2,401 research outputs found
Should We Learn Probabilistic Models for Model Checking? A New Approach and An Empirical Study
Many automated system analysis techniques (e.g., model checking, model-based
testing) rely on first obtaining a model of the system under analysis. System
modeling is often done manually, which is often considered as a hindrance to
adopt model-based system analysis and development techniques. To overcome this
problem, researchers have proposed to automatically "learn" models based on
sample system executions and shown that the learned models can be useful
sometimes. There are however many questions to be answered. For instance, how
much shall we generalize from the observed samples and how fast would learning
converge? Or, would the analysis result based on the learned model be more
accurate than the estimation we could have obtained by sampling many system
executions within the same amount of time? In this work, we investigate
existing algorithms for learning probabilistic models for model checking,
propose an evolution-based approach for better controlling the degree of
generalization and conduct an empirical study in order to answer the questions.
One of our findings is that the effectiveness of learning may sometimes be
limited.Comment: 15 pages, plus 2 reference pages, accepted by FASE 2017 in ETAP
Polarized Radiation Signals from Highly Magnetized Neutron Star Surfaces
The surfaces of neutron stars are likely sources of strongly polarized soft X
rays due to the presence of strong magnetic fields. Scattering transport in the
surface layers is critical to the determination of the emergent anisotropy of
light intensity, and is strongly influenced by the complicated interplay
between linear and circular polarization information. We have developed a
magnetic Thomson scattering simulation to model the outer layers of
fully-ionized atmospheres in such compact objects. Here we summarize emergent
intensities and polarizations from extended atmospheric simulations, spanning
considerable ranges of magnetic colatitudes. General relativistic propagation
of light from the surface to infinity is fully included. The net polarization
degrees are moderate and not very small when summing over a variety of field
directions. These results provide an important foundation for observations of
magnetars to be acquired by NASA's new IXPE X-ray polarimeter and future X-ray
polarimetry missions.Comment: 4 pages, 3 figures, accepted for publication in the proceedings of
the IAU Symposium 363, Neutron Star Astrophysics at the Crossroads: Magnetars
and the Multimessenger Revolution, eds. E. Troja & M. G. Barin
Level densities and -ray strength functions in Yb
Level densities and radiative strength functions in Yb and Yb
nuclei have been measured using the
Yb(He,He)Yb and
Yb(He,)Yb reactions. New data on Yb
are compared to a previous measurement for Yb from the
Yb(He,)Yb reaction. Systematics of level
densities and radiative strength functions in Yb are
established. The entropy excess in Yb relative to the even-even nuclei
Yb due to the unpaired neutron quasiparticle is found to be
approximately 2. Results for the radiative strength function from the two
reactions lead to consistent parameters characterizing the ``pygmy''
resonances. Pygmy resonances in the Yb populated by the
(He,) reaction appear to be split into two components for both of
which a complete set of resonance parameters are obtained.Comment: 8 pages, 7 figure
Randomized crossover comparison of proportional assist ventilation and patient-triggered ventilation in extremely low birth weight infants with evolving chronic lung disease
Background: Refinement of ventilatory techniques remains a challenge given the persistence of chronic lung disease of preterm infants. Objective: To test the hypothesis that proportional assist ventilation ( PAV) will allow to lower the ventilator pressure at equivalent fractions of inspiratory oxygen (FiO(2)) and arterial hemoglobin oxygen saturation in ventilator-dependent extremely low birth weight infants in comparison with standard patient-triggered ventilation ( PTV). Methods: Design: Randomized crossover design. Setting: Two level-3 university perinatal centers. Patients: 22 infants ( mean (SD): birth weight, 705 g ( 215); gestational age, 25.6 weeks ( 2.0); age at study, 22.9 days ( 15.6)). Interventions: One 4- hour period of PAV was applied on each of 2 consecutive days and compared with epochs of standard PTV. Results: Mean airway pressure was 5.64 ( SD, 0.81) cm H2O during PAV and 6.59 ( SD, 1.26) cm H2O during PTV ( p < 0.0001), the mean peak inspiratory pressure was 10.3 ( SD, 2.48) cm H2O and 15.1 ( SD, 3.64) cm H2O ( p < 0.001), respectively. The FiO(2) ( 0.34 (0.13) vs. 0.34 ( 0.14)) and pulse oximetry readings were not significantly different. The incidence of arterial oxygen desaturations was not different ( 3.48 ( 3.2) vs. 3.34 ( 3.0) episodes/ h) but desaturations lasted longer during PAV ( 2.60 ( 2.8) vs. 1.85 ( 2.2) min of desaturation/ h, p = 0.049). PaCO2 measured transcutaneously in a subgroup of 12 infants was similar. One infant met prespecified PAV failure criteria. No adverse events occurred during the 164 cumulative hours of PAV application. Conclusions: PAV safely maintains gas exchange at lower mean airway pressures compared with PTV without adverse effects in this population. Backup conventional ventilation breaths must be provided to prevent apnea-related desaturations. Copyright (c) 2007 S. Karger AG, Base
High-Resolution X-Ray Spectroscopy of the Bursting Pulsar GRO J1744-28
The bursting pulsar GRO J1744-28 is a Galactic low-mass X-ray binary that
distinguishes itself by displaying type-II X-ray bursts: brief, bright flashes
of X-ray emission that likely arise from spasmodic accretion. Combined with its
coherent 2.1 Hz X-ray pulsations and relatively high estimated magnetic field,
it is a particularly interesting source to study the physics of accretion flows
around neutron stars. Here we report on Chandra/HETG observations obtained near
the peak of its bright 2014 accretion outburst. Spectral analysis suggests the
presence of a broad iron emission line centered at E_l ~ 6.7 keV. Fits with a
