7,682 research outputs found
Relativistic Iron Lines in Galactic Black Holes: Recent Results and Lines in the ASCA Archive
Recent observations with Chandra and XMM-Newton, aided by broad-band spectral
coverage from RXTE, have revealed skewed relativistic iron emission lines in
stellar-mass Galactic black hole systems. Such systems are excellent
laboratories for testing General Relativity, and relativistic iron lines
provide an important tool for making such tests. In this contribution to the
Proceedings of the 10th Annual Marcel Grossmann Meeting on General Relativity,
we briefly review recent developments and present initial results from fits to
archival ASCA observations of Galactic black holes. It stands to reason that
relativistic effects, if real, should be revealed in many systems (rather than
just one or two); the results of our archival work have borne-out this
expectation. The ASCA spectra reveal skewed, relativistic lines in XTE
J1550-564, GRO J1655-40, GRS 1915+105, and Cygnus X-1.Comment: to appear in the proc. of the 10th Annual Marcel Grossmann Meeting on
General Relativity, 5 pages, 1 figure, uses specific .cls and .sty file
A search for X-ray pulsations from the galactic center
Data from the SAS-3 satellite were used in a search for X-ray pulsations from the direction of the galactic center. No periodic X-ray behavior was detected in the frequency interval 0.6 Hz to 0.0006 Hz and energy range 2.5 - 35 keV. For periods less than 60 sec, the upper limit to the amplitude of any pulsation in the 2.5 - 10 keV band is approximately .0017 cts/sq cm/s. This corresponds to a pulsed fraction of approximately 1.3 percent of the total GCX flux. Somewhat higher limits apply for longer periods and for energies greater than 10 keV
Chromosome Segregation Is Biased by Kinetochore Size
Chromosome missegregation during mitosis or meiosis is a hallmark of cancer and the main cause of prenatal death in humans. The gain or loss of specific chromosomes is thought to be random, with cell viability being essentially determined by selection. Several established pathways including centrosome amplification, sister-chromatid cohesion defects, or a compromised spindle assembly checkpoint can lead to chromosome missegregation. However, how specific intrinsic features of the kinetochore—the critical chromosomal interface with spindle microtubules—impact chromosome segregation remains poorly understood. Here we used the unique cytological attributes of female Indian muntjac, the mammal with the lowest known chromosome number (2n = 6), to characterize and track individual chromosomes with distinct kinetochore size throughout mitosis. We show that centromere and kinetochore functional layers scale proportionally with centromere size. Measurement of intra-kinetochore distances, serial-section electron microscopy, and RNAi against key kinetochore proteins confirmed a standard structural and functional organization of the Indian muntjac kinetochores and revealed that microtubule binding capacity scales with kinetochore size. Surprisingly, we found that chromosome segregation in this species is not random. Chromosomes with larger kinetochores bi-oriented more efficiently and showed a 2-fold bias to congress to the equator in a motor-independent manner. Despite robust correction mechanisms during unperturbed mitosis, chromosomes with larger kinetochores were also strongly biased to establish erroneous merotelic attachments and missegregate during anaphase. This bias was impervious to the experimental attenuation of polar ejection forces on chromosome arms by RNAi against the chromokinesin Kif4a. Thus, kinetochore size is an important determinant of chromosome segregation fidelity
XMM-Newton Spectroscopy of the Accretion-Driven Millisecond X-ray Pulsar XTE J1751-305 in Outburst
We present an analysis of the first high-resolution spectra measured from an
accretion-driven millisecond X-ray pulsar in outburst. We observed XTE
J1751-305 with XMM-Newton on 2002 April 7 for approximately 35 ksec. Using a
simple absorbed blackbody plus power-law model, we measure an unabsorbed flux
of (6.6 +/- 0.1) * 10^(-10) erg/cm^2/s (0.5--10.0 keV). A hard power-law
component (Gamma = 1.44 +/- 0.01) contributes 83% of the unabsorbed flux in the
0.5-10.0 keV band, but a blackbody component (kT = 1.05 +/- 0.01 keV) is
required. We find no clear evidence for narrow or broad emission or absorption
