44,957 research outputs found
The Planet Nine Hypothesis
Over the course of the past two decades, observational surveys have unveiled
the intricate orbital structure of the Kuiper Belt, a field of icy bodies
orbiting the Sun beyond Neptune. In addition to a host of readily-predictable
orbital behavior, the emerging census of trans-Neptunian objects displays
dynamical phenomena that cannot be accounted for by interactions with the known
eight-planet solar system alone. Specifically, explanations for the observed
physical clustering of orbits with semi-major axes in excess of AU,
the detachment of perihelia of select Kuiper belt objects from Neptune, as well
as the dynamical origin of highly inclined/retrograde long-period orbits remain
elusive within the context of the classical view of the solar system. This
newly outlined dynamical architecture of the distant solar system points to the
existence of a new planet with mass of , residing on
a moderately inclined orbit () with semi-major axis AU and eccentricity between . This paper
reviews the observational motivation, dynamical constraints, and prospects for
detection of this proposed object known as Planet Nine.Comment: 92 pages, 28 figures, published in Physics Report
A new model for the X-ray continuum of the magnetized accreting pulsars
Accreting highly magnetized pulsars in binary systems are among the brightest
X-ray emitters in our Galaxy. Although a number of high statistical quality
broad-band (0.1-100 keV) X-ray observations are available, the spectral energy
distribution of these sources is usually investigated by adopting pure
phenomenological models, rather than models linked to the physics of accretion.
In this paper, a detailed spectral study of the X-ray emission recorded from
the high-mass X-ray binary pulsars Cen X-3, 4U 0115+63, and Her X-1 is carried
out by using BeppoSAX and joined Suzaku+NuStar data, together with an advanced
version of the compmag model. The latter provides a physical description of the
high energy emission from accreting pulsars, including the thermal and bulk
Comptonization of cyclotron and bremsstrahlung seed photons along the neutron
star accretion column. The compmag model is based on an iterative method for
solving second-order partial differential equations, whose convergence
algorithm has been improved and consolidated during the preparation of this
paper. Our analysis shows that the broad-band X-ray continuum of all considered
sources can be self-consistently described by the compmag model. The cyclotron
absorption features, not included in the model, can be accounted for by using
Gaussian components. From the fits of the compmag model to the data we inferred
the physical properties of the accretion columns in all sources, finding values
reasonably close to those theoretically expected according to our current
understanding of accretion in highly magnetized neutron stars. The updated
version of the compmag model has been tailored to the physical processes that
are known to occur in the columns of highly magnetized accreting neutron stars
and it can thus provide a better understanding of the high energy radiation
from these sources.Comment: 19 pages, 10 figures, accepted for publication in A&
Transcript of My Father’s Heroics
This story is an excerpt from a longer interview that was collected as part of the Launching through the Surf: The Dory Fleet of Pacific City project. In this story, Sid Fisher recounts how his father, Walt Fisher, saved him from rolling his dory
Analytical ground state for the three-band Hubbard model
For the calculation of charge excitations as those observed in, e.g.,
photo-emission spectroscopy or in electron-energy loss spectroscopy, a correct
description of ground-state charge properties is essential. In strongly
correlated systems like the undoped cuprates this is a highly non-trivial
problem. In this paper we derive a non-perturbative analytical approximation
for the ground state of the three-band Hubbard model on an infinite, half
filled CuO_2 plane. By comparison with Projector Quantum Monte Carlo
calculations it is shown that the resulting expressions correctly describe the
charge properties of the ground state. Relations to other approaches are
discussed. The analytical ground state preserves size consistency and can be
generalized for other geometries, while still being both easy to interpret and
to evaluate.Comment: REVTeX, 8 pages, 6 figures, to appear in Phys. Rev.
Interference effects in above-threshold ionization from diatomic molecules: determining the internuclear separation
We calculate angle-resolved above-threshold ionization spectra for diatomic
molecules in linearly polarized laser fields, employing the strong-field
approximation. The interference structure resulting from the individual
contributions of the different scattering scenarios is discussed in detail,
with respect to the dependence on the internuclear distance and molecular
orientation. We show that, in general, the contributions from the processes in
which the electron is freed at one center and rescatters off the other obscure
the interference maxima and minima obtained from single-center processes.
However, around the boundary of the energy regions for which rescattering has a
classical counterpart, such processes play a negligible role and very clear
interference patterns are observed. In such energy regions, one is able to
infer the internuclear distance from the energy difference between adjacent
interference minima.Comment: 10 pages, 8 figures; discussions slightly modified and an additional
figure inserted for clarit
The Deep Lens Survey Transient Search I : Short Timescale and Astrometric Variability
We report on the methodology and first results from the Deep Lens Survey
transient search. We utilize image subtraction on survey data to yield all
sources of optical variability down to 24th magnitude. Images are analyzed
immediately after acquisition, at the telescope and in near-real time, to allow
for followup in the case of time-critical events. All classes of transients are
posted to the web upon detection. Our observing strategy allows sensitivity to
variability over several decades in timescale. The DLS is the first survey to
classify and report all types of photometric and astrometric variability
detected, including solar system objects, variable stars, supernovae, and short
timescale phenomena. Three unusual optical transient events were detected,
flaring on thousand-second timescales. All three events were seen in the B
passband, suggesting blue color indices for the phenomena. One event (OT
20020115) is determined to be from a flaring Galactic dwarf star of spectral
type dM4. From the remaining two events, we find an overall rate of \eta = 1.4
events deg-2 day-1 on thousand-second timescales, with a 95% confidence limit
of \eta < 4.3. One of these events (OT 20010326) originated from a compact
precursor in the field of galaxy cluster Abell 1836, and its nature is
uncertain. For the second (OT 20030305) we find strong evidence for an extended
extragalactic host. A dearth of such events in the R passband yields an upper
95% confidence limit on short timescale astronomical variability between 19.5 <
R < 23.4 of \eta_R < 5.2. We report also on our ensemble of astrometrically
variable objects, as well as an example of photometric variability with an
undetected precursor.Comment: 24 pages, 12 figures, 3 tables. Accepted for publication in ApJ.
