131,373 research outputs found
Fermi LAT View of a Sample of Flaring gamma-Ray AGNs
No full textIn the first 3.5 years of operations, Fermi detected several sources whose flaring activity brought them to exceed daily fluxes brighter than F(E > 100MeV) > 10(-6) ph cm(-2) s(-1). These episodes were promptly reported to the scientific community by the Fermi collaboration by means of astronomer telegrams (ATels). We focus our attention on the sample composed by these flaring sources, most of which are blazars, known to be extremely variable over the whole electromagnetic spectrum, from radio to gamma-ray energies. We study properties of the selected sample and compare them to general characteristics of the Fermi source catalogue
MeSH term explosion and author rank improve expert recommendations
No full textInformation overload is an often-cited phenomenon that reduces the productivity, efficiency and efficacy of scientists. One challenge for scientists is to find appropriate collaborators in their research. The literature describes various solutions to the problem of expertise location, but most current approaches do not appear to be very suitable for expert recommendations in biomedical research. In this study, we present the development and initial evaluation of a vector space model-based algorithm to calculate researcher similarity using four inputs: 1) MeSH terms of publications; 2) MeSH terms and author rank; 3) exploded MeSH terms; and 4) exploded MeSH terms and author rank. We developed and evaluated the algorithm using a data set of 17,525 authors and their 22,542 papers. On average, our algorithms correctly predicted 2.5 of the top 5/10 coauthors of individual scientists. Exploded MeSH and author rank outperformed all other algorithms in accuracy, followed closely by MeSH and author rank. Our results show that the accuracy of MeSH term-based matching can be enhanced with other metadata such as author rank
Going Beyond Counting First Authors in Author Co-citation Analysis
No full textThe 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
"Closing the R&D Gap, Evaluating the Sources of R&D Spending"
No full textBoth spending and tax policies have been implemented in the United States with the goal of stimulating private sector research and development (R&D). Karier questions whether current R&D policy, especially the research and experimentation tax credit, can contribute to closing the gap between nondefense expenditures on R&D in the United States and such expenditures in other countries, such as Japan and Germany. He also explores possible changes to our current R&D policy to make it more effective.
FERMI LAT view of a sample of flaring gamma-ray AGN
No full textRadio-loud AGNs, especially blazars, are sources known to be extremely variable over the whole electromagnetic spectrum, from radio to γ-ray energies. Up to now, the Large Area Telescope (LAT), the primary instrument on board the Fermi satellite, has detected more than 850 sources that could be associated with AGNs. Several of them underwent at least one flaring episode at γ-rays during the time spanned by the Fermi observations. We present and discuss a list of flaring AGNs detected during the first 3.5 years of Fermi operations with fluxes brighter than F(E > 100 MeV) > 10−6 ph cm −2 s −1, the threshold set by the LAT Collaboration for issuing an Astronomer Telegram. More, the general characteristics of the selected sample are investigated
A novel approach for pre-filtering event sources using the von Mises–Fisher distribution
No full textSearching for as yet undetected γ-ray sources is one of the main stated goals of the Fermi Large Area Telescope Collaboration. In this paper, we explore the capability of a filtering method based on a finite mixture of von Mises–Fisher distributions. The proposed procedure is specifically designed to handle data with support on the unit sphere. The assumption of a parametric model for each high energy emitting source allows us to derive an explicit expression for both the direction of the sources and their angular resolutions. The corresponding measures are based on the directional mean and the quantiles of the single mixture components. Sound criteria of model selection can provide an automatic way to determine the number of detected sources. Additionally, a likelihood-ratio test is developed to evaluate their significance. The procedure is tested on simulated data sets of photon emissions from high energy sources within the energy range [10 − 1 , 000] GeV. A real data example consisting of a sample of the Fermi LAT data collected over a period of about 7.2 years within the energy range [10 − 1 , 000] GeV , in a subregion of the γ-ray sky, is furthermore provided
A novel approach for pre-filtering event sources using the von Mises–Fisher distribution
No full textThree Years of Fermi LAT Flare Advocate Activity
No full textThe Fermi Flare Advocate (also known as Gamma-ray Sky Watcher, FA-GSW)
service provides for a daily quicklook analysis and review of the
high-energy gamma-ray sky seen by the Fermi Gamma-ray Space Telescope.
The duty offers alerts for potentially new gamma-ray sources,
interesting transients and relevant flares. A public weekly digest
containing the main highlights about the GeV gamma-ray sky is published
in the web-based Fermi Sky Blog. During the first 3 years of all-sky
survey, more than 150 Astronomical Telegrams, several alerts to the TeV
Cherenkov telescopes, and targets of opportunity to Swift and other
observatories have been realized. This increased the rate of
simultaneous multi-frequency observing campaigns and the level of
international cooperation. Many gamma-ray flares from blazars (like
extraordinary outbursts of 3C 454.3, intense flares of PKS 1510-089, 4C
21.35, PKS 1830-211, AO 0235+164, PKS 1502+106, 3C 279, 3C 273, PKS
1622-253), short/long flux duty cycles, unidentified transients near the
Galactic plane (like J0910-5041, J0109+6134, the Galactic center
region), flares associated to Galactic sources (like the Crab nebula,
the nova V407 Cyg, the microquasar Cyg X-3), emission of the quiet and
active sun, were observed by Fermi and communicated by FA-GSWs
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