7,449 research outputs found
Is the Compact Source at the Center of Cas A Pulsed?
A 50 ksec observation of the Supernova Remnant Cas A was taken using the
Chandra X-Ray Observatory High Resolution Camera (HRC) to search for periodic
signals from the compact source located near the center. Using the HRC-S in
imaging mode, problems with correctly assigning times to events were overcome,
allowing the period search to be extended to higher frequencies than possible
with previous observations. In an extensive analysis of the HRC data, several
possible candidate signals are found using various algorithms, including
advanced techniques developed by Ransom to search for low significance periodic
signals. Of the candidate periods, none is at a high enough confidence level to
be particularly favored over the rest. When combined with other information,
however (e.g., spectra, total energetics, and the historical age of the
remnant), a 12 ms candidate period seems to be more physically plausible than
the others, and we use it for illustrative purposes in discussing the possible
properties of a putative neutron star in the remnant. We emphasize that this is
not necessarily the true period, and that a follow-up observation, scheduled
for the fall of 2001, is required.
A 50 ksec Advanced CCD Imaging Spectrometer (ACIS) observation was taken, and
analysis of these data for the central object shows that the spectrum is
consistent with several forms, and that the emitted X-ray luminosity in the 0.1
-10 keV band is 10^{33}-10^{35}erg cm^{-2}sec^{-1} depending on the spectral
model and the interstellar absorption along the line of sight to the source.Comment: 14 pages, 3 figures Submitted to ApJ 2001 June 2
Persistent junk solutions in time-domain modeling of extreme mass ratio binaries
In the context of metric perturbation theory for non-spinning black holes,
extreme mass ratio binary (EMRB) systems are described by distributionally
forced master wave equations. Numerical solution of a master wave equation as
an initial boundary value problem requires initial data. However, because the
correct initial data for generic-orbit systems is unknown, specification of
trivial initial data is a common choice, despite being inconsistent and
resulting in a solution which is initially discontinuous in time. As is well
known, this choice leads to a "burst" of junk radiation which eventually
propagates off the computational domain. We observe another unintended
consequence of trivial initial data: development of a persistent spurious
solution, here referred to as the Jost junk solution, which contaminates the
physical solution for long times. This work studies the influence of both types
of junk on metric perturbations, waveforms, and self-force measurements, and it
demonstrates that smooth modified source terms mollify the Jost solution and
reduce junk radiation. Our concluding section discusses the applicability of
these observations to other numerical schemes and techniques used to solve
distributionally forced master wave equations.Comment: Uses revtex4, 16 pages, 9 figures, 3 tables. Document reformatted and
modified based on referee's report. Commentary added which addresses the
possible presence of persistent junk solutions in other approaches for
solving master wave equation
Learning from Examples with Unspecified Attribute Values
We introduce the UAV learning model in which some of the attributes in the examples are unspecified. In our model, an example x is classified positive (resp., negative) if all possible assignments for the unspecified attributes result in a positive (resp., negative) classification. Otherwise the classificatoin given to x is ? (for unknown). Given an example x in which some attributes are unspecified, the oracle UAV-MQ responds with the classification of x. Given a hypothesis h, the oracle UAV-EQ returns an example x (that could have unspecified attributes) for which h(x) is incorrect. We show that any class learnable in the exact model using the MQ and EQ oracles is also learnable in the UAV model using the MQ and UAV-EQ oracles as long as the counterexamples provided by the UAV-EQ oracle have a logarithmic number of unspecified attributes. We also show that any class learnable in the exact model using the MQ and EQ oracles is also learnable in the UAV model using the UAV-MQ and UAV-EQ oracles as well as an oracle to evaluate a given boolean formula on an example with unspecified attributes. (For some hypothesis classes such as decision trees and unate formulas the evaluation can be done in polynomial time without an oracle.) We also study the learnability of a universal class of decision trees under the UAV model and of DNF formulas under a representation-dependent variation of the UAV model
Agnostic Learning of Geometric Patterns
Goldberg, Goldman, and Scott demonstrated how the problem of recognizing a landmark from a one-dimensional visual image can be mapped to that of learning a one-dimensional geometric pattern and gave a PAC algorithm to learn that class. In this paper, we present an efficient on-line agnostic learning algorithm for learning the class of constant-dimension geometric patterns. Our algorithm can tolerate both classification and attribute noise. By working in higher dimensional spaces we can represent more features from the visual image in the geometric pattern. Our mapping of the data to a geometric pattern, and hence our learning algorithm, is applicable to any data representable as a constant-dimensional array of values, e.g. sonar data, temporal difference information, or amplitudes of a waveform. To our knowledge, these classes of patterns are more complex than any class of geometric patterns previously studied. Also, our results are easily adapted to learn the union of fixed-dimensional boxes from multiple-instance examples. Finally, our algorithms are tolerant of concept shift
Strong-field tidal distortions of rotating black holes: Formalism and results for circular, equatorial orbits
Tidal coupling between members of a compact binary system can have an
interesting and important influence on that binary's dynamical inspiral. Tidal
coupling also distorts the binary's members, changing them (at lowest order)
from spheres to ellipsoids. At least in the limit of fluid bodies and Newtonian
gravity, there are simple connections between the geometry of the distorted
ellipsoid and the impact of tides on the orbit's evolution. In this paper, we
develop tools for investigating tidal distortions of rapidly rotating black
holes using techniques that are good for strong-field, fast-motion binary
orbits. We use black hole perturbation theory, so our results assume extreme
mass ratios. We develop tools to compute the distortion to a black hole's
curvature for any spin parameter, and for tidal fields arising from any bound
orbit, in the frequency domain. We also develop tools to visualize the
horizon's distortion for black hole spin (leaving the more
complicated case to a future analysis). We then study how a
Kerr black hole's event horizon is distorted by a small body in a circular,
equatorial orbit. We find that the connection between the geometry of tidal
distortion and the orbit's evolution is not as simple as in the Newtonian
limit.Comment: 37 pages, 8 figures. Accepted for publication to Physical Review D.
