219 research outputs found
The Effelsberg Search for Pulsars in the Galactic Centre
We report the status of a search for pulsars in the Galactic Centre, using acompletely revised and improved high-sensitivity double-horn system at4.85-GHz. We also present calculations about the success rate of periodicitysearches for such a survey, showing that in contrast to conclusions in recentliterature pulsars can be indeed detected at the chosen search frequency
Nanostructure and properties of a Cu-Cr composite processed by severe plastic deformation
A Cu-Cr composite was processed by severe plastic deformation to investigate
the role of interphase boundaries on the grain size reduction mechanisms. The
as-deformed material exhibits a grain size of only 20nm. This gives rise to a
dramatic increase of the hardness. Some deformation induced Cu super saturated
solid solutions were clearly exhibited and it is shown that they decrease the
hardness. The formation of such supersaturated solid solution and their
influence on the mechanical properties are discussed
Long term monitoring of mode switching for PSR B0329+54
The mode switching phenomenon of PSR B0329+54 is investigated based on the
long-term monitoring from September 2003 to April 2009 made with the Urumqi 25m
radio telescope at 1540 MHz. At that frequency, the change of relative
intensity between the leading and trailing components is the predominant
feature of mode switching. The intensity ratios between the leading and
trailing components are measured for the individual profiles averaged over a
few minutes. It is found that the ratios follow normal distributions, where the
abnormal mode has a wider typical width than the normal mode, indicating that
the abnormal mode is less stable than the normal mode. Our data show that 84.9%
of the time for PSR B0329+54 was in the normal mode and 15.1% was in the
abnormal mode. From the two passages of eight-day quasi-continuous observations
in 2004, and supplemented by the daily data observed with 15 m telescope at 610
MHz at Jodrell Bank Observatory, the intrinsic distributions of mode timescales
are constrained with the Bayesian inference method. It is found that the gamma
distribution with the shape parameter slightly smaller than 1 is favored over
the normal, lognormal and Pareto distributions. The optimal scale parameters of
the gamma distribution is 31.5 minutes for the abnormal mode and 154 minutes
for the normal mode. The shape parameters have very similar values, i.e.
0.75^{+0.22}_{-0.17} for the normal mode and 0.84^{+0.28}_{-0.22} for the
abnormal mode, indicating the physical mechanisms in both modes may be the
same. No long-term modulation of the relative intensity ratios was found for
both the modes, suggesting that the mode switching was stable. The intrinsic
timescale distributions, for the first time constrained for this pulsar,
provide valuable information to understand the physics of mode switching.Comment: 31 pages,12 figures, Accepted by the Ap
The Geometry of PSR B0031-07
PSR B0031-07 is well known to exhibit three different modes of drifting
sub-pulses (mode A, B and C). It has recently been shown that in a
multifrequency observation, consisting of 2700 pulses, all driftmodes were
visible at low frequencies, while at 4.85 GHz only mode-A drift or non-drifting
emission was detected. This suggests that modes A and B are emitted in
sub-beams, rotating at a fixed distance from the magnetic axis, with the mode-B
sub-beams being closer to the magnetic axis than the mode-A sub-beams. Diffuse
emission between the sub-beams can account for the non-drifting emission. Using
the results of an analysis of simultaneous multifrequency observations of PSR
B0031-07, we set out to construct a geometrical model that includes emission
from both sub-beams and diffuse emission and describes the regions of the radio
emission of PSR B0031-07 at each emission frequency for driftmodes A and B.
Based on the vertical spacing between driftbands, we have determined the
driftmode of each sequence of drift. To restrict the model, we calculated
average polarisation and intensity characteristics for each driftmode and at
each frequency. The model reproduces the observed polarisation and intensity
characteristics, suggesting that diffuse emission plays an important role in
the emission properties of PSR B0031-07. The model further suggests that the
emission heights of this pulsar range from a few kilometers to a little over 10
kilometers above the pulsar surface. We also find that the relationships
between height and frequency of emission that follow from curvature radiation
and from plasma-frequency emission could not be used to reproduce the observed
frequency dependence of the width of the average intensity profiles.Comment: 15 pages, 9 figures, 8 tables, accepted for publication in A&
A Quantitative Model for Drifting Subpulses in PSR B0809+74
In this paper we analyze high time resolution single pulse data of PSR
B0809+74 at 820 MHz. We compare the subpulse phase behavior, undocumented at
820 MHz, with previously published results. The subpulse period changes over
time and we measure a subpulse phase jump, when visible, that ranges from 95 to
147 degrees. We find a correlation between the subpulse modulation, subpulse
phase, and orthogonal polarization modes. This variety of complicated behavior
is not well understood and is not easily explained within the framework of
existing models, most of which are founded on the drifting spark model of
Ruderman & Sutherland (1975). We quantitatively fit our data with a non-radial
oscillation model (Clemens & Rosen 2008) and show that the model can accurately
reproduce the drifting subpulses, orthogonal polarization modes, subpulse phase
jump, and can explain the correlation between all these features.Comment: 33 pages, 13 figures, 3 tables, Accepted for publication in Ap
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
Simultaneous multi-frequency single-pulse properties of AXP XTE J1810-197
We have used the 76-m Lovell, 94-m equivalent WSRT and 100-m Effelsberg radio
telescopes to investigate the simultaneous single-pulse properties of the radio
emitting magnetar AXP XTE J1810-197 at frequencies of 1.4, 4.8 and 8.35 GHz
during May and July 2006. We study the magnetar's pulse-energy distributions
which are found to be very peculiar as they are changing on time-scales of days
and cannot be fit by a single statistical model. The magnetar exhibits strong
spiky single giant-pulse-like subpulses, but they do not fit the definition of
the giant pulse or giant micropulse phenomena. Measurements of the
longitude-resolved modulation index reveal a high degree of intensity
fluctuations on day-to-day time-scales and dramatic changes across pulse phase.
We find the frequency evolution of the modulation index values differs
significantly from what is observed in normal radio pulsars. We find that no
regular drifting subpulse phenomenon is present at any of the observed
frequencies at any observing epoch. However, we find a quasi-periodicity of the
subpulses present in the majority of the observing sessions. A correlation
analysis indicates a relationship between components from different
frequencies. We discuss the results of our analysis in light of the emission
properties of normal radio pulsars and a recently proposed model which takes
radio emission from magnetars into consideration.Comment: 15 pages, 11 figures, accepted for publication by MNRA
Placing limits on the stochastic gravitational-wave background using European Pulsar Timing Array data
Direct detection of low-frequency gravitational waves (
Hz) is the main goal of pulsar timing array (PTA) projects. One of the main
targets for the PTAs is to measure the stochastic background of gravitational
waves (GWB) whose characteristic strain is expected to approximately follow a
power-law of the form , where is the
gravitational-wave frequency. In this paper we use the current data from the
European PTA to determine an upper limit on the GWB amplitude as a function
of the unknown spectral slope with a Bayesian algorithm, by modelling
the GWB as a random Gaussian process. For the case , which is
expected if the GWB is produced by supermassive black-hole binaries, we obtain
a 95% confidence upper limit on of , which is 1.8 times
lower than the 95% confidence GWB limit obtained by the Parkes PTA in 2006. Our
approach to the data analysis incorporates the multi-telescope nature of the
European PTA and thus can serve as a useful template for future
intercontinental PTA collaborations.Comment: 14 pages, 8 figures, 3 tables, mnras accepte
- …