1,721,301 research outputs found
Probing the inner regions of supermassive black hole accretion disks and jets
Full text linkActive galactic nuclei (AGN), which host actively accreting supermassive black holes (SMBHs), present multiple mysteries. Primary among these are the physics of inner accretion disks and the production of relativistic plasma jets. Observations of two types of AGN, Seyfert galaxies and blazars, provide some of the most revealing clues toward solving these mysteries. I examine the inner accretion flow of Seyfert galaxies within ~10 Schwarzschild radii of the SMBH and the jet and its environment of blazars within ~10,000 Schwarzschild radii of the SMBH based on their X-ray, optical, and radio emission.
Through measuring the relativistic distortion of the X-ray emission from the inner accretion disk, important properties of the inner accretion disk and black hole can be determined, including the black hole spin. I present a measurement of the spin of the SMBH at the center of the Seyfert Galaxy NGC 4151 through X-ray spectroscopy with the NuSTAR and Suzaku X-ray observatories, showing that it has a near-maximal value.
Through probing blazar jets on parsec (pc) scales with high-resolution Very Long Baseline Array (VLBA) observations, fundamental details of AGN jets can be determined. I present a multi-frequency study of ten blazar jets to probe the jet shape and magnetic field on pc scales with the VLBA. I show that the jet of 3C 273 is consistent with being conical with adiabatic lateral expansion on ~pc scales. From estimating the SMBH masses in these sources with observations of emission lines with the Discovery Channel Telescope, I show that the regions probed with the VLBA are at a distance comparable to the Bondi radius.
I discuss my results on the Seyfert galaxy NGC 4151 relative to the SMBH spin census. I compare NGC 4151 with a blazar, 3C 273, to put my results in broader context of AGN. I discuss the future of these studies with high spectral resolution X-ray missions such as Athena and with high frequency, higher angular resolution interferometric arrays such as the Event Horizon Telescope
Blazar variability at high temporal resolution across the electromagnetic spectrum
Full text linkBlazars are a subclass of active galactic nuclei whose observable characteristics are generated by relativistic jets of high-energy plasma with trajectories closely aligned to the line of sight. This orientation results in extreme observed phenomena, such as ultraluminous emission, high amplitudes of variability, and high degrees of optical linear polarization. Furthermore, blazars are the most common extragalactic sources of γ-ray photons and have been proposed as sources of high-energy neutrinos.
Long-timescale monitoring of blazars has revealed relatively quiescent states interspersed with active states featuring dramatic brightening events, with timescales ranging from months to years. New, high-time-resolution observations of blazars are revealing dramatic variability on timescales as short as several minutes, the physical drivers of which are not well understood. In this dissertation, I focus on observational signatures that can potentially identify these mechanisms. To characterize this variability, I combine observations of blazars from many telescopes, obtaining a comprehensive view of events occurring in the jets.
Using the Very Long Baseline Array, I investigate moving and quasi-stationary features in the parsec-scale jets of 38 blazars. I find that the flow speeds, orientation, brightness temperatures, and opening angles of the jets in flat-spectrum radio quasars (FSRQs), BL Lacertae type objects (BL Lac objects), and radio galaxies (RGs) are statistically different, with FSRQs having the highest speeds and smallest viewing angles. Focusing on a typical source of each subclass, I characterize the optical variability observed at 2-min cadence (with the Transiting Exoplanet Survey Satellite), at gamma-ray energies (Fermi Large Area Telescope), X-ray energies (NICER, NuSTAR, and Swift satellites), and optical (ground-based instruments, especially, the Perkins telescope), including linear polarization measurements.
I find that the minimum timescale of RG variability is longer than those of the FSRQ and BL Lac object, and is likely caused by changes in the accretion disk. In contrast, the variability in the FSRQ and BL Lac object can be associated with shocks, magnetic reconnections, and turbulence in the jet. The unprecedented availability of data, which promises to expand in the future, represents a new phase of observational astronomy that provides valuable information on changes in the jets of blazars
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
Multi-waveband Behavior of Blazars
No full textThe author reviews recent progress toward understanding blazars that multi-waveband monitoring observations have advanced. The primary techniques include the compilation of multi-waveband light curves, multi-epoch VLBI images at radio wavelengths, plots of linear polarization vs. time at radio through optical wavelengths, and spectral energy distributions (SEDs). Correlations and the coincidence or lag of events across wavebands and in the images indicate where the events take place relative to the “core” that lies ≳ 0.5 pc from the central engine. Rotations of the polarization electric vector suggest a helical geometry of the magnetic field upstream of the millimeter-wave core, while rapid fluctuations in degree and position angle of polarization imply that the jet plasma is turbulent in and downstream of the core. The author is developing a numerical model that simulates the emission from such turbulence as it interacts with a conical standing shock in the core region
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
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
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