3,788 research outputs found
Intervention or Collaboration?:Rethinking Information and Communication Technologies for Development
Over the past decades information system developers and knowledge engineers in ICT projects in wealthy regions of the world have come to realize that technical work can only be successful when situated in a broader organizational context. However, for low-resource environments (or example rural Africa), where contextual embedding is even more demanding given the complexity of these environments, practical, context-oriented methodologies how to "do" information systems engineering are still lacking. This book gives a basic but thorough insight how to develop information systems and services for people in low resource environments, from a socio-technical, information systems engineering perspective, presenting field-validated methods that cover the complete lifecycle of information systems engineering, with emphasis on context analysis, needs assessment, use case and requirements analysis and (business) sustainability analysis. Since technical development does not go without critical reflection, this book also investigates which (tacit) assumptions affect the way technologies are implemented in poor, low-resource environments. Linking collaborative sociotechnical development with theories of complexity and social networks of innovation, this book offers a reflective and critical approach to information and communication technologies for development
Quasars: from the Physics of Line Formation to Cosmology
Quasars accreting matter at very high rates (known as extreme Population A
[xA] or super-Eddington accreting massive black holes) provide a new class of
distance indicators covering cosmic epochs from the present-day Universe up to
less than 1 Gyr from the Big Bang. The very high accretion rate makes it
possible that massive black holes hosted in xA quasars radiate at a stable,
extreme luminosity-to-mass ratio. This in turns translates into stable physical
and dynamical conditions of the mildly ionized gas in the quasar low-ionization
line emitting region. In this contribution, we analyze the main optical and UV
spectral properties of extreme Population A quasars that make them easily
identifiable in large spectroscopic surveys at low-z (z < 1) and intermediate-z
(2 < z < 2.6), and the physical conditions that are derived for the formation
of their emission lines. Ultimately, the analysis supports the possibility of
identifying a virial broadening estimator from low-ionization line widths, and
the conceptual validity of the redshift-independent luminosity estimates based
on virial broadening for a known luminosity-to-mass ratio.Comment: 13 pages, 5 figures. Invited lecture at SPIG 2018, Belgrade. To
appear in Ato
The First Spectroscopically Resolved Sub-parsec Orbit of a Supermassive Binary Black Hole
One of the most intriguing scenarios proposed to explain how active galactic
nuclei are triggered involves the existence of a supermassive binary black hole
system in their cores. Here we present an observational evidence for the first
spectroscopically resolved sub-parsec orbit of a such system in the core of
Seyfert galaxy NGC 4151. Using a method similar to those typically applied for
spectroscopic binary stars we obtained radial velocity curves of the
supermassive binary system, from which we calculated orbital elements and made
estimates about the masses of components. Our analysis shows that periodic
variations in the light and radial velocity curves can be accounted for an
eccentric, sub-parsec Keplerian orbit of a 15.9-year period. The flux maximum
in the lightcurve correspond to the approaching phase of a secondary component
towards the observer. According to the obtained results we speculate that the
periodic variations in the observed H{\alpha} line shape and flux are due to
shock waves generated by the supersonic motion of the components through the
surrounding medium. Given the large observational effort needed to reveal this
spectroscopically resolved binary orbital motion we suggest that many such
systems may exist in similar objects even if they are hard to find. Detecting
more of them will provide us with insight into black hole mass growth process.Comment: 29 pages, 10 figures, published in ApJ, 759, 11
Black hole mass estimates in quasars - A comparative analysis of high- and low-ionization lines
The inter-line comparison between high- and low-ionization emission lines has
yielded a wealth of information on the quasar broad line region (BLR) structure
and dynamics, including perhaps the earliest unambiguous evidence in favor of a
disk + wind structure in radio-quiet quasars. We carried out an analysis of the
CIV 1549 and Hbeta line profiles of 28 Hamburg-ESO high luminosity quasars and
of 48 low-z, low luminosity sources in order to test whether the
high-ionization line CIV 1549 width could be correlated with Hbeta and be used
as a virial broadening estimator. We analyze intermediate- to high-S/N,
moderate resolution optical and NIR spectra covering the redshifted CIV and
H over a broad range of luminosity log L ~ 44 - 48.5 [erg/s] and
redshift (0 - 3), following an approach based on the quasar main sequence. The
