs in 1FLE is higher than in 3FGL or 3FHL. There are 11 sources that do not have associations in the 3FGL. Most of the unassociated sources either come from regions of bright diffuse emission or have several known 3FGL sources in the vicinity, which can lead to source confusion. The remaining unassociated sources have significance less than 4σ
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
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
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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