21,348 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
Spherical Orbifolds for Cosmic Topology
Harmonic analysis is a tool to infer cosmic topology from the measured
astrophysical cosmic microwave background CMB radiation. For overall positive
curvature, Platonic spherical manifolds are candidates for this analysis. We
combine the specific point symmetry of the Platonic manifolds with their deck
transformations. This analysis in topology leads from manifolds to orbifolds.
We discuss the deck transformations of the orbifolds and give eigenmodes for
the harmonic analysis as linear combinations of Wigner polynomials on the
3-sphere. These provide new tools for detecting cosmic topology from the CMB
radiation.Comment: 17 pages, 9 figures. arXiv admin note: substantial text overlap with
arXiv:1011.427
The 2-10 keV emission properties of PSR B1937+21
We present the results of a BeppoSAX observation of the fastest pulsar known:
PSR B1937+21. The ~ 200 ks observation (78.5 (34) ks MECS (LECS) exposure
times) allowed us to investigate with high statistical significance both the
spectral properties and the pulse profile shape. The absorbed power law
spectral model gave a photon index of ~ 1.7 and N_H ~ 2.3 x 10^22 cm^-2. These
values explain both a) the ROSAT non-detection and b) the deviant estimate of a
photon index of ~ 0.8 obtained by ASCA. The pulse profile appears, for the
first time, clearly double peaked with the main component much stronger than
the other. The statistical significance is 10 sigma (main peak) and 5 sigma
(secondary peak). The 1.6-10 keV pulsed fraction is consistent with 100%; only
in the 1.6-4 keV band there is a ~ 2 sigma indication for a DC component. The
secondary peak is detected significantly only for energies above 3 / 4 keV. The
unabsorbed (2-10 keV) flux is F_2-10 = 3.7 x 10^-13 erg cm^-2 s^-1, implying a
luminosity of L_X = 4.6 x 10^31 Theta (d/3.6 kpc)^2 erg s^-1 and an X-ray
efficiency of eta = 4 x 10^-5 Theta, where Theta is the solid angle spanned by
the emission beam. These results are in agreement with those obtained by ASCA.Comment: 4 pages, 4 figures, 2 tables. To appear in the Proceedings of the
270. WE-Heraeus Seminar on Neutron Stars, Pulsars and Supernova Remnants,
Jan. 21-25, 2002, Physikzentrum Bad Honnef, eds W. Becker, H. Lesch & J.
Truemper. Proceedings are available as MPE-Report 27
An investigation of pulsar searching techniques with the Fast Folding Algorithm
Here we present an in-depth study of the behaviour of the Fast Folding
Algorithm, an alternative pulsar searching technique to the Fast Fourier
Transform. Weaknesses in the Fast Fourier Transform, including a susceptibility
to red noise, leave it insensitive to pulsars with long rotational periods (P >
1 s). This sensitivity gap has the potential to bias our understanding of the
period distribution of the pulsar population. The Fast Folding Algorithm, a
time-domain based pulsar searching technique, has the potential to overcome
some of these biases. Modern distributed-computing frameworks now allow for the
application of this algorithm to all-sky blind pulsar surveys for the first
time. However, many aspects of the behaviour of this search technique remain
poorly understood, including its responsiveness to variations in pulse shape
and the presence of red noise. Using a custom CPU-based implementation of the
Fast Folding Algorithm, ffancy, we have conducted an in-depth study into the
behaviour of the Fast Folding Algorithm in both an ideal, white noise regime as
well as a trial on observational data from the HTRU-S Low Latitude pulsar
survey, including a comparison to the behaviour of the Fast Fourier Transform.