present analysis indicates that the line width of CIV 1549 is not immediately
offering a virial broadening estimator equivalent to H. At the same time
a virialized part of the BLR appears to be preserved even at the highest
luminosities. We suggest a correction to FWHM(CIV) for Eddington ratio (using
the CIV blueshift as a proxy) and luminosity effects that can be applied over
more than four dex in luminosity. Great care should be used in estimating
high-L black hole masses from CIV 1549 line width. However, once corrected
FWHM(CIV) values are used, a CIV-based scaling law can yield unbiased MBH
values with respect to the ones based on H with sample standard
deviation ~ 0.3 dex.Comment: 43 pages, 15 Figures, submitted to A&
Improving Teaching and Learning in Polytechnics with ICT
This paper presents the use of ICT as a tool for improving teaching and learning in the polytechnics of Ghana. It looks at the need for polytechnics to ‘catch up’ with the global trends in ICT deployment, utilization and exploitation in support of its academic activities. The paper also looks at the present state of ICT use in polytechnics in relation to initiatives, projects, opportunities, challenges, players and stakeholders’ roles and finally what the implications are to policy makers, management, teachers and learners within the polytechnic environment
Exploring possible relations between optical variability time scales and broad emission line shapes in AGN
Here we investigate the connection of broad emission line shapes and
continuum light curve variability time scales of type-1 Active Galactic Nuclei
(AGN). We developed a new model to describe optical broad emission lines as an
accretion disk model of a line profile with additional ring emission. We
connect ring radii with orbital time scales derived from optical light curves,
and using Kepler's third law, we calculate mass of central supermassive black
hole (SMBH). The obtained results for central black hole masses are in a good
agreement with {other methods. This indicates that the variability time scales
of AGN may not be stochastic, but rather connected to the orbital time scales
which depend on the central SMBH mass.Comment: 13 pages, 3 figures, Accepted on 11 May 2018 Front. Astron. Space Sc
Stratification of the elements in the atmospheres of blue horizontal-branch stars
Blue horizontal-branch (BHB) stars with approximately larger
than 11500 K show several observational anomalies. In globular clusters, they
exhibit low rotational velocities, abundance anomalies (as compared to cluster
abundances), photometric jumps and gaps and spectroscopic gravities lower than
predicted by canonical models. It is commonly believed that the low rotational
velocities of these stars permit atomic diffusion to be efficient in their
atmosphere thereby causing the observed anomalies. Recent detections of
vertical stratification of iron (and some other chemical elements) in several
BHB stars concur with this framework. In this paper, improved model atmospheres
that include the vertical stratification of the elements are applied to BHB
stars to verify if they can explain their observational anomalies. The results
from theoretical model atmospheres are consistent with the photometric jumps
and gaps observed for BHB stars in globular clusters. It is found that iron
stratification in the theoretical models and that obtained from observations
have similar tendancies. Our results also show that the spectroscopic gravities
obtained while using chemically homogeneous model atmospheres to fit
observations are underestimated. These results significantly strengthen the
belief that atomic diffusion is responsible for these BHB-star anomalies.Comment: 6 pages, 6 figure
A mechanistic investigation of Pickering emulsion polymerization
Pickering emulsion polymerization offers a versatile way of synthetising hybrid core–shell latexes where a polymer core is surrounded by an armour of inorganic nanoparticles. A mechanistic understanding of the polymerization process is limited which restricts the use of the technique in the fabrication of more complex, multilayered colloids. In this paper clarity is provided through an in-depth investigation into the Pickering emulsion polymerization of methyl methacrylate (MMA) in the presence of nano-sized colloidal silica (Ludox TM-40). Mechanistic insights are discussed by studying both the adsorption of the stabiliser to the surface of the latex particles and polymerization kinetics. The adhesion of the Pickering nanoparticles was found not to be spontaneous, as confirmed by cryo-TEM analysis of MMA droplets in water and monomer-swollen PMMA latexes. This supports the theory that the inorganic particles are driven towards the interface as a result of a heterocoagulation event in the water phase with a growing oligoradical. The emulsion polymerizations were monitored by reaction calorimetry in order to establish accurate values for monomer conversion and the overall rate of polymerizations (Rp). Rp increased for higher initial silica concentrations and the polymerizations were found to follow pseudo-bulk kinetics
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