19,150 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
Skills and knowledge needs assessment of current and future library professionals in the state of Qatar
Purpose: The purpose of this paper is to investigate and document the skills and knowledge needs of future library professionals in Qatar and to use the outcomes of this research to help develop or refine focused library and information studies course curricula that meet the needs of the local workforce and also guide or improve national or local professional development programmes. / Design/methodology/approach: A skills and knowledge needs assessment survey was sent to library professionals, LIS students and library managers in Qatar. A total of 109 respondents completed the survey, a representation of around 25 per cent of the current LIS workforce in Qatar. / Findings: Findings indicated that respondents felt that the most needed future job roles included more client focused positions such as research librarians, information services librarians and subject librarians, as well as technical roles such as Arabic cataloguers, electronic resources librarians and system librarians. The largest amount of needed positions was also felt to be in school libraries. Respondents to the survey also felt that there was a lack of opportunities for professional development in Qatar and that the most needed area of skills training was information literacy, followed by copyright training and technical skills including RDA and Arabic cataloguing. One further finding identified from the survey was the concern felt by respondents about the lack of a professional body in Qatar that represented LIS professionals. / Practical implications: This paper provides data on future roles, skills and knowledge needed by library professionals working in international and culturally diverse workforces. It also provides findings that can be used to develop LIS curriculum and professional development programmes in international LIS environments. / Originality/value: A detailed needs assessment of this kind has not previously been undertaken in Qatar. The library and information sector in Qatar is an emerging field with a largely international workforce. This situation provides a distinct perspective on the needs of an emerging library sector that is a blend of different cultures, workplace practices and differing expectations and understandings of the role and skills needed to be a LIS professional
Timing models for the long-orbital period binary pulsar PSR B1259-63
The pulsar PSR B1259-63 is in a highly eccentric 3.4-yr orbit with the Be
star SS 2883. Timing observations of this pulsar, made over a 7-yr period using
the Parkes 64-m radio telescope, cover two periastron passages, in 1990 August
and 1994 January. The timing data cannot be fitted by the normal pulsar and
Keplerian binary parameters. A timing solution including a (non-precessing)
Keplerian orbit and timing noise (represented as a polynomial of fifth order in
time) provide a satisfactory fit to the data. However, because the Be star
probably has a significant quadrupole moment, we prefer to interpret the data
by a combination of timing noise, dominated by a cubic phase term, and
and terms. We show that the and are
likely to be a result of a precessing orbit caused by the quadrupole moment of
the tilted companion star. We further rule out a number of possible physical
effects which could contribute to the timing data of PSR B1259-63 on a
measurable level.Comment: LaTeX, 9 pages, 8 figures, accepted for publication in MNRA
Feynman Propagator for a Free Scalar Field on a Causal Set
The Feynman propagator for a free bosonic scalar field on the discrete
spacetime of a causal set is presented. The formalism includes scalar field
operators and a vacuum state which define a scalar quantum field theory on a
causal set. This work can be viewed as a novel regularisation of quantum field
theory based on a Lorentz invariant discretisation of spacetime.Comment: 4 pages, 2 plots. Minor updates to match published versio
Magnetic properties of strongly disordered electronic systems
We present a unified, global perspective on the magnetic properties of
strongly disordered electronic systems, with special emphasis on the case where
the ground state is metallic. We review the arguments for the instability of
the disordered Fermi liquid state towards the formation of local magnetic
moments, and argue that their singular low temperature thermodynamics are the
``quantum Griffiths'' precursors of the quantum phase transition to a metallic
spin glass; the local moment formation is therefore not directly related to the
metal-insulator transition. We also review the the mean-field theory of the
disordered Fermi liquid to metallic spin glass transition and describe the
separate regime of ``non-Fermi liquid'' behavior at higher temperatures near
the quantum critical point. The relationship to experimental results on doped
semiconductors and heavy-fermion compounds is noted.Comment: 25 pages; Contribution to the Royal Society Discussion Meeting on
"The Metal-Non Metal Transition in Macroscopic and Microscopic Systems",
March 5-6, 199
Experimental and theoretical electronic structure of EuRh2As2
The Fermi surfaces (FS's) and band dispersions of EuRh2As2 have been
investigated using angle-resolved photoemission spectroscopy. The results in
the high-temperature paramagnetic state are in good agreement with the full
potential linearized augmented plane wave calculations, especially in the
context of the shape of the two-dimensional FS's and band dispersion around the
Gamma (0,0) and X (pi,pi) points. Interesting changes in band folding are
predicted by the theoretical calculations below the magnetic transition
temperature Tn=47K. However, by comparing the FS's measured at 60K and 40K, we
did not observe any signature of this transition at the Fermi energy indicating
a very weak coupling of the electrons to the ordered magnetic moments or strong
fluctuations. Furthermore, the FS does not change across the temperature (~
25K) where changes are observed in the Hall coefficient. Notably, the Fermi
surface deviates drastically from the usual FS of the superconducting
iron-based AFe2As2 parent compounds, including the absence of nesting between
the Gamma and X FS pockets.Comment: 4 pages, 4 figure
Discovery of two pulsars towards the Galactic Centre
We report the discovery of two highly dispersed pulsars in the direction of
the Galactic Centre made during a survey at 3.1 GHz with the Parkes radio
telescope. Both PSRs J1745-2912 and J1746-2856 have an angular separation from
the Galactic Centre of less than 0.3 degrees and dispersion measures in excess
of 1100 cm-3pc, placing them in the top 10 pulsars when ranked on this value.
The frequency dependence of the scatter-broadening in PSR J1746-2856 is much
shallower than expected from simple theory. We believe it likely that the
pulsars are located between 150 and 500 pc from the Galactic Centre on the near
side, and are part of an excess population of neutron stars associated with the
Centre itself. A second survey made at 8.4 GHz did not detect any pulsars. This
implies either that there are not many bright, long-period pulsars at the
Galactic Centre or that the scattering is more severe at high frequencies than
current models would suggest.Comment: Submitted to MNRAS Letter
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