Variability data available at http://dls.bell-labs.com/transients.htm
Report on the capability assessment and improvement workshop (CAIW) at iPres 2013
While Digital Preservation is making progress in terms of tool development, progressive establishment of standards and increasing activity in user communities, there is a lack of approaches to systematically assessing, comparing and improving how organizations go about achieving their preservation goals. This currently presents a challenge to many organizations for whom digital stewardship is a concern and reveals a substantial gap between theory and practice. To provide an interactive, focused venue for those interested in systematic approaches for assessment and improvement, we organized the first Capability Assessment and Improvement Workshop (CAIW) in Lisbon, on September 5, 2013, as part of the 10th International Conference on Digital Preservation (iPres 2013). This article provides a report on the issues discussed and attempts to synthesize the main conclusions with the intention of stimulating further discussion in the community on this topic
Laparoscopy Pneumoperitoneum Fuzzy Modeling
Abstract: Gas volume to intra-peritoneal pressure fuzzy modeling for evaluating pneumoperitoneum in videolaparoscopic surgery is proposed in this paper. The proposed approach innovates in using fuzzy logic and fuzzy set theory for evaluating the accuracy of the prognosis value in order to minimize or avoid iatrogenic injuries due to the blind needle puncture. In so doing, it demonstrates the feasibility of fuzzy analysis to contribute to medicine and health care. Fuzzy systems is employed here in synergy with artificial neural network based on backpropaga tion, multilayer perceptron architecture for building up numerical functions. Experimental data employed for analysis were collected in the accomplishment of the pneumoperitoneum in a random population of patients submitted to videolaparoscopic surgeries. Numerical results indicate that the proposed fuzzy mapping for describing the relation from the intra peritoneal pressure measures as function injected gas volumes succeeded in determinining a fuzzy model for this nonlinear system when compared to the statistical model
Chandra X-Ray Observations of Nineteen Millisecond Pulsars in the Globular Cluster 47 Tucanae
We present spectral and long-timescale variability analyses of
\textit{Chandra} ACIS-S observations of the 19 millisecond pulsars (MSPs) with
precisely known positions in the globular cluster 47 Tucanae. The X-ray
emission of the majority of these MSPs is well described by a thermal
(blackbody or neutron star hydrogen atmosphere) spectrum with a temperature
K, emission radius km,
and luminosity ergs s. For several MSPs, there is
indication that a second thermal component is required, similar to what is seen
in some nearby field MSPs. The radio-eclipsing binary MSPs 47 Tuc J, O, and W
show a significant non-thermal component, with photon index ,
which may originate in an shock formed due to interaction between the
relativistic pulsar wind and matter from the stellar companion. We re-examine
the X-ray--spindown luminosity relation () and find that due to
the large uncertainties in both parameters the result is consistent with both
the linear relation and the flatter
predicted by polar cap heating models. In terms of X-ray properties, we find no
clear systematic differences between MSPs in globular clusters and in the field
of the Galaxy.Comment: 13 pages, 6 figures, accepted for publication in the Astrophysical
Journa
Enhanced gravity-wave activity and interhemispheric coupling during the MaCWAVE/MIDAS northern summer program 2002
We present new sensitivity experiments that link observed anomalies of the mesosphere and lower thermosphere at high latitudes during the MaCWAVE/MIDAS summer program 2002 to enhanced planetary Rossby-wave activity in the austral winter troposphere. <P style="line-height: 20px;"> We employ the same general concept of a GCM having simplified representations of radiative and latent heating as in a previous study by Becker et al.&nbsp;(2004). In the present version, however, the model includes no gravity wave (GW) parameterization. Instead we employ a high vertical and a moderate horizontal resolution in order to describe GW effects explicitly. This is supported by advanced, nonlinear momentum diffusion schemes that allow for a self-consistent generation of inertia and mid-frequency GWs in the lower atmosphere, their vertical propagation into the mesosphere and lower thermosphere, and their subsequent dissipation which is induced by prescribed horizontal and vertical mixing lengths as functions of height. <P style="line-height: 20px;"> The main anomalies in northern summer&nbsp;2002 consist of higher temperatures than usual above 82 km, an anomalous eastward mean zonal wind between 70 and 90 km, an altered meridional flow, enhanced turbulent dissipation below 80 km, and enhanced temperature variations associated with GWs. These signals are all reasonably described by differences between two long-integration perpetual model runs, one with normal July conditions, and another run with modified latent heating in the tropics and Southern Hemisphere to mimic conditions that correspond to the unusual austral winter&nbsp;2002. The model response to the enhanced winter hemisphere Rossby-wave activity has resulted in both an interhemispheric coupling through a downward shift of the GW-driven branch of the residual circulation and an increased GW activity at high summer latitudes. Thus a quantitative explanation of the dynamical state of the northern mesosphere and lower thermosphere during June-August&nbsp;2002 requires an enhanced Lorenz energy cycle and correspondingly enhanced GW sources in the troposphere, which in the model show up in both hemispheres
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