239 research outputs found
On Pair Production in the Crab Pulsar
We consider the widespread assumption that coherent pulsar radio emission is
based on extended pair production leading to plasma densities highly exceeding
the Goldreich-Julian density. We show as an example that the observed low
frequency (160 MHz) emission of the Crab pulsar is incompatible to the model of
extended pair production. Our results rule out significant pair production if a
plasma process is responsible for coherence and the radio emission originates
from inside the light cylinder.Comment: accepted for publication in ApJ Letters; 4 pages, no figure
Predicting language learners' grades in the L1, L2, L3 and L4: the effect of some psychological and sociocognitive variables
This study of 89 Flemish high-school students' grades for L1 (Dutch), L2 (French), L3 (English) and L4 (German) investigates the effects of three higher-level personality dimensions (psychoticism, extraversion, neuroticism), one lower-level personality dimension (foreign language anxiety) and sociobiographical variables (gender, social class) on the participants' language grades. Analyses of variance revealed no significant effects of the higher-level personality dimensions on grades. Participants with high levels of foreign language anxiety obtained significantly lower grades in the L2 and L3. Gender and social class had no effect. Strong positive correlations between grades in the different languages could point to an underlying sociocognitive dimension. The implications of these findings are discussed
The characteristics of millisecond pulsar emission: I. Spectra, pulse shapes and the beaming fraction
We have monitored a large sample of millisecond pulsars using the 100-m
Effelsberg radio telescope in order to compare their radio emission properties
to the slowly rotating population. With some notable exceptions, our findings
suggest that the two groups of objects share many common properties. A
comparison of the spectral indices between samples of normal and millisecond
pulsars demonstrates that millisecond pulsar spectra are not significantly
different from those of normal pulsars. There is evidence, however, that
millisecond pulsars are slightly less luminous and less efficient radio
emitters compared to normal pulsars. We confirm recent suggestions that a
diversity exists among the luminosities of millisecond pulsars with the
isolated millisecond pulsars being less luminous than the binary millisecond
pulsars. There are indications that old millisecond pulsars exhibit somewhat
flatter spectra than the presumably younger ones. We present evidence that
millisecond pulsar profiles are only marginally more complex than those found
among the normal pulsar population. Moreover, the development of the profiles
with frequency is rather slow, suggesting very compact magnetospheres. The
profile development seems to anti-correlate with the companion mass and the
spin period, again suggesting that the amount of mass transfer in a binary
system might directly influence the emission properties. The angular radius of
radio beams of millisecond pulsars does not follow the scaling predicted from a
canonical pulsar model which is applicable for normal pulsars. Instead they are
systematically smaller. The smaller inferred luminosity and narrower emission
beams will need to be considered in future calculations of the birth-rate of
the Galactic population.Comment: 40 pages, 14 figures, accepted for publication in Ap
Precision Measurement of Optical Pulsation using a Cherenkov Telescope
During 2003, a camera designed to measure the optical pulsations of pulsars
was installed on a telescope of the H.E.S.S. array. The array is designed for
gamma-ray astronomy in the ~100 GeV - 100 TeV energy regime. The aims of this
exercise were two-fold: to prove the pulsar timing capabilities of H.E.S.S. on
all relevant time-scales, and to explore the possibility of performing
sensitive optical pulsar measurements using the ~100 m^2 mirror of a Cherenkov
telescope. Measurements of the Crab pulsar with this instrument demonstrate an
order of magnitude sensitivity improvement over previous attempts using
Cherenkov telescopes. Here we describe the design and performance of the system
and discuss design considerations for future instruments of this type.Comment: Accepted for publication in Astroparticle Physics, 14 pages, 7
figure
The international pulsar timing array project: using pulsars as a gravitational wave detector
The International Pulsar Timing Array project combines observations of
pulsars from both Northern and Southern hemisphere observatories with the main
aim of detecting ultra-low frequency (~10^-9 to 10^-8 Hz) gravitational waves.
