10,735 research outputs found
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
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
Pure phase-encoded MRI and classification of solids
Here, the authors combine a pure phase-encoded magnetic resonance imaging (MRI) method with a new tissue-classification technique to make geometric models of a human tooth. They demonstrate the feasibility of three-dimensional imaging of solids using a conventional 11.7-T NMR spectrometer. In solid-state imaging, confounding line-broadening effects are typically eliminated using coherent averaging methods. Instead, the authors circumvent them by detecting the proton signal at a fixed phase-encode time following the radio-frequency excitation. By a judicious choice of the phase-encode time in the MRI protocol, the authors differentiate enamel and dentine sufficiently to successfully apply a new classification algorithm. This tissue-classification algorithm identifies the distribution of different material types, such as enamel and dentine, in volumetric data. In this algorithm, the authors treat a voxel as a volume, not as a single point, and assume that each voxel may contain more than one material. They use the distribution of MR image intensities within each voxel-sized volume to estimate the relative proportion of each material using a probabilistic approach. This combined approach, involving MRI and data classification, is directly applicable to bone imaging and hard-tissue contrast-based modeling of biological solids
Leptogenesis in the type III seesaw mechanism
It is shown that the type III seesaw mechanism proposed recently can have
certain advantages over the conventional (or type I) seesaw mechanism for
leptogenesis. In particular a resonant enhancement of leptogenesis via heavy
quasi-Dirac right-handed neutrino pairs can occur without a special flavor form
or "texture" of the mass matrices being assumed. Some of the requirements for
neutrino mixing and leptogenesis are effectively decoupled.Comment: 12 pages including one figure, several references adde
Polarization studies of Rotating Radio Transients
We study the polarization properties of 22 known rotating radio transients
(RRATs) with the 64-m Parkes radio telescope and present the Faraday rotation
measures (RMs) for the 17 with linearly polarized flux exceeding the off-pulse
noise by 3. Each RM was estimated using a brute-force search over trial
RMs that spanned the maximum measurable range (in steps of 1 ), followed by an
iterative refinement algorithm. The measured RRAT RMs are in the range |RM|
to rad m with an average linear polarization
fraction of per cent. Individual single pulses are observed to be up
to 100 per cent linearly polarized. The RMs of the RRATs and the corresponding
inferred average magnetic fields (parallel to the line-of-sight and weighted by
the free electron density) are observed to be consistent with the Galactic
plane pulsar population. Faraday rotation analyses are typically performed on
accumulated pulsar data, for which hundreds to thousands of pulses have been
integrated, rather than on individual pulses. Therefore, we verified the
iterative refinement algorithm by performing Monte Carlo simulations of
artificial single pulses over a wide range of S/N and RM. At and above a S/N of
17 in linearly polarized flux, the iterative refinement recovers the simulated
RM value 100 per cent of the time with a typical mean uncertainty of
rad m. The method described and validated here has also been
successfully used to determine reliable RMs of several fast radio bursts (FRBs)
discovered at Parkes.Comment: Submitted to MNRAS, 10 pages, 6 figure
Evidence for Nonlinear X-ray Variability from the Broad-line Radio Galaxy 3C 390.3
We present analysis of the light curve from the ROSAT HRI monitoring
observations of the broad-line radio galaxy 3C 390.3. Observed every three days
for about 9 months, this is the first well sampled X-ray light curve on these
time scales. The flares and quiescent periods in the light curve suggest that
the variability is nonlinear, and a statistical test yields a detection with >6
sigma confidence. The structure function has a steep slope ~0.7, while the
periodogram is much steeper with a slope ~2.6, with the difference partially
due to a linear trend in the data. The non-stationary character of the light
curve could be evidence that the variability power spectrum has not turned over
to low frequencies, or it could be an essential part of the nonlinear process.
Evidence for X-ray reprocessing suggests that the X-ray emission is not from
the compact radio jet, and the reduced variability before and after flares
suggests there cannot be two components contributing to the X-ray short term
variability. Thus, these results cannot be explained easily by simple models
for AGN variability, including shot noise which may be associated with flares
in disk-corona models or active regions on a rotating disk, because in those
models the events are independent and the variability is therefore linear. The
character of the variability is similar to that seen in Cygnus X-1, which has
been explained by a reservoir or self-organized criticality model. Inherently
nonlinear, this model can reproduce the reduced variability before and after
large flares and the steep PDS seen generally from AGN. The 3C 390.3 light
curve presented here is the first support for such models to explain AGN
variability on intermediate time scales from a few days to months.Comment: 10 pages using (AASTeX) aaspp4.sty and 3 Postscript figures.
Astrophysical Journal Letters, in pres
Similar non-vertebral antifracture efficacy demonstrated with monthly ibandronate versus weekly bisphosphonate therapy: A retrospective cohort study
peer reviewe
The X-ray Spectrum of the Rapid Burster using the Chandra HETGS
We present observations of the Rapid Burster (RB, also known as MXB 1730-335)
using the Chandra High Energy Transmission Grating Spectrometer. The average
interval between type II (accretion) bursts was about 40 s. There was one type
I (thermonuclear flash) burst and about 20 "mini-bursts" which are probably
type II bursts whose peak flux is 10-40% of the average peak flux of the other
type II bursts. The time averaged spectra of the type II bursts are well fit by
a blackbody with a temperature of kT = 1.6 keV, a radius of 8.9 km for a
distance of 8.6 kpc, and an interstellar column density of 1.7e22 per sq. cm.
No narrow emission or absorption lines were clearly detected. The 3 sigma upper
limits to the equivalent widths of any features are < 10 eV in the 1.1-7.0 keV
band and as small as 1.5 eV near 1.7 keV. We suggest that Comptonization
destroys absorption features such as the resonance line of Fe XXVI.Comment: 10 pages, 4 figures, accepted for publication in AJ (with minor
changes and enhanced discussion of the instrument configuration
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