1,446 research outputs found
Study of HST counterparts to Chandra X-ray sources in the Globular Cluster M71
We report on archival Hubble Space Telescope (HST) observations of the
globular cluster M71 (NGC 6838). These observations, covering the core of the
globular cluster, were performed by the Advanced Camera for Surveys (ACS) and
the Wide Field Planetary Camera 2 (WFPC2). Inside the half-mass radius (r_h =
1.65') of M71, we find 33 candidate optical counterparts to 25 out of 29
Chandra X-ray sources while outside the half-mass radius, 6 possible optical
counterparts to 4 X-ray sources are found. Based on the X-ray and optical
properties of the identifications, we find 1 certain and 7 candidate
cataclysmic variables (CVs). We also classify 2 and 12 X-ray sources as certain
and potential chromospherically active binaries (ABs), respectively. The only
star in the error circle of the known millisecond pulsar (MSP) is inconsistent
with being the optical counterpart. The number of X-ray faint sources with
L_x>4x10^{30} ergs/s (0.5-6.0 keV) found in M71 is higher than extrapolations
from other clusters on the basis of either collision frequency or mass. Since
the core density of M71 is relatively low, we suggest that those CVs and ABs
are primordial in origin.Comment: 12 pages, 6 figures. Accepted for publication in Astronomy and
Astrophysic
Study of material homogeneity in the long fiber thermoset injection molding process by image texture analysis
To quantify the homogeneity of fiber dispersion in short fiber-reinforced polymer composites, a method for image texture analysis of 3-dimensional X-ray micro computed tomography (µCT) images is presented in this work. The adaption of the method to the specific requirements of the composite material is accomplished using a statistical region merging approach. Subsequently, the method is applied for evaluating the homogeneity of specimens from an intermediate step of the long fiber thermoset injection molding process as well as molded parts. This new injection molding process enables the manufacturing of parts with a flexible combination of short and long glass fibers. By using a newly developed screw element based on the Maddock mixing element design, the material homogeneity of parts molded in the long fiber injection molding process is improved
Superconductivity in Organic Compounds with Pseudo-Triangular Lattice
We study spin fluctuation (SF) mediated superconductivity (SC) in a
half-filled square lattice Hubbard model with the transfer matrices -t between
nearest neighbor sites and -t' between a half of next nearest neighbor sites
neighboring along only one of the directions, considering application of
this model to organic kappa-(BEDT-TTF)2X compounds. Varying the t'/t value from
0 to 1, one can interpolate between a square and an equilateral triangular
lattice, the latter giving frustration to antiferromagnetically (AF) coupled
spin systems. Within the fluctuation exchange (FLEX) approximation, we
calculate chi(q,omega), Tc and the SC order parameter for various model
parameter values and find that both AF and SC are suppressed as one approaches
the frustration geometry or |(t'/t)-1| \to 0. The SC phase, however, extends
beyond the AF phase boundary fairly close to t'/t=1 for realistic U/t values.
The order parameter is of x2-y2-type for t'/t1.Comment: 4 pages, 5 eps figures, to appear in J. Phys. Soc. Jp
High-resolution x-ray telescopes
High-energy astrophysics is a relatively young scientific field, made
possible by space-borne telescopes. During the half-century history of x-ray
astronomy, the sensitivity of focusing x-ray telescopes-through finer angular
resolution and increased effective area-has improved by a factor of a 100
million. This technological advance has enabled numerous exciting discoveries
and increasingly detailed study of the high-energy universe-including accreting
(stellar-mass and super-massive) black holes, accreting and isolated neutron
stars, pulsar-wind nebulae, shocked plasma in supernova remnants, and hot
thermal plasma in clusters of galaxies. As the largest structures in the
universe, galaxy clusters constitute a unique laboratory for measuring the
gravitational effects of dark matter and of dark energy. Here, we review the
history of high-resolution x-ray telescopes and highlight some of the
scientific results enabled by these telescopes. Next, we describe the planned
next-generation x-ray-astronomy facility-the International X-ray Observatory
(IXO). We conclude with an overview of a concept for the next next-generation
facility-Generation X. The scientific objectives of such a mission will require
very large areas (about 10000 m2) of highly-nested lightweight
grazing-incidence mirrors with exceptional (about 0.1-arcsecond) angular
resolution. Achieving this angular resolution with lightweight mirrors will
likely require on-orbit adjustment of alignment and figure.Comment: 19 pages, 11 figures, SPIE Conference 7803 "Adaptive X-ray Optics",
part of SPIE Optics+Photonics 2010, San Diego CA, 2010 August 2-
Changes in the long term intensity variations in Cyg X-2 and LMC X-3
We report the detection of changes in the long-term intensity variations in
two X-ray binaries, Cyg X-2 and LMC X-3. In this work, we have used the
long-term light curves obtained with the All-Sky Monitors (ASMs) of the Rossi
X-Ray Timing Explorer (RXTE), Ginga, Ariel 5, and Vela 5B and the scanning
modulation collimator of HEAO 1. It is found that in the light curves of both
the sources, obtained with these instruments at various times over the last 30
years, more than one periodic or quasi-periodic component is always present.
