6,704 research outputs found
Sub-Saharan Africa at a crossroads: a quantitative analysis of regional development
This repository item contains a single issue of The Pardee Papers, a series papers that began publishing in 2008 by the Boston University Frederick S. Pardee Center for the Study of the Longer-Range Future. The Pardee Papers series features working papers by Pardee Center Fellows and other invited authors. Papers in this series explore current and future challenges by anticipating the pathways to human progress, human development, and human well-being. This series includes papers on a wide range of topics, with a special emphasis on interdisciplinary perspectives and a development orientation.Sub-Saharan Africa is at a crossroads of development. Despite a quarter of a century of economic reforms propagated by national policies and international financial agencies and institutions, sub-Saharan Africa is still lagging in development. In this paper, the authors adopt two techniques using both qualitative (e.g. governance) and quantitative factors (e.g., GDP) to examine regional patterns of development in sub-Saharan Africa. More specifically, they examine and analyze similarities and differences among the countries in this region using a multivariate statistical technique, Principal Component Analysis (PCA), and a unsupervised neural network called Kohonen’s Self-Organizing Map (SOM) to cluster levels of development. PCA serves as a tool for determining regional patterns while SOM is more useful for determining continental patterns in development. Both PCA and SOM results show a “developed” cluster in Southern Africa (South Africa, Namibia, Botswana, and Gabon). SOM exhibits a cluster of least developed countries in southern Western Africa and western Central Africa. The results demonstrate that the applied techniques are highly effective to compress multidimensional qualitative and quantitative data sets to extract relevant information about development from a policy perspective. Our analysis indicates the significance of governance variables in some clusters while a combination of variables explains other regional clusters.
Zachary Tyler works for a consulting firm in Massachusetts that conducts program evaluations for energy efficiency programs, and he continues to work on statistical and geospatial analyses of human development issues. In 2010, he will receive a master’s degree in energy and environmental analysis from Boston University.
Sucharita Gopal is Professor and Director of Graduate Studies in the Department of Geography and Environment and a member of the Cognitive & Neural Systems (CNS) Technology Lab at Boston University. She teaches and conducts research in geographical information systems (GIS), spatial analysis and modeling, and remote sensing for environmental and public health applications. Her recent research includes the development of a marin integrated decision analysis system (MIDAS) for Belize, Panama, and Massachusetts, and a post-disaster geospatial risk model for Haiti.
This paper is part of the Africa 2060 Project, a Pardee Center program of research, publications, and symposia exploring African futures in various aspects related to development on continental and regional scales. For more information, visit www-staging.bu.edu/pardee/research/
Size and Orientation of the `Z' in ZRGs
Some X-shaped radio galaxies (XRGs) show a Z-symmetric morphology in the less
luminous secondary lobes. Our geometrical arguments strongly support a merger
of two galaxies as mechanism for the formation of these sources (ZRG). They
also strengthen the conjecture that a jet is aligned with the spin of the BH at
its base and that the jet flips into the direction of the orbital angular
momentum of the pre-merger binary black hole (BHB). We could also restrict the
distance where the pre-merger jet is bent into Z-shape by the inspiralling
galaxy to the range of 30-100 kpc. One of three possible orientations of the
jet relative to our line of sight is more likely than the others and allows us
to deduce the direction of the spin of the merged BH. The existence of XRGs and
ZRGs proves that the binary has merged, contrary to previous speculations that
after a merger of two galaxies the decay of the BHB stalls due to loss cone
depletion. In ZRGs the black holes probably merge on timescales of some 10^8 yr
after the bending of the jet in a distance of about 50 kpc. Thus, in a way, the
bending starts a stop watch for the rest of the merger.Comment: 2 pages, to appear in the proceedings of ESO Astrophysics Symposia:
Groups of Galaxies in the Nearby Universe, Santiago de Chile, Dec. 200
Clear Evidence for Intranight Optical Variability in Radio-quiet Quasars
We present new clues to the problem of the radio loudness dichotomy arising
from an extensive search for intranight optical variability in seven sets of
optically luminous radio-quiet quasars and (radio-loud) BL Lacertae objects,
which are matched in optical luminosity and redshift. Our monitoring of
radio-quiet quasars has for the first time clearly detected such intranight
variability, with peak-to-peak amplitudes ~1%, occurring with a duty cycle of ~
1/6. The matched BL Lacs have both higher variability amplitudes and duty
cycles when observed in the same fashion. We show that the much less pronounced
intranight variability of the radio-quiet quasars relative to BL Lacs can be
understood in terms of a modest misalignment of the jets in radio-quiet quasars
from the line-of-sight. We thus infer that relativistic particle jets may well
also emerge from radio-quiet quasars, but while traversing the short
optical-emitting distances, they could be snuffed out, possibly through inverse
Compton losses in the nuclear region.Comment: 13 pages, 3 figures, in press in ApJ Letters (20 March 2003
Large Miscibility Gap in the Ba(Mn_xFe_{1-x})2As2 System
The compounds BaMn2As2 and BaFe2As2 both crystallize in the
body-centered-tetragonal ThCr2Si2-type (122-type) structure at room temperature
but exhibit quite different unit cell volumes and very different magnetic and
electronic transport properties. Evidently reflecting these disparities, we
have discovered a large miscibility gap in the system Ba(Mn_xFe_{1-x})2As2.