disk reflection model yield an inclination angle of i ~ 52 degrees and an inner
disk radius of R_in ~ 85 GM/c^2, which is much further out than typically found
for neutron star low-mass X-ray binaries. Assuming that the disk is truncated
at the magnetospheric radius of the neutron star, we estimate a magnetic field
strength of B ~ (2-6)E10 G. Furthermore, we identify an absorption feature near
~6.85 keV could correspond to blue-shifted Fe xxv and point to a fast disk wind
with an outflow velocity of v_out ~ (7.5-8.2)E3 km/s (~0.025c-0.027c). If the
covering fraction and filling factor are large, this wind could be
energetically important and perhaps account for the fact that the companion
star lost significant mass while the magnetic field of the neutron star
remained strong.Comment: Accepted for publication in ApJ Letters, 6 pages, 4 figures, 1 tabl
Application of the Fisher-Rao metric to ellipse detection
The parameter space for the ellipses in a two dimensional image is a five dimensional manifold, where each point of the manifold corresponds to an ellipse in the image. The parameter space becomes a Riemannian manifold under a Fisher-Rao metric, which is derived from a Gaussian model for the blurring of ellipses in the image. Two points in the parameter space are close together under the Fisher-Rao metric if the corresponding ellipses are close together in the image. The Fisher-Rao metric is accurately approximated by a simpler metric under the assumption that the blurring is small compared with the sizes of the ellipses under consideration. It is shown that the parameter space for the ellipses in the image has a finite volume under the approximation to the Fisher-Rao metric. As a consequence the parameter space can be replaced, for the purpose of ellipse detection, by a finite set of points sampled from it. An efficient algorithm for sampling the parameter space is described. The algorithm uses the fact that the approximating metric is flat, and therefore locally Euclidean, on each three dimensional family of ellipses with a fixed orientation and a fixed eccentricity. Once the sample points have been obtained, ellipses are detected in a given image by checking each sample point in turn to see if the corresponding ellipse is supported by the nearby image pixel values. The resulting algorithm for ellipse detection is implemented. A multiresolution version of the algorithm is also implemented. The experimental results suggest that ellipses can be reliably detected in a given low resolution image and that the number of false detections
can be reduced using the multiresolution algorithm
Evolution of level density step structures from 56,57-Fe to 96,97-Mo
Level densities have been extracted from primary gamma spectra for 56,57-Fe
and 96,97-Mo nuclei using (3-He,alpha gamma) and (3-He,3-He') reactions on
57-Fe and 97-Mo targets. The level density curves reveal step structures above
the pairing gap due to the breaking of nucleon Cooper pairs. The location of
the step structures in energy and their shapes arise from the interplay between
single-particle energies and seniority-conserving and seniority-non-conserving
interactions.Comment: 9 pages, including 5 figure
Detection of Very Low-Frequency Quasi-Periodic Oscillations in the 2015 Outburst of V404 Cygni
In June 2015, the black hole X-ray binary (BHXRB) V404 Cygni went into
outburst for the first time since 1989. Here, we present a comprehensive search
for quasi-periodic oscillations (QPOs) of V404 Cygni during its recent
outburst, utilizing data from six instruments on board five different X-ray
missions: Swift/XRT, Fermi/GBM, Chandra/ACIS, INTEGRAL's IBIS/ISGRI and JEM-X,
and NuSTAR. We report the detection of a QPO at 18 mHz simultaneously with both
Fermi/GBM and Swift/XRT, another example of a rare but slowly growing new class
of mHz-QPOs in BHXRBs linked to sources with a high orbital inclination.
Additionally, we find a duo of QPOs in a Chandra/ACIS observation at 73 mHz and
1.03 Hz, as well as a QPO at 136 mHz in a single Swift/XRT observation that can
be interpreted as standard Type-C QPOs. Aside from the detected QPOs, there is
significant structure in the broadband power, with a strong feature observable
in the Chandra observations between 0.1 and 1 Hz. We discuss our results in the
context of current models for QPO formation.Comment: 17 pages, 9 figures, published in Ap
Evidence for an abundant old population of Galactic ultra long period magnetars and implications for fast radio bursts
Two recent discoveries, namely PSR J0901-4046 and GLEAM-X J162759.5-523504.3
(hereafter GLEAM-X J1627), have corroborated an extant population of radio-loud
periodic sources with long periods (76 s and 1091 s respectively) whose
emission can hardly be explained by rotation losses. We argue that GLEAM-X
J1627 is a highly-magnetized object consistent with a magnetar (an ultra long
period magnetar - ULPM), and demonstrate it is unlikely to be either a
magnetically or a rotationally-powered white dwarf. By studying these sources
together with previously detected objects, we find there are at least a handful
of promising candidates for Galactic ULPMs. The detections of these objects
imply a substantial number, and for PSR
J0901--4046 like and GLEAM-X J1627 like objects, respectively, within our
Galaxy. These source densities, as well as cooling age limits from
non-detection of thermal X-rays, Galactic offsets, timing stability and dipole
spindown limits, all imply the ULPM candidates are substantially older than
confirmed Galactic magnetars and that their formation channel is a common one.
Their existence implies widespread survival of magnetar-like fields for several
Myr, distinct from the inferred behaviour in confirmed Galactic magnetars.
ULPMs may also constitute a second class of FRB progenitors which could
naturally exhibit very long periodic activity windows. Finally, we show that
existing radio campaigns are biased against detecting objects like these and
discuss strategies for future radio and X-ray surveys to identify more such
objects. We estimate that more such objects should be detected
with SKA-MID and DSA-2000.Comment: 22 pages, 10 figures. Published in MNRA
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