3,471 research outputs found
Choose the school, choose the performance. New evidence on the determinants of student performance in eight European countries
This study aims to identify the main determinants of student performance in reading and
maths across eight European Union countries (Austria, Croatia, Germany, Hungary, Italy,
Portugal, Slovakia, and Slovenia). Based on student-level data from the OECD’s PISA
2018 survey and by means of the application of efficient algorithms, we highlight that the
number of books at home and a variable combining the type and location of their school
represent the most important predictors of student performance in all of the analysed
countries, while other school characteristics are rarely relevant. Econometric results show
that students attending vocational schools perform significantly worse than those in
general schools, except in Portugal. Considering only general school students, the
differences between big and small cities are not statistically significant, while among
students in vocational schools, those in a small city tend to perform better than those in a
big city. Through the Gelbach decomposition method, which allows measuring the
relative importance of observable characteristics in explaining a gap, we show that the
differences in test scores between big and small cities depend on school characteristics,
while the differences between general and vocational schools are mainly explained by
family social statu
Synthetic Observations of Simulated Radio Galaxies I: Radio and X-ray Analysis
We present an extensive synthetic observational analysis of numerically-
simulated radio galaxies designed to explore the effectiveness of conventional
observational analyses at recovering physical source properties. These are the
first numerical simulations with sufficient physical detail to allow such a
study. The present paper focuses on extraction of magnetic field properties
from nonthermal intensity information. Synchrotron and inverse-Compton
intensities provided meaningful information about distributions and strengths
of magnetic fields, although considerable care was called for. Correlations
between radio and X-ray surface brightness correctly revealed useful dynamical
relationships between particles and fields. Magnetic field strength estimates
derived from the ratio of X-ray to radio intensity were mostly within about a
factor of two of the RMS field strength along a given line of sight. When
emissions along a given line of sight were dominated by regions close to the
minimum energy/equipartition condition, the field strengths derived from the
standard power-law-spectrum minimum energy calculation were also reasonably
close to actual field strengths, except when spectral aging was evident.
Otherwise, biases in the minimum- energy magnetic field estimation mirrored
actual differences from equipartition. The ratio of the inverse-Compton
magnetic field to the minimum-energy magnetic field provided a rough measure of
the actual total energy in particles and fields in most instances, within an
order of magnitude. This may provide a practical limit to the accuracy with
which one may be able to establish the internal energy density or pressure of
optically thin synchrotron sources.Comment: 43 pages, 14 figures; accepted for publication in ApJ, v601 n2
February 1, 200
Electron-nuclei spin dynamics in II-VI semiconductor quantum dots
We report on the dynamics of optically induced nuclear spin polarization in
individual CdTe/ZnTe quantum dots loaded with one electron by modulation
doping. The fine structure of the hot trion (charged exciton with an
electron in the -shell) is identified in photoluminescence excitation
spectra. A negative polarisation rate of the photoluminescence, optical pumping
of the resident electron and the built-up of dynamic nuclear spin polarisation
(DNSP) are observed in time-resolved optical pumping experiments when the
quantum dot is excited at higher energy than the hot trion triplet state. The
time and magnetic field dependence of the polarisation rate of the
emission allows to probe the dynamics of formation of the DNSP in the optical
pumping regime. We demonstrate using time-resolved measurements that the
creation of a DNSP at B=0T efficiently prevents longitudinal spin relaxation of
the electron caused by fluctuations of the nuclear spin bath. The DNSP is built
in the microsecond range at high excitation intensity. A relaxation time of the
DNSP in about 10 microseconds is observed at and significantly increases
under a magnetic field of a few milli-Tesla. We discuss mechanisms responsible
for the fast initialisation and relaxation of the diluted nuclear spins in this
system
Deep LOFAR 150 MHz imaging of the Bo\"otes field: Unveiling the faint low-frequency sky
We have conducted a deep survey (with a central rms of )
with the LOw Frequency ARray (LOFAR) at 120-168 MHz of the Bo\"otes field, with
an angular resolution of , and obtained a sample of
10091 radio sources ( limit) over an area of .
The astrometry and flux scale accuracy of our source catalog is investigated.