The multiple prominent peaks in the periodograms have frequencies unrelated to
each other. In Cyg X-2, RXTE-ASM data show strong peaks at 40.4 and 68.8 days,
and Ginga-ASM data show strong peaks at 53.7 and 61.3 days. Multiple peaks are
also observed in LMC X-3. The various strong peaks in the periodograms of LMC
X-3 appear at 104, 169, and 216 days (observed with RXTE-ASM) and 105, 214, and
328 days (observed with Ginga-ASM). The present results, when compared with the
earlier observations of periodicities in these two systems, demonstrate the
absence of any stable long period. The 78 day periodicity detected earlier in
Cyg X-2 was probably due to the short time base in the RXTE data that were
used, and the periodicity of 198 days in LMC X-3 was due to a relatively short
duration of observation with HEAO 1.Comment: 11 pages, 7 postscript figures include
Jupiter's X-ray and EUV auroras monitored by Chandra, XXM-Newton, and Hisaki satellite
Jupiter's X-ray auroral emission in the polar cap region results from particles which have undergone strong field-aligned acceleration into the ionosphere. The origin of precipitating ions and electrons and the time variability in the X-ray emission are essential to uncover the driving mechanism for the high-energy acceleration. The magnetospheric location of the source field line where the X-ray is generated is likely affected by the solar wind variability. However, these essential characteristics are still unknown because the long-term monitoring of the X-rays and contemporaneous solar wind variability has not been carried out. In April 2014, the first long-term multiwavelength monitoring of Jupiter's X-ray and EUV auroral emissions was made by the Chandra X-ray Observatory, XMM-Newton, and Hisaki satellite. We find that the X-ray count rates are positively correlated with the solar wind velocity and insignificantly with the dynamic pressure. Based on the magnetic field mapping model, a half of the X-ray auroral region was found to be open to the interplanetary space. The other half of the X-ray auroral source region is magnetically connected with the prenoon to postdusk sector in the outermost region of the magnetosphere, where the Kelvin-Helmholtz (KH) instability, magnetopause reconnection, and quasiperiodic particle injection potentially take place. We speculate that the high-energy auroral acceleration is associated with the KH instability and/or magnetopause reconnection. This association is expected to also occur in many other space plasma environments such as Saturn and other magnetized rotators
Polarization of Thermal X-rays from Isolated Neutron Stars
Since the opacity of a magnetized plasma depends on polarization of
radiation, the radiation emergent from atmospheres of neutron stars with strong
magnetic fields is expected to be strongly polarized. The degree of linear
polarization, typically ~10-30%, depends on photon energy, effective
temperature and magnetic field. The spectrum of polarization is more sensitive
to the magnetic field than the spectrum of intensity. Both the degree of
polarization and the position angle vary with the neutron star rotation period
so that the shape of polarization pulse profiles depends on the orientation of
the rotational and magnetic axes. Moreover, as the polarization is
substantially modified by the general relativistic effects, observations of
polarization of X-ray radiation from isolated neutron stars provide a new
method for evaluating the mass-to-radius ratio of these objects, which is
particularly important for elucidating the properties of the superdense matter
in the neutron star interiors.Comment: 7 figures, to be published in Ap
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