2,019 research outputs found
Gravitational radiation from a particle plunging into a Schwarzschild black hole: frequency-domain and semi-relativistic analyses
We revisit the classic problem of gravitational wave emission by a test
particle plunging into a Schwarzschild black hole both in the frequency-domain
Regge-Wheeler-Zerilli formalism and in the semi-relativistic approximation. We
use, and generalize, a transformation due to Nakamura, Sasaki and Shibata to
improve the fall-off of the source term of the Zerilli function. The faster
decay improves the numerical convergence of quantities of interest, such as the
energy radiated at spatial infinity through gravitational waves. As a test of
the method, we study the gravitational radiation produced by test particles
that plunge into the black hole with impact parameters close to the threshold
for scattering. We recover and expand upon previous results that were obtained
using the Sasaki-Nakamura equation. In particular, we study the relative
contributions to the total energy radiated due to waves of axial and polar
parity, and uncover an universal behavior in the waveforms at late times. We
complement our study with a semi-relativistic analysis of the problem, and we
compare the two approaches. The generalized Nakamura-Sasaki-Shibata
transformation presented here is a simple and practical alternative for the
analysis of gravitational-wave emission by unbound orbits in the Schwarzschild
spacetime using the frequency-domain Regge-Wheeler-Zerilli formalism.Comment: 16 pages, 10 figure
The surface of rapidly-rotating neutron stars: implications to neutron star parameter estimation
The Neutron star Interior Composition Explorer (NICER) is currently observing
the x-ray pulse profiles emitted by hot spots on the surface of rotating
neutron stars allowing for an inference of their radii with unprecedented
precision. A critical ingredient in the pulse profile model is an analytical
formula for the oblate shape of the star. These formulas require a fitting over
a large ensemble of neutron star solutions, which cover a wide set of equations
of state, stellar compactnesses and rotational frequencies. However, this
procedure introduces a source of systematic error, as (i) the fits do not
describe perfectly the surface of all stars in the ensemble and (ii) neutron
stars are described by a single equation of state, whose influence on the
surface shape is averaged out during the fitting procedure. Here we perform a
first study of this systematic error, finding evidence that it is subdominant
relative to the statistical error in the radius inference by NICER. We also
find evidence that the formula currently used by NICER can be used in the
inference of the radii of rapidly rotating stars, outside of the formula's
domain of validity. Moreover, we employ an accurate enthalpy-based method to
locate the surface of numerical solutions of rapidly rotating neutron stars and
a new highly-accurate formula to describe their surfaces. These results can be
used in applications that require an accurate description of oblate surfaces of
rapidly rotating neutron stars.Comment: 19 pages, 12 figures, 7 tables, 1 appendi
Hadron properties in the nuclear medium
The QCD vacuum shows the dynamical breaking of chiral symmetry. In the
hot/dense QCD medium, the chiral order parameter such as is
expected to change as function of temperature and density of the
medium, and its experimental detection is one of the main challenges in modern
hadron physics. In this article, we discuss theoretical expectations for the
in-medium hadron spectra associated with partial restoration of chiral symmetry
and the current status of experiments with an emphasis on the measurements of
properties of mesons produced in near-ground-state nuclei.Comment: 40 pages, submitted to Reviews of Modern Physic
Recent topics of mesic atoms and mesic nuclei -- mesic nuclei exist ?--
We study -meson production in nuclei to investigate the in-medium
modification of the -meson spectral function at finite density. We
consider (), () and () reactions to produce a
-meson inside the nucleus and evaluate the effects of the medium
modifications to reaction cross sections. The structures of the bound states,
-mesic nuclei, are also studied. For strong absorptive interaction cases,
we need to know the spectrum shape in a wide energy region to deduce the
properties of .Comment: Talk given at EXA08, Vienna, September 2008. To be published in the
Proceedings, Hyperfine Interactions. 6 pages, 6 figure
Effect of the curvature and the {\beta} parameter on the nonlinear dynamics of a drift tearing magnetic island
We present numerical simulation studies of 2D reduced MHD equations
investigating the impact of the electronic \beta parameter and of curvature
effects on the nonlinear evolution of drift tearing islands. We observe a
bifurcation phenomenon that leads to an amplification of the pressure energy,
the generation of E \times B poloidal flow and a nonlinear diamagnetic drift
that affects the rotation of the magnetic island. These dynamical modifications
arise due to quasilinear effects that generate a zonal flow at the onset point
of the bifurcation. Our simulations show that the transition point is
influenced by the \beta parameter such that the pressure gradient through a
curvature effect strongly stabilizes the transition. Regarding the modified
rotation of the island, a model for the frequency is derived in order to study
its origin and the effect of the \beta parameter. It appears that after the
transition, an E \times B poloidal flow as well as a nonlinear diamagnetic
drift are generated due to an amplification of the stresses by pressure
effects
Temperature Dependence of Zero-Bias Resistances of a Single Resistance-Shunted Josephson Junction
Zero-bias resistances of a single resistance-shunted Josephson junction are
calculated as a function of the temperature by means of the path-integral Monte
Carlo method in case a charging energy is comparable with a
Josephson energy . The low-temperature behavior of the zero-bias
resistance changes around , where is
a shunt resistance and . The temperature dependence of the
zero-bias resistance shows a power-law-like behavior whose exponent depends on
. These results are compared with the experiments on
resistance-shunted Josephson junctions
Keck/LRIS Spectroscopic Confirmation of Coma Cluster Dwarf Galaxy Membership Assignments
Keck/LRIS multi-object spectroscopy has been carried out on 140 of some of
the lowest and highest surface brightness faint (19 < R < 22) dwarf galaxy
candidates in the core region of the Coma Cluster. These spectra are used to
measure redshifts and establish membership for these faint dwarf populations.
