736 research outputs found
High-resolution single-pulse studies of the Vela Pulsar
We present high-resolution multi-frequency single-pulse observations of the
Vela pulsar, PSR B0833-45, aimed at studying micro-structure, phase-resolved
intensity fluctuations and energy distributions at 1.41 and 2.30 GHz. We show
that the micro-pulse width in pulsars has a period dependence. Like individual
pulses, Vela's micro-pulses are highly elliptically polarized. There is a
strong correlation between Stokes parameters V and I in the micro-structure. We
show that the V/I distribution is Gaussian with a narrow width and that this
width appears to be constant as a function of pulse phase. The phase-resolved
intensity distributions of I are best fitted with log-normal statistics. Extra
emission components, i.e.``bump'' and ``giant micro-pulses'', discovered by
Johnston et al.(2001) are also present at 2.3 GHz. The bump component seems to
be an extra component superposed on the main pulse profile but does not appear
periodically. The giant micro-pulses are time-resolved and have significant
jitter in their arrival times. Their flux density distribution is best fitted
by a power-law, indicating a link between these features and ``classical''
giant pulses as observed for the Crab pulsar, (PSR B0531+21), PSR B1937+21 and
PSR B1821-24. We find that Vela contains a mixture of emission properties
representing both ``classical'' properties of radio pulsars (e.g.
micro-structure, high degree of polarization, S-like position angle swing,
orthogonal modes) and features which are most likely related to high-energy
emission (e.g. extra profile components, giant micro-pulses). It hence
represents an ideal test case to study the relationship between radio and
high-energy emission in significant detail.Comment: accepted for publication in MNRAS (11 pages, 10 figures
PSRCHIVE and PSRFITS: Definition of the Stokes Parameters and Instrumental Basis Conventions
This paper defines the mathematical convention adopted to describe an
electromagnetic wave and its polarisation state, as implemented in the PSRCHIVE
software and represented in the PSRFITS definition. Contrast is made between
the convention that has been widely accepted by pulsar astronomers and the
IAU/IEEE definitions of the Stokes parameters. The former is adopted as the
PSR/IEEE convention, and a set of useful parameters are presented for
describing the differences between the PSR/IEEE standard and the conventions
(either implicit or explicit) that form part of the design of observatory
instrumentation. To aid in the empirical determination of instrumental
convention parameters, well-calibrated average polarisation profiles of PSR
J0304+1932 and PSR J0742-2822 are presented at radio wavelengths of
approximately 10, 20, and 40 cm.Comment: 7 pages, 2 figures, to be published in PAS
Are the distributions of Fast Radio Burst properties consistent with a cosmological population?
High time resolution radio surveys over the last few years have discovered a
population of millisecond-duration transient bursts called Fast Radio Bursts
(FRBs), which remain of unknown origin. FRBs exhibit dispersion consistent with
propagation through a cold plasma and dispersion measures indicative of an
origin at cosmological distances. In this paper we perform Monte Carlo
simulations of a cosmological population of FRBs, based on assumptions
consistent with observations of their energy distribution, their spatial
density as a function of redshift and the properties of the interstellar and
intergalactic media. We examine whether the dispersion measures, fluences,
inferred redshifts, signal-to-noises and effective widths of known FRBs are
consistent with a cosmological population. Statistical analyses indicate that
at least 50 events at Parkes are required to distinguish between a constant
co-moving FRB density, and a FRB density that evolves with redshift like the
cosmological star formation rate density.Comment: 11 pages, 7 figures, 3 table
High signal-to-noise ratio observations and the ultimate limits of precision pulsar timing
We demonstrate that the sensitivity of high-precision pulsar timing
experiments will be ultimately limited by the broadband intensity modulation
that is intrinsic to the pulsar's stochastic radio signal. That is, as the peak
flux of the pulsar approaches that of the system equivalent flux density,
neither greater antenna gain nor increased instrumental bandwidth will improve
timing precision. These conclusions proceed from an analysis of the covariance
matrix used to characterise residual pulse profile fluctuations following the
template matching procedure for arrival time estimation. We perform such an
analysis on 25 hours of high-precision timing observations of the closest and
brightest millisecond pulsar, PSR J0437-4715. In these data, the standard
deviation of the post-fit arrival time residuals is approximately four times
greater than that predicted by considering the system equivalent flux density,
mean pulsar flux and the effective width of the pulsed emission. We develop a
technique based on principal component analysis to mitigate the effects of
shape variations on arrival time estimation and demonstrate its validity using
a number of illustrative simulations. When applied to our observations, the
method reduces arrival time residual noise by approximately 20%. We conclude
that, owing primarily to the intrinsic variability of the radio emission from
PSR J0437-4715 at 20 cm, timing precision in this observing band better than 30
- 40 ns in one hour is highly unlikely, regardless of future improvements in
antenna gain or instrumental bandwidth. We describe the intrinsic variability
of the pulsar signal as stochastic wideband impulse modulated self-noise
(SWIMS) and argue that SWIMS will likely limit the timing precision of every
millisecond pulsar currently observed by Pulsar Timing Array projects as larger
and more sensitive antennae are built in the coming decades.Comment: 16 pages, 9 figures, accepted for publication in MNRAS. Updated
version: added DOI and changed manuscript to reflect changes in the final
published versio
Radio Astronomical Polarimetry and Point-Source Calibration
A mathematical framework is presented for use in the experimental
determination of the polarimetric response of observatory instrumentation.
Elementary principles of linear algebra are applied to model the full matrix
description of the polarization measurement equation by least-squares
estimation of non-linear, scalar parameters. The formalism is applied to
calibrate the center element of the Parkes Multibeam receiver using
observations of the millisecond pulsar, PSR J0437-4715, and the radio galaxy,
3C 218 (Hydra A).Comment: 8 pages, 4 figures, to be published in ApJ
Large atom number Bose-Einstein condensate of sodium
We describe the setup to create a large Bose-Einstein condensate containing
more than 120x10^6 atoms. In the experiment a thermal beam is slowed by a
Zeeman slower and captured in a dark-spot magneto-optical trap (MOT). A typical
dark-spot MOT in our experiments contains 2.0x10^10 atoms with a temperature of
320 microK and a density of about 1.0x10^11 atoms/cm^3. The sample is spin
polarized in a high magnetic field, before the atoms are loaded in the magnetic
trap. Spin polarizing in a high magnetic field results in an increase in the
transfer efficiency by a factor of 2 compared to experiments without spin
polarizing. In the magnetic trap the cloud is cooled to degeneracy in 50 s by
evaporative cooling. To suppress the 3-body losses at the end of the
evaporation the magnetic trap is decompressed in the axial direction.Comment: 11 pages, 12 figures, submitted to Review Of Scientific Instrument
Rotation measure variations for 20 millisecond pulsars
We report on variations in the mean position angle of the 20 millisecond
pulsars being observed as part of the Parkes Pulsar Timing Array (PPTA)
project. It is found that the observed variations are dominated by changes in
the Faraday rotation occurring in the Earth's ionosphere. Two ionospheric
models are used to correct for the ionospheric contribution and it is found
that one based on the International Reference Ionosphere gave the best results.
Little or no significant long-term variation in interstellar RM was found with
limits typically about 0.1 rad m yr in absolute value. In a few
cases, apparently significant RM variations over timescales of a few 100 days
or more were seen. These are unlikely to be due to localised magnetised regions
crossing the line of sight since the implied magnetic fields are too high. Most
probably they are statistical fluctuations due to random spatial and temporal
variations in the interstellar electron density and magnetic field along the
line of sight.Comment: Accepted for publication in Astrophysics & Space Scienc
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