We are able to both confirm and expand upon earlier studies that demonstrate
the ability of the Fast Folding Algorithm to outperform the Fast Fourier
Transform under ideal white noise conditions, and demonstrate a significant
improvement in sensitivity to long-period pulsars in real observational data
through the use of the Fast Folding Algorithm.Comment: 19 pages, 15 figures, 3 table
Low-Frequency Spectral Turn-Overs in Millisecond Pulsars Studied from Imaging Observations
Measurements of pulsar flux densities are of great importance for
understanding the pulsar emission mechanism and for predictions of pulsar
survey yields and the pulsar population at large. Typically these flux
densities are determined from phase-averaged "pulse profiles", but this method
has limited applicability at low frequencies because the observed pulses can
easily be spread out by interstellar effects like scattering or dispersion,
leading to a non-pulsed continuum component that is necessarily ignored in this
type of analysis. In particular for the class of the millisecond pulsars (MSPs)
at frequencies below 200MHz, such interstellar effects can seriously compromise
de- tectability and measured flux densities. In this paper we investigate MSP
spectra based on a complementary approach, namely through investigation of
archival con- tinuum imaging data. Even though these images lose sensitivity to
pulsars since the on-pulse emission is averaged with off-pulse noise, they are
insensitive to effects from scattering and provide a reliable way to determine
the flux density and spectral indices of MSPs based on both pulsed and unpulsed
components. Using the 74MHz VLSSr as well as the 325MHz WENSS and 1.4GHz NVSS
catalogues, we investigate the imaging flux densities of MSPs and evaluate the
likelihood of spectral turn-overs in this population. We determine three new
MSP spectral indices and identify six new MSPs with likely spectral turn-overs.Comment: 10 pages, 4 figures, 3 tables, accepted for publication in MNRA
Long-term Observations of Three Nulling Pulsars
We present an analysis of approximately 200 hours of observations of the
pulsars J16345107, J17174054 and J18530505, taken over the course of
14.7 yr. We show that all of these objects exhibit long term nulls and
radio-emitting phases (i.e. minutes to many hours), as well as considerable
nulling fractions (NFs) in the range . PSR J17174054 is
also found to exhibit short timescale nulls () and burst phases
() during its radio-emitting phases. This behaviour acts to
modulate the NF, and therefore the detection rate of the source, over
timescales of minutes. Furthermore, PSR J18530505 is shown to exhibit a weak
emission state, in addition to its strong and null states, after sufficient
pulse integration. This further indicates that nulls may often only represent
transitions to weaker emission states which are below the sensitivity
thresholds of particular observing systems. In addition, we detected a
peak-to-peak variation of in the spin-down rate of PSR
J17174054, over timescales of hundreds of days. However, no long-term
correlation with emission variation was found.Comment: 10 pages, 8 figures, accepted for publication in MNRA
Hysteresis in the de Haas-van Alphen Effect
A hysteresis loop is observed for the first time in the de Haas-van Alphen
(dHvA) effect of beryllium at low temperatures and quantizing magnetic field
applied parallel to the hexagonal axis of the single crystal. The irreversible
behavior of the magnetization occurs at the paramagnetic part of the dHvA
period in conditions of Condon domain formation arising by strong enough dHvA
amplitude. The resulting extremely nonlinear response to a very small
modulation field offers the possibility to find in a simple way the Condon
domain phase diagram. From a harmonic analysis, the shape and size of the
hysteresis loop is constructed.Comment: 4 pages, 5 figures, submitted to PR
Prospects for probing strong gravity with a pulsar-black hole system
The discovery of a pulsar (PSR) in orbit around a black hole (BH) is expected
to provide a superb new probe of relativistic gravity and BH properties. Apart
from a precise mass measurement for the BH, one could expect a clean
verification of the dragging of space-time caused by the BH spin. In order to
measure the quadrupole moment of the BH for testing the no-hair theorem of
general relativity (GR), one has to hope for a sufficiently massive BH. In this
respect, a PSR orbiting the super-massive BH in the center of our Galaxy would
be the ultimate laboratory for gravity tests with PSRs. But even for gravity
theories that predict the same properties for BHs as GR, a PSR-BH system would
constitute an excellent test system, due to the high grade of asymmetry in the
strong field properties of these two components. Here we highlight some of the
potential gravity tests that one could expect from different PSR-BH systems,
utilizing present and future radio telescopes, like FAST and SKA.Comment: Proceedings of IAUS 291 "Neutron Stars and Pulsars: Challenges and
Opportunities after 80 years", J. van Leeuwen (ed.); 6 pages, 3 figure
Anderson transitions in three-dimensional disordered systems with randomly varying magnetic flux
The Anderson transition in three dimensions in a randomly varying magnetic
flux is investigated in detail by means of the transfer matrix method with high
accuracy. Both, systems with and without an additional random scalar potential
are considered. We find a critical exponent of with random
scalar potential. Without it, is smaller but increases with the system
size and extrapolates within the error bars to a value close to the above. The
present results support the conventional classification of universality classes
due to symmetry.Comment: 4 pages, 2 figures, to appear in Phys. Rev.
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