19,873 research outputs found
Polarisation profiles of southern pulsars at 3.1 GHz
We present polarisation profiles for 48 southern pulsars observed with the
new 10-cm receiver at the Parkes telescope. We have exploited the low system
temperature and high bandwidth of the receiver to obtain profiles which have
good signal to noise for most of our sample at this relatively high frequency.
Although, as expected, a number of profiles are less linearly polarised at 3.1
GHz than at lower frequencies, we identify some pulsars and particular
components of profiles in other pulsars which have increased linear
polarisation at this frequency. We discuss the dependence of linear
polarisation with frequency in the context of a model in which emission
consists of the superposition of two, orthogonally polarised modes. We show
that a simple model, in which the orthogonal modes have different spectral
indices, can explain many of the observed properties of the frequency evolution
of both the linear polarisation and the total power, such as the high degree of
linear polarisation seen at all frequencies in some high spin-down, young
pulsars. Nearly all the position angle profiles show deviations from the
rotating vector model; this appears to be a general feature of high-frequency
polarisation observations.Comment: Accepted for publication in MNRA
13 Years of Timing of PSR B1259-63
This paper summarizes the results of 13 years of timing observations of a
unique binary pulsar, PSR B125963, which has a massive B2e star companion.
The data span encompasses four complete orbits and includes the periastron
passages in 1990, 1994, 1997 and 2000. Changes in dispersion measure occurring
around the 1994, 1997 and 2000 periastrons are measured and accounted for in
the timing analysis. There is good evidence for a small glitch in the pulsar
period in 1997 August, not long after the 1997 periastron, and a significant
frequency second derivative indicating timing noise. We find that spin-orbit
coupling with secular changes in periastron longitude and projected semi-major
axis () cannot account for the observed period variations over the whole
data set. While fitting the data fairly well, changes in pulsar period
parameters at each periastron seem ruled out both by X-ray observations and by
the large apparent changes in pulsar frequency derivative. Essentially all of
the systematic period variations are accounted for by a model consisting of the
1997 August glitch and step changes in at each periastron. These changes
must be due to changes in the orbit inclination, but we can find no plausible
mechanism to account for them. It is possible that timing noise may mask the
actual changes in orbital parameters at each periastron, but the good fit to
the data of the step-change model suggests that short-term timing noise is
not significant.Comment: 9 pages, 7 figures, accepted by MNRA
The 2000 Periastron Passage of PSR B1259-63
We report here on a sequence of 28 observations of the binary pulsar system
PSR B1259-63/SS2883 at four radio frequencies made with the Australia Telescope
Compact Array around the time of the 2000 periastron passage. Observations made
on 2000 Sep 1 show that the pulsar's apparent rotation measure (RM) reached a
maximum of rad m, some 700 times the value measured
away from periastron, and is the largest astrophysical RM measured. This value,
combined with the dispersion measure implies a magnetic field in the Be star's
wind of 6 mG. We find that the light curve of the unpulsed emission is similar
to that obtained during the 1997 periastron but that differences in detail
imply that the emission disc of the Be star is thicker and/or of higher
density. The behaviour of the light curve at late times is best modelled by the
adiabatic expansion of a synchrotron bubble formed in the pulsar/disc
interaction. The expansion rate of the bubble km s is
surprisingly low but the derived magnetic field of 1.6 G close to that
expected.Comment: 8 pages, 6 figures, 3 tables, LaTeX (mn.sty). Accepted for
publication in the Monthly Notices of the Royal Astronomical Society. Also
available at http://astronomy.swin.edu.au/staff/tconnors/publications.htm
NMR Determination of an Incommensurate Helical Antiferromagnetic Structure in EuCo2As2
We report Eu, As and Co nuclear magnetic resonance
(NMR) results on EuCoAs single crystal. Observations of Eu and
As NMR spectra in zero magnetic field at 4.3 K below an
antiferromagnetic (AFM) ordering temperature = 45 K and its
external magnetic field dependence clearly evidence an incommensurate helical
AFM structure in EuCoAs. Furthermore, based on Co NMR data in
both the paramagnetic and the incommensurate AFM states, we have determined the
model-independent value of the AFM propagation vector = (0, 0, 0.73
0.07)2/ where is the lattice parameter. Thus the
incommensurate helical AFM state was characterized by only NMR data with
model-independent analyses, showing NMR to be a unique tool for determination
of the spin structure in incommensurate helical AFMs.Comment: 6 pages, 4 figures, accepted for publication in Phys.Rev.
EuCo2P2: A Model Molecular-Field Helical Heisenberg Antiferromagnet
The metallic compound EuCo2P2 with the body-centered tetragonal ThCr2Si2
structure containing Eu spins 7/2 was previously shown from single-crystal
neutron diffraction measurements to exhibit a helical antiferromagnetic (AFM)
structure below TN = 66.5 K with the helix axis along the c axis and with the
ordered moments aligned within the ab-plane. Here we report crystallography,
electrical resistivity, heat capacity, magnetization and magnetic
susceptibility measurements on single crystals of this compound. We demonstrate
that EuCo2P2 is a model molecular-field helical Heisenberg antiferromagnet from
comparisons of the anisotropic magnetic susceptibility chi, high-field
magnetization and magnetic heat capacity of EuCo2P2 single crystals at
temperature T < TN with the predictions of our recent formulation of molecular
field theory. Values of the Heisenberg exchange interactions between the Eu
spins are derived from the data. The low-T magnetic heat capacity ~ T^3 arising
from spin-wave excitations with no anisotropy gap is calculated and found to be
comparable to the lattice heat capacity. The density of states at the Fermi
energy of EuCo2P2 and the related compound BaCo2P2 are found from the heat
capacity data to be large, 10 and 16 states/eV per formula unit for EuCo2P2 and
BaCo2P2, respectively. These values are enhanced by a factor of ~2.5 above
those found from DFT electronic structure calculations for the two compounds.
The calculations also find ferromagnetic Eu-Eu exchange interactions within the
ab-plane and AFM interactions between nearest- and next-nearest planes, in
agreement with the MFT analysis of chi{ab}(T < TN).Comment: 20 pages, 17 figures, 3 tables, 46 references. This is an extended
replacement of arXiv:1512.02958 with an additional coautho
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