1,685 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
DSPSR: Digital Signal Processing Software for Pulsar Astronomy
DSPSR is a high-performance, open-source, object-oriented, digital signal
processing software library and application suite for use in radio pulsar
astronomy. Written primarily in C++, the library implements an extensive range
of modular algorithms that can optionally exploit both multiple-core processors
and general-purpose graphics processing units. After over a decade of research
and development, DSPSR is now stable and in widespread use in the community.
This paper presents a detailed description of its functionality, justification
of major design decisions, analysis of phase-coherent dispersion removal
algorithms, and demonstration of performance on some contemporary
microprocessor architectures.Comment: 15 pages, 10 figures, to be published in PAS
The Big Picture of a Career Behind the Microscope: Life Lessons from a Thirty-Something Woman in Medicine
I consider myself fortunate to pursue a career whose steps were laid out before me. The umbrella of medicine is broad, however, and many major life events occur between the time a young person starts college and a career truly materializes. Looking back on the past fifteen years since graduating high school, Iâve learned more about people and the world around me than any single topic taught in a classroom. From one detour after finishing my undergraduate degree, to another after graduating medical school, my path to becoming a pathologist was not the direct one I envisioned. Since graduating from NSU, I am at the cusp of my career in pathology, am married, and have started a family. Iâm in the middle of my first fellowship in cytology, and anticipating the start of my second fellowship in breast pathology across the country at Mayo Clinic in Minnesota. Iâm due with my second child any day, and supporting a husband whose career is just as demanding. Iâm still learning, but know that the period of time spent in college is developmentally as important as early childhood. That time is truly a crossroads and the few years spent pursuing an undergraduate degree should be a time of exploration, not procrastination. Join me as I elaborate on truths like â30 is not the new 20â, âyou can have it all, just not all at onceâ, and ânothing in the world takes the place of persistenceâ. I look forward to meeting you
Enhanced heat flow in the hydrodynamic-collisionless regime
We study the heat conduction of a cold, thermal cloud in a highly asymmetric
trap. The cloud is axially hydrodynamic, but due to the asymmetric trap
radially collisionless. By locally heating the cloud we excite a thermal dipole
mode and measure its oscillation frequency and damping rate. We find an
unexpectedly large heat conduction compared to the homogeneous case. The
enhanced heat conduction in this regime is partially caused by atoms with a
high angular momentum spiraling in trajectories around the core of the cloud.
Since atoms in these trajectories are almost collisionless they strongly
contribute to the heat transfer. We observe a second, oscillating hydrodynamic
mode, which we identify as a standing wave sound mode.Comment: Sumitted to Phys. Rev. Letters, 4 pages, 4 figure
Analysis of Photoassociation Spectra for Giant Helium Dimers
We perform a theoretical analysis to interpret the spectra of purely
long-range helium dimers produced by photoassociation (PA) in an ultra-cold gas
of metastable helium atoms. The experimental spectrum obtained with the PA
laser tuned closed to the atomic line has been
reported in a previous Letter. Here, we first focus on the corrections to be
applied to the measured resonance frequencies in order to infer the molecular
binding energies. We then present a calculation of the vibrational spectra for
the purely long-range molecular states, using adiabatic potentials obtained
from perturbation theory. With retardation effects taken into account, the
agreement between experimental and theoretical determinations of the spectrum
for the purely long-range potential well is very good. The results
yield a determination of the lifetime of the atomic state
Reaching the hydrodynamic regime in a Bose-Einstein condensate by suppression of avalanche
We report the realization of a Bose-Einstein condensate (BEC) in the
hydrodynamic regime. The hydrodynamic regime is reached by evaporative cooling
at a relative low density suppressing the effect of avalanches. With the
suppression of avalanches a BEC containing 120.10^6 atoms is produced. The
collisional opacity can be tuned from the collisionless regime to a collisional
opacity of more than 3 by compressing the trap after condensation. In the
collisional opaque regime a significant heating of the cloud at time scales
shorter than half of the radial trap period is measured. This is direct proof
that the BEC is hydrodynamic.Comment: Article submitted for Phys. Rev. Letters, 6 figure
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
Radio astronomical polarimetry and phase-coherent matrix convolution
A new phase-coherent technique for the calibration of polarimetric data is
presented. Similar to the one-dimensional form of convolution, data are
multiplied by the response function in the frequency domain. Therefore, the
system response may be corrected with arbitrarily high spectral resolution,
effectively treating the problem of bandwidth depolarization. As well, the
original temporal resolution of the data is retained. The method is therefore
particularly useful in the study of radio pulsars, where high time resolution
and polarization purity are essential requirements of high-precision timing. As
a demonstration of the technique, it is applied to full-polarization baseband
recordings of the nearby millisecond pulsar, PSR J0437-4715.Comment: 8 pages, 4 figures, accepted for publication in Ap
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