lines in the time-averaged spectra, and the sensitivity of this observation has
allowed us to set constraining upper-limits on the strength of important
features. The lack of line features is at odds with spectra measured from some
other X-ray binaries which share some similarities with XTE J1751-305. We
discuss the implications of these findings on the accretion flow geometry in
XTE J1751-305.Comment: 5 pages, 3 figures (2 color). ApJ Letters, accepted. Uses
emulateapj.st
Energy Cost to Make a Hole in the Fermi Sea
The change in energy of an ideal Fermi gas when a local one-body potential is
inserted into the system, or when the density is changed locally, are important
quantities in condensed matter physics. We show that they can be rigorously
bounded from below by a universal constant times the value given by the
semiclassical approximation.Comment: 4 pages, final version published in Phys. Rev. Let
Coulomb Charging Effects for Finite Channel Number
We consider quantum fluctuations of the charge on a small metallic grain
caused by virtual electron tunneling to a nearby electrode. The average
electron number and the effective charging energy are determined by means of
perturbation theory in the tunneling Hamiltonian. In particular we discuss the
dependence of charging effects on the number N of tunneling channels. Earlier
results for N>>1 are found to be approached rather rapidly with increasing N.Comment: 6 pages, 5 figure
Carbon release by selective alloying of transition metal carbides
We have performed first principles density functional theory calculations on
TiC alloyed on the Ti sublattice with 3d transition metals ranging from Sc to
Zn. The theory is accompanied with experimental investigations, both as regards
materials synthesis as well as characterization. Our results show that by
dissolving a metal with a weak ability to form carbides, the stability of the
alloy is lowered and a driving force for the release of carbon from the carbide
is created. During thin film growth of a metal carbide this effect will favor
the formation of a nanocomposite with carbide grains in a carbon matrix. The
choice of alloying elements as well as their concentrations will affect the
relative amount of carbon in the carbide and in the carbon matrix. This can be
used to design the structure of nanocomposites and their physical and chemical
properties. One example of applications is as low-friction coatings. Of the
materials studied, we suggest the late 3d transition metals as the most
promising elements for this phenomenon, at least when alloying with TiC.Comment: 9 pages, 6 figure
XTE J2123-058: A New Neutron Star X-Ray Transient
We report on optical and RXTE observations of a new high-latitude bursting
X-ray transient, XTE J2123-058. We identified the optical counterpart, measured
the 5.9573 hr orbital period and constrained the binary inclination and the
source distance. The distance lower limit indicates that the source is at least
2.6 kpc from the Galactic plane, which is unusual for an LMXB. RXTE
observations were made between June and August 1998 during the first detected
outburst from this source. We describe correlations between X-ray timing and
spectral properties and discuss the possibility that the propeller mechanism
turned on during the decay of the outburst. During one of the RXTE
observations, we detect a pair of high frequency QPOs at 847.1 +/- 5.5 Hz and
1102 +/- 13 Hz simultaneously. According to the beat frequency model, the QPO
separation implies a neutron star spin period near 3.9 ms. As the X-ray flux
decreases at the end of the outburst, the amplitude of the optical modulation
increases significantly. This behavior can be explained if the size of the
accretion disk decreases during the decay of the outburst.Comment: 24 pages, 9 figures, accepted by Ap
Comparing P-stars with Observations
P-stars are compact stars made of up and down quarks in -equilibrium
with electrons in a chromomagnetic condensate. P-stars are able to account for
compact stars as well as stars with radius comparable with canonical neutron
stars. We compare p-stars with different available observations. Our results
indicate that p-stars are able to reproduce in a natural manner several
observations from isolated and binary pulsars.Comment: 15 pages, 2 figures; accepted for publication in Astrophysical
Journa
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