Rietveld refinements of powder x-ray diffraction (XRD) measurements on samples
slow-cooled from 1000 C to room temperature (RT) reveal a two-phase mixture of
BaMn2As2 and Ba(Mn_{0.12}Fe_{0.88})2As2 phases together with impurity phases
for x = 0.2, 0.4, 0.5, 0.6 and 0.8. We infer that there exists a miscibility
gap in this system at 300 K with composition limits 0.12 < x < 1. For samples
quenched from 1000 C to 77 K, the refinements of RT XRD data indicate that the
miscibility gap at RT narrows at 1000 C to 0.2 < x < 0.8. Samples with x=0.4,
0.5 and 0.6 quenched from 1100-1400 C to 77 K contain a single 122-type phase
together with significant amounts of Fe_{1-x}Mn_xAs and FeAs2 impurity phases.
These results indicate that the system is not a pseudo-binary system over the
whole composition range and that the 122-type phase has a significant
homogeneity range at these temperatures. Magnetic susceptibility, electrical
resistivity and heat capacity measurements versus temperature of the
single-phase quenched polycrystalline samples with x = 0.2 and 0.8 and for
lightly doped BaMn2As2 crystals are reported.Comment: 14 pages, 16 figures, 3 tables; published versio
Unstable topography of biphasic surfactant monolayers
We study the conformation of a heterogeneous surfactant monolayer at a
fluid-fluid interface, near a boundary between two lateral regions of differing
elastic properties. The monolayer attains a conformation of shallow, steep
`mesas' with a height difference of up to 10 nm. If the monolayer is
progressively compressed (e.g. in a Langmuir trough), the profile develops
overhangs and finally becomes unstable at a surface tension of about K(delta
c_0)^2, where (delta c_0) is the difference in spontaneous curvature and K a
bending stiffness. We discuss the relevance of this instability to recently
observed folding behavior in lung surfactant monolayers, and to the absence of
domain structures in films separating oil and water in emulsions.Comment: 7 pages, 7 figures, LaTex using epl.cls, accepted for Europhys Let
Further evidence for intra-night optical variability of radio-quiet quasars
Although well established for BL Lac objects and radio-loud quasars, the
occurrence of intra-night optical variability (INOV) in radio-quiet quasars is
still debated, primarily since only a handful of INOV events with good
statistical significance, albeit small amplitude, have been reported so far.
This has motivated us to continue intra-night optical monitoring of bona-fide
radio-quiet quasars (RQQs). Here we present the results for a sample of 11 RQQs
monitored by us on 19 nights. On 5 of these nights a given RQQ was monitored
simultaneously from two well separated observatories. In all, two clear cases
and two probable case of INOV were detected. From these data, we estimate an
INOV duty cycle of 8% for RQQs which would increase to 19% if the
`probable variable' cases are also included. Such comparatively small INOV duty
cycles for RQQs, together with the small INOV amplitudes (1%), are in
accord with the previously deduced characteristics of this phenomenon.Comment: 15 Pages, 4 Tables, 24 Figures; Accepted in BAS
Improved characterisation of intra-night optical variability of prominent AGN classes
The incidence of intra-night optical variability (INOV) is known to to differ
significantly among different classes of powerful active galactic nuclei (AGN).
A number of statistical methods have been employed in the literature for
testing the presence of INOV in the light curves, sometimes leading to
discordant results. In this paper we compare the INOV characteristics of six
prominent classes of AGN, as evaluated using three commonly used statistical
tests, namely the test, the modified test and the test, which
has recently begun to gain popularity. The AGN classes considered are:
radio-quiet quasars (RQQs), radio-intermediate quasars (RIQs), lobe-dominated
quasars (LDQs), low optical polarization core-dominated quasars (LPCDQs), high
optical polarization core-dominated quasars (HPCDQs), and TeV blazars. Our
analysis is based on a large body of AGN monitoring data, involving 262
sessions of intra-night monitoring of a total 77 AGN, using 1-2 metre class
optical telescopes located in India. In order to compare the usefulness of the
statistical tests, we have also subjected them to a `sanity check' by comparing
the number of false positives yielded by each test with the corresponding
statistical prediction. The present analysis is intended to serve as a
benchmark for future INOV studies of AGN of different classes.Comment: 26 pages, 2 tables, Accepted for publication in MNRA
- …