1,721,065 research outputs found
VLBA observations of radio faint Fermi-LAT sources above 10 GeV
Full text linkContext. The first Fermi-LAT high-energy source catalog (1FHL), containing γ-ray sources detected above 10 GeV, is an ideal sample for characterizing the physical properties of the most extreme γ-ray sources.
Aims. We investigate the pc scale properties of a subsample of radio faint 1FHL sources with the aim of confirming the proposed blazar associations by revealing a compact high brightness temperature radio core, and we propose new low-frequency counterparts for the unassociated γ-ray sources (UGS). Moreover, we increase the number of 1FHL sources with high-resolution observations to explore the possible connection between radio and γ rays at E> 10 GeV.
Methods. We observed 84 1FHL sources, mostly blazars of high synchrotron peaked (HSP) type, in the northern sky with the Very Long Baseline Array (VLBA) at 5 GHz. These sources lack high-resolution radio observations and have at least one NRAO VLA sky survey counterpart within the 95% confidence radius. For those sources without a well-identified radio counterpart we exploit the VLBA multiple phase-center correlation capability to identify the possible low-frequency candidates.
Results. For ~93% of the sources in our sample we reveal a compact high brightness temperature radio core, which supports their proposed blazar association. The vast majority of the detected sources are radio weak, with a median VLBI flux density value of 16.3 mJy. For the detected sources we obtain an average brightness temperature on the order of 2 × 1010 K. We find a compact component for 16 UGS, for which we propose a new low-frequency association.
Conclusions. We find brightness temperature values that do not require high Doppler factors, and are in agreement with the expected values for the equipartition of energy between particles and magnetic field. We find strong indications for the blazar nature of all of the detected UGS, for which we propose new low-frequency associations. The characterization of the physical properties of this emerging population is relevant in view of the construction of the new generation Cherenkov Telescope Array
On the Time Variable Rotation Measure in the Core Region of Markarian 421
No full textIn this conference contribution, we discuss and interpret the time variable rotation measure (RM) detected in the core region of the TeV blazar Markarian 421 (Mrk 421). We monitored Mrk 421 during 2011 with one observing run per month at 15, 24, and 43 GHz with the American Very Long Baseline Array. We explore the possible connection between the RM and the accretion rate, and we investigate the Faraday screen properties and its location with respect to the jet emitting region. Among the various scenarios, the jet sheath is the most promising candidate for being the main source of Faraday rotation. We interpret the RM sign reversals observed during the one-year monitoring within the context of the magnetic tower models by invoking the presence of two nested helical magnetic fields in the relativistic jet with opposite helicities, originating through the Poynting–Robertson cosmic battery effect. The net observed RM values result from the relative contribution of both inner and outer helical fields
Key Science Goals for the Next-Generation Event Horizon Telescope
No full textFull list of the authors: Johnson, Michael D.; Akiyama, Kazunori; Blackburn, Lindy; Bouman, Katherine L.; Broderick, Avery E.; Cardoso, Vitor; Fender, Rob P.; Fromm, Christian M.; Galison, Peter; Gómez, José L.; Haggard, Daryl; Lister, Matthew L.; Lobanov, Andrei P.; Markoff, Sera; Narayan, Ramesh; Natarajan, Priyamvada; Nichols, Tiffany; Pesce, Dominic W.; Younsi, Ziri; Chael, Andrew; Chatterjee, Koushik; Chaves, Ryan; Doboszewski, Juliusz; Dodson, Richard; Doeleman, Sheperd S.; Elder, Jamee; Fitzpatrick, Garret; Haworth, Kari; Houston, Janice; Issaoun, Sara; Kovalev, Yuri Y.; Levis, Aviad; Lico, Rocco; Marcoci, Alexandru; Martens, Niels C. M.; Nagar, Neil M.; Oppenheimer, Aaron; Palumbo, Daniel C. M.; Ricarte, Angelo; Rioja, María J.; Roelofs, Freek; Thresher, Ann C.; Tiede, Paul; Weintroub, Jonathan; Wielgus, MaciekThe Event Horizon Telescope (EHT) has led to the first images of a supermassive black hole, revealing the central compact objects in the elliptical galaxy M87 and the Milky Way. Proposed upgrades to this array through the next-generation EHT (ngEHT) program would sharply improve the angular resolution, dynamic range, and temporal coverage of the existing EHT observations. These improvements will uniquely enable a wealth of transformative new discoveries related to black hole science, extending from event-horizon-scale studies of strong gravity to studies of explosive transients to the cosmological growth and influence of supermassive black holes. Here, we present the key science goals for the ngEHT and their associated instrument requirements, both of which have been formulated through a multi-year international effort involving hundreds of scientists worldwide. © 2023 by the authorsThe ngEHT design studies are funded by National Science Foundation grants AST-1935980
and AST-2034306 and the Gordon and Betty Moore Foundation (GBMF-10423). This work was
supported by the Black Hole Initiative at Harvard University, which is funded by grants from the
John Templeton Foundation and the Gordon and Betty Moore Foundation to Harvard University.
This work was supported by Volkswagen Foundation, VILLUM Foundation (grant no. VIL37766)
and the DNRF Chair program (grant no. DNRF162) by the Danish National Research Foundation. We
acknowledge financial support provided under the European Union’s H2020 ERC Advanced Grant
“Black holes: gravitational engines of discovery” grant agreement no. Gravitas–101052587. NCMM
acknowledges support from the European Union’s Horizon Europe research and innovation programme
for the funding received under the Marie Skłodowska-Curie grant agreement No. 101065772
(PhilDarkEnergy) and the ERC Starting Grant agreement No. 101076402 (COSMO-MASTER). Views
and opinions expressed are however those of the author only and do not necessarily reflect those of
the European Union or the European Research Council. Neither the European Union nor the granting
authority can be held responsible for them. NN acknowledges funding from TITANs NCN19-058
and Fondecyt 1221421.Peer reviewe
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
The Event Horizon Telescope journey from the Earth to the heart of the Milky Way
No full textIn this talk I will provide a review of the science process, imaging analysis and theoretical interpretation that led to the publication of the first Event Horizon Telescope (EHT) images of the supermassive black hole at the center of the Milky Way, known as Sagittarius A*. These EHT 1.3mm observations, carried out in 2017, allowed us to resolve the compact emission region and to reveal a morphology dominated by a bright and thick ring with a central dark depression, the so-called black hole shadow, with a diameter of ~52uas. As supported by a comprehensive suite of numerical simulations, the observed brightness distribution and morphology are consistent with the expected appearance of a ~4.3 million solar masses Kerr black hole viewed at low inclination (<50deg)
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