This version corrects a number of typographical errors found when reviewing
the page proof
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Anti-CD20 therapy depletes activated myelin-specific CD8+ T cells in multiple sclerosis.
CD8+ T cells are believed to play an important role in multiple sclerosis (MS), yet their role in MS pathogenesis remains poorly defined. Although myelin proteins are considered potential autoantigenic targets, prior studies of myelin-reactive CD8+ T cells in MS have relied on in vitro stimulation, thereby limiting accurate measurement of their ex vivo precursor frequencies and phenotypes. Peptide:MHC I tetramers were used to identify and validate 5 myelin CD8+ T cell epitopes, including 2 newly described determinants in humans. The validated tetramers were used to measure the ex vivo precursor frequencies and phenotypes of myelin-specific CD8+ T cells in the peripheral blood of untreated MS patients and HLA allele-matched healthy controls. In parallel, CD8+ T cell responses against immunodominant influenza epitopes were also measured. There were no differences in ex vivo frequencies of tetramer-positive myelin-specific CD8+ T cells between MS patients and control subjects. An increased proportion of myelin-specific CD8+ T cells in MS patients exhibited a memory phenotype and expressed CD20 compared to control subjects, while there were no phenotypic differences observed among influenza-specific CD8+ T cells. Longitudinal assessments were also measured in a subset of MS patients subsequently treated with anti-CD20 monoclonal antibody therapy. The proportion of memory and CD20+ CD8+ T cells specific for certain myelin but not influenza epitopes was significantly reduced following anti-CD20 treatment. This study, representing a characterization of unmanipulated myelin-reactive CD8+ T cells in MS, indicates these cells may be attractive targets in MS therapy
A New Search Technique for Short Orbital Period Binary Pulsars
We describe a new and efficient technique which we call sideband or
phase-modulation searching that allows one to detect short period binary
pulsars in observations longer than the orbital period. The orbital motion of
the pulsar during long observations effectively modulates the phase of the
pulsar signal causing sidebands to appear around the pulsar spin frequency and
its harmonics in the Fourier transform. For the majority of binary radio
pulsars or Low-Mass X-ray Binaries (LMXBs), large numbers of sidebands are
present, allowing efficient searches using Fourier transforms of short portions
of the original power spectrum. Analysis of the complex amplitudes and phases
of the sidebands can provide enough information to solve for the Keplerian
orbital parameters. This technique is particularly applicable to radio pulsar
searches in globular clusters and searches for coherent X-ray pulsations from
LMXBs and is complementary to more standard ``acceleration'' searches.Comment: 22 pages. 8 figures. Submitted to Ap
Discontinuous Galerkin method for computing gravitational waveforms from extreme mass ratio binaries
Gravitational wave emission from extreme mass ratio binaries (EMRBs) should
be detectable by the joint NASA-ESA LISA project, spurring interest in
analytical and numerical methods for investigating EMRBs. We describe a
discontinuous Galerkin (dG) method for solving the distributionally forced 1+1
wave equations which arise when modeling EMRBs via the perturbation theory of
Schwarzschild blackholes. Despite the presence of jump discontinuities in the
relevant polar and axial gravitational "master functions", our dG method
achieves global spectral accuracy, provided that we know the instantaneous
position, velocity, and acceleration of the small particle. Here these
variables are known, since we assume that the particle follows a timelike
geodesic of the Schwarzschild geometry. We document the results of several
numerical experiments testing our method, and in our concluding section discuss
the possible inclusion of gravitational self-force effects.Comment: Revised in response to referees' reports. Several minus sign errors
corrected, and improved tables. Uses revtex4, 27 pages, 6 figures, 3 table
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