The primary goal of the low surface brightness sample is to test our ability to
use morphological and surface brightness criteria to distinguish between Coma
Cluster members and background galaxies using high resolution HST/ACS images.
Candidates were rated as expected members, uncertain, or expected background.
From 93 spectra, 51 dwarf galaxy members and 20 background galaxies are
identified. Our morphological membership estimation success rate is ~100% for
objects expected to be members and better than ~90% for galaxies expected to be
in the background. We confirm that low surface brightness is a very good
indicator of cluster membership. High surface brightness galaxies are almost
always background with confusion arising only from the cases of the rare
compact elliptical galaxies. The more problematic cases occur at intermediate
surface brightness. Many of these galaxies are given uncertain membership
ratings, and these were found to be members about half of the time. Including
color information will improve membership determination but will fail for some
of the same objects that are already mis-identified when using only surface
brightness and morphology criteria. Compact elliptical galaxies with B-V colors
~0.2 magnitudes redward of the red sequence in particular require spectroscopic
follow-up. In a sample of 47 high surface brightness, UCD candidates, 19
objects have redshifts which place them in the Coma Cluster. Redshift
measurements are presented and the use of indirect means for establishing
cluster membership is discussed.Comment: Accepted for publication in ApJ, 25 pages, 15 figure
Bacterial contamination of anesthesia machinesâ internal breathing-circuit-systems
Background: Bacterial contamination of anesthesia breathing machines and their potential hazard for pulmonary infection and cross-infection among anesthetized patients has been an infection control issue since the 1950s. Disposable equipment and bacterial filters have been introduced to minimize this risk. However, the machinesâ internal breathing-circuit-system has been considered to be free of micro-organisms without providing adequate data supporting this view. The aim of the study was to investigate if any micro-organisms can be yielded from used internal machinesâ breathing-circuit-system. Based on such results objective reprocessing intervals could be defined
Narrow band imaging in [OIII] and Halpha to search for ICPNe in the Virgo cluster
We have identified intracluster planetary nebulae in a Virgo cluster core
field by imaging with the Subaru Suprime-Cam through two narrow band filters
centered at the redshifted wavelengths of the [OIII] \AA and the
H \AA lines; broad-band images in V and R were
acquired to check for emission in the adjacent continuum. Emission line objects
in Virgo are then selected from the two-color diagram ([OIII] -- Halpha) vs.
([OIII] -- (V+R)), which is calibrated using PNe in M84 (Jacoby et al. 1990).
Using both [OIII] and Halpha allows us to distinguish bona-fide planetary
nebulae from high redshift emission-line galaxies at the bright end of the
[OIII] luminosity function. Spectroscopic observations of a subsample of these
objects were made at the TNG and at the VLT, in a region around M84 and in an
intracluster field respectively. The observations confirm the efficiency of the
combined [OIII]+H imaging to identify true PNe. We also obtained the
first spectrum of an intracluster PN which shows the [OIII] doublet with S/N >
10 and its Halpha emission. From the results based on the spectroscopic
follow-up, we derive a lower limit to the fraction of the Virgo cluster light
contributed by the intracluster stars at the surveyed position in the cluster
core: it amounts to 10%.Comment: 32 pages,11 figures, in press on AJ, February 200
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