The resolution bias, incompleteness and other systematic effects that could
affect our source counts are discussed and accounted for. The derived 150 MHz
source counts present a flattening below sub-mJy flux densities, that is in
agreement with previous results from high- and low- frequency surveys. This
flattening has been argued to be due to an increasing contribution of
star-forming galaxies and faint active galactic nuclei. Additionally, we use
our observations to evaluate the contribution of cosmic variance to the scatter
in source counts measurements. The latter is achieved by dividing our Bo\"otes
mosaic into 10 non-overlapping circular sectors, each one with an approximate
area of The counts in each sector are computed in the
same way as done for the entire mosaic. By comparing the induced scatter with
that of counts obtained from depth observations scaled to 150MHz, we find that
the scatter due to cosmic variance is larger than the Poissonian
errors of the source counts, and it may explain the dispersion from previously
reported depth source counts at flux densities . This work
demonstrates the feasibility of achieving deep radio imaging at low-frequencies
with LOFAR.Comment: A\&A in press. 15 pages, 16 figure
Improved sensitivity of H.E.S.S.-II through the fifth telescope focus system
The Imaging Atmospheric Cherenkov Telescope (IACT) works by imaging the very
short flash of Cherenkov radiation generated by the cascade of relativistic
charged particles produced when a TeV gamma ray strikes the atmosphere. This
energetic air shower is initiated at an altitude of 10-30 km depending on the
energy and the arrival direction of the primary gamma ray. Whether the best
image of the shower is obtained by focusing the telescope at infinity and
measuring the Cherenkov photon angles or focusing on the central region of the
shower is a not obvious question. This is particularly true for large size IACT
for which the depth of the field is much smaller. We address this issue in
particular with the fifth telescope (CT5) of the High Energy Stereoscopic
System (H.E.S.S.); a 28 m dish large size telescope recently entered in
operation and sensitive to an energy threshold of tens of GeVs. CT5 is equipped
with a focus system, its working principle and the expected effect of focusing
depth on the telescope sensitivity at low energies (50-200 GeV) is discussed.Comment: In Proceedings of the 33rd International Cosmic Ray Conference
(ICRC2013), Rio de Janeiro (Brazil
Preliminary analysis of the drive system of the CTA LST Telescope and its integration in the whole PLC architecture
This work aims to present a preliminary analysis of the drive system configuration for the CTA telescopes array and more specifically a possible architecture for the sub-array of Large Size Telescopes - LSTs. The first part of this document is focused on the control command architecture of the drive system dedicated to the LST including a view on some mechanical aspects concerning the telescopes of this class. In particular the current investigation on the interfaces between the drive system and the automatic system in charge of the camera mast control system (e.g. the arch damping) is presented. In the second part of this work the issue of the integration of the telescope drive system within a global PLC (Programmable Logic Controller) architecture for the CTA array is addressed with the corresponding links to the control software layer
Random Graph-Homomorphisms and Logarithmic Degree
A graph homomorphism between two graphs is a map from the vertex set of one
graph to the vertex set of the other graph, that maps edges to edges. In this
note we study the range of a uniformly chosen homomorphism from a graph G to
the infinite line Z. It is shown that if the maximal degree of G is
`sub-logarithmic', then the range of such a homomorphism is super-constant.
Furthermore, some examples are provided, suggesting that perhaps for graphs
with super-logarithmic degree, the range of a typical homomorphism is bounded.
In particular, a sharp transition is shown for a specific family of graphs
C_{n,k} (which is the tensor product of the n-cycle and a complete graph, with
self-loops, of size k). That is, given any function psi(n) tending to infinity,
the range of a typical homomorphism of C_{n,k} is super-constant for k = 2
log(n) - psi(n), and is 3 for k = 2 log(n) + psi(n)
Continuum surveys with LOFAR and synergy with future large surveys in the 1-2 GHz band
Radio astronomy is entering the era of large surveys. This paper describes
the plans for wide surveys with the LOw Frequency ARray (LOFAR) and their
synergy with large surveys at higher frequencies (in particular in the 1-2 GHz
band) that will be possible using future facilities like Apertif or ASKAP. The
LOFAR Survey Key Science Project aims at conducting large-sky surveys at 15,
30, 60, 120 and 200 MHz taking advantage of the wide instantaneous field of
view and of the unprecedented sensitivity of this instrument. Four topics have
been identified as drivers for these surveys covering the formation of massive
galaxies, clusters and black holes using z>6 radio galaxies as probes, the
study of the intercluster magnetic fields using diffuse radio emission and
Faraday rotation measures in galaxy clusters as probes and the study of star
formation processes in the early Universe using starburst galaxies as probes.
The fourth topic is the exploration of new parameter space for serendipitous
discovery taking advantage of the new observational spectral window open up by
LOFAR. Here, we briefly discuss the requirements of the proposed surveys to
address these (and many others!) topics as well as the synergy with other wide
area surveys planned at higher frequencies (and in particular in the 1-2 GHz
band) with new radio facilities like ASKAP and Apertif. The complementary
information provided by these surveys will be crucial for detailed studies of
the spectral shape of a variety of radio sources (down to sub-mJy sources) and
for studies of the ISM (in particular HI and OH) in nearby galaxies.Comment: to appear in the proceedings of "Panoramic Radio Astronomy:
Wide-field 1-2 GHz research on galaxy evolution", G. Heald and P. Serra eds.,
8 pages, 3 figure
Construcció i desenvolupament d'una plataforma per a l'estudi de comportaments en robòtica mòbil
Al llarg d’aquest projecte es desenvolupa la construcció d’un microrobot i d’una plataforma per a l’estudi de comportaments en robòtica mòbil que té com ús final la docència a partir d’una placa de desenvolupament de sistemes basada en el microcontrolador 80c552.
Per a la consecució dels objectius s’ha dissenyat un prototipus dotat d’actuadors (dos servomotors que accionen una configuració cinemàtica diferencial mitjançant dues rodes motrius) i sensors (dos sensors d’infrarojos, sis sensors de contacte i un transreceptor per a una comunicació mitjançant ones de ràdio).
Per la seva part la plataforma consta de tres parts diferenciades: un carregador de bateries de NiMH, un mòdul de ràdio per a la comunicació half-duplex entre el microrobot i una computadora que disposa de port sèrie i una petita interfície de control per a poder monitoritzar l’estat del robot.
Les solucions presentades al llarg del projecte no són úniques, ja que en tot moment s’ha intentat arribar a un compromís que equilibrés el triangle format pel cost econòmic dels components utilitzats, els recursos disponibles de la placa de desenvolupament basada en el 80C552 i les prestacions ofertes per la plataforma i el microrobot.
Tant els codis del robot com de la plataforma són oberts. El primer es programa en C ANSI adaptat per a la família de microcontroladors C51, la interfície gràfica de la plataforma està realitzada amb Visual C++, i tots dos poden ser ampliats, modificats i millorats en funció de les noves prestacions amb que es vulguin dotar tant el robot com la plataforma.
També s’ha realitzat el control manual del robot com a demostració del funcionament del mateix mitjançant una funcionalitat de la plataforma i que utilitza un típic Joystick emprat en jocs i videoconsoles d’oci
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