Here we introduce the project, review the methods used to search for
gravitational waves emitted from coalescing supermassive binary black-hole
systems in the centres of merging galaxies and discuss the status of the
project.Comment: accepted by Classical and Quantum Gravity. Review talk for the
Amaldi8 conference serie
Formation of the Radio Profile Components of the Crab Pulsar
The induced Compton scattering of radio emission off the particles of the
ultrarelativistic electron-positron plasma in the open field line tube of a
pulsar is considered. We examine the scattering of a bright narrow radio beam
into the background over a wide solid angle and specifically study the
scattering in the transverse regime, which holds in a moderately strong
magnetic field. Making use of the angular distribution of the scattered
intensity and taking into account the effect of rotational aberration in the
scattering region, we simulate the profiles of the backscattered components as
applied to the Crab pulsar. It is suggested that the interpulse (IP), the
high-frequency interpulse (IP') and the pair of the so-called high-frequency
components (HFC1 and HFC2) result from the backward scattering of the main
pulse (MP), precursor (PR) and the low-frequency component (LFC), respectively.
The components of the high-frequency profiles, the IP' and HFCs, are
interpreted for the first time. The HFC1 and HFC2 are argued to be a single
component split by the rotational aberration close to the light cylinder. It is
demonstrated that the observed spectral and polarization properties of the
profile components of the Crab pulsar as well as the giant pulse phenomenon
outside of the MP can be explained in terms of our model.Comment: Accepted for publication in MNRA
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Teaching modern foreign languages in multilingual classrooms: an examination of Key Stage 2 teachers’ experiences
The statutory inclusion of modern foreign languages (MFL) into the Key Stage 2 curriculum in England in 2014 aimed to raise the language skills of younger learners in preparation for their secondary education. This change to the curriculum has occurred at a time in which the linguistic diversity within primary schools across the country has been consistently increasing. This study used Interpretative Phenomenological Analysis to qualitatively examine the impact of the curriculum change on teachers
implementing it in multilingual classrooms in Greater Manchester. Six teachers with varying experience in teaching MFL participated in semistructured interviews focussing on different aspects of the curriculum change. This paper focuses on the teaching of MFL, as well as on
teachers’ perceptions of English as an Additional Language (EAL) pupils’ aptitude for language learning in comparison to their monolingual peers. The superordinate themes identified from the data included the inconsistent delivery of MFL in primary schools, and the role of
multilingual classrooms as opportunities for augmented MFL provision. The findings from this study will have implications for teachers, head teachers, and policy-makers regarding the effectiveness of the initial
implementation of MFL into the primary curriculum, with specific reference to the EAL school population
Rectification of the Water Permeability in COS-7 Cells at 22, 10 and 0°C
The osmotic and permeability parameters of a cell membrane are essential physico-chemical properties of a cell and particularly important with respect to cell volume changes and the regulation thereof. Here, we report the hydraulic conductivity, Lp, the non-osmotic volume, Vb, and the Arrhenius activation energy, Ea, of mammalian COS-7 cells. The ratio of Vb to the isotonic cell volume, Vc iso, was 0.29. Ea, the activation energy required for the permeation of water through the cell membrane, was 10,700, and 12,000 cal/mol under hyper- and hypotonic conditions, respectively. Average values for Lp were calculated from swell/shrink curves by using an integrated equation for Lp. The curves represented the volume changes of 358 individually measured cells, placed into solutions of nonpermeating solutes of 157 or 602 mOsm/kg (at 0, 10 or 22°C) and imaged over time. Lp estimates for all six combinations of osmolality and temperature were calculated, resulting in values of 0.11, 0.21, and 0.10 µm/min/atm for exosmotic flow and 0.79, 1.73 and 1.87 µm/min/atm for endosmotic flow (at 0, 10 and 22°C, respectively). The unexpected finding of several fold higher Lp values for endosmotic flow indicates highly asymmetric membrane permeability for water in COS-7. This phenomenon is known as rectification and has mainly been reported for plant cell, but only rarely for animal cells. Although the mechanism underlying the strong rectification found in COS-7 cells is yet unknown, it is a phenomenon of biological interest and has important practical consequences, for instance, in the development of optimal cryopreservation
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