8,060 research outputs found
Confirming the Detection of an Intergalactic X-ray Absorber Toward PKS 2155-304
We present new observations on PKS 2155-304 with the Chandra Low Energy
Transmission Grating Spectrometer (LETG), using the Advanced CCD Imaging
Spectrometer (ACIS). We confirm the detection of an absorption line plausibly
identified as OVIII Ly-alpha from the warm-hot intergalactic medium associated
with a small group of galaxies along the line of sight, as originally reported
by Fang et al. 2002 (here after FANG02). Combining the previous observations in
FANG02 and five new, long observations on the same target, we increase the
total exposure time by a factor of three, and the total counts per resolution
element by a factor of five. The measured line equivalent width is smaller than
that observed in FANG02, but still consistent at 90% confidence. We also
analyze the XMM-Newton observations on the same target, as well as observations
using the Chandra LETG and the High Resolution Camera (HRC) combination. These
observations have been used to challenge our reported detection. While no line
is seen in either the XMM-Newton and the Chandra LETG+HRC data, we find that
our result is consistent with the upper limits from both data sets. We
attribute the non-detection to (1) higher quality of the Chandra LETG+ACIS
spectrum, and (2) the rather extended wings of the line spread functions of
both the XMM RGS and the Chandra LETG+HRC. We discuss the implication of our
observation on the temperature and density of the absorber. We also confirm the
detection of z ~ 0 OVII absorption and, comparing with previous Chandra
analysis, we obtain much tighter constraints on the line properties.Comment: 10 pages, 8 figures, accepted for publication in Ap
Diamond chains with multiple-spin exchange interactions
We study the phase diagram of a symmetric spin-1/2 Heisenberg diamond chain
with additional cyclic four-spin exchange interactions. The presented analysis
supplemented by numerical exact-diagonalization results for finite periodic
clusters implies a rich phase diagram containing, apart from standard magnetic
and spin-liquid phases, two different tetramer-dimer phases as well as an
exotic four-fold degenerate dimerized phase. The characteristics of the
established spin phases as well as the nature of quantum phase transitions are
discussed, as well.Comment: 6 PRB pages, Added reference
Systematic Investigation of Possibilities for New Physics Effects in b --> s Penguin Processes
Although recent experimental results in b-->s penguin process seem to be
roughly consistent with the standard model predictions, there may be still
large possibilities of new physics hiding in this processes. Therefore, here we
investigate systematically the potential new physics effects that may appear in
time-dependent CP asymmetries of B --> phi K^0, B--> eta^\prime K^0 and B-->
K^0 \pi^0 decay modes, by classifying the cases for the values of the
mixing-induced indirect CP asymmetries, S_{phi K^0}, S_{eta^\prime K^0}, S_{K^0
pi^0} which are compared to S_{J/psi K^0}. We also show that several B_s decay
modes may help to resolve the ambiguities in such an analysis. Through
combining analysis with the time-dependent CP asymmetries of B_s decay modes
such as B_s --> phi eta^\prime, B_s--> eta^\prime pi^0 and B_s --> K^0
bar{K}^0, we can determine where the new CP phases precisely come from.Comment: 17 pages, version to be published in Prog.Theor.Phy
On polymorphic logical gates in sub-excitable chemical medium
In a sub-excitable light-sensitive Belousov-Zhabotinsky chemical medium an
asymmetric disturbance causes the formation of localized traveling
wave-fragments. Under the right conditions these wave-fragment can conserve
their shape and velocity vectors for extended time periods. The size and life
span of a fragment depend on the illumination level of the medium. When two or
more wave-fragments collide they annihilate or merge into a new wave-fragment.
In computer simulations based on the Oregonator model we demonstrate that the
outcomes of inter-fragment collisions can be controlled by varying the
illumination level applied to the medium. We interpret these wave-fragments as
values of Boolean variables and design collision-based polymorphic logical
gates. The gate implements operation XNOR for low illumination, and it acts as
NOR gate for high illumination. As a NOR gate is a universal gate then we are
able to demonstrate that a simulated light sensitive BZ medium exhibits
computational universality
Proto-clusters in the Lambda CDM Universe
We compare the highly clustered populations of very high redshift galaxies
with proto-clusters identified numerically in a standard CDM universe
() simulation. We evolve 256^3 dark matter
particles in a comoving box of side 150h^{-1}Mpc. By the present day there are
63 cluster sized objects of mass in excess of 10^{14}h^{-1}Mo in this box. We
trace these clusters back to higher redshift finding that their progenitors at
z=4--5 are extended regions of typically 20--40 Mpc (comoving) in size, with
dark halos of mass in excess of 10^{12}h^{-1}Mo and are overdense by typically
1.3--13 times the cosmological mean density. Comparison with the observation of
Lyman alpha emitting (LAEs) galaxies at z=4.86 and at z=4.1 indicates that the
observed excess clustering is consistent with that expected for a proto-cluster
region if LAEs typically correspond to massive dark halos of more than
10^{12}h^{-1}Mo. We give a brief discussion on the relation between high
redshift concentration of massive dark halos and present day rich clusters of
galaxies.Comment: 4 pages, 5 figures, Accepted for publication in ApJ Letter
Exact static solutions for discrete models free of the Peierls-Nabarro barrier: Discretized first integral approach
We propose a generalization of the discrete Klein-Gordon models free of the
Peierls-Nabarro barrier derived in Nonlinearity {\bf 12}, 1373 (1999) and Phys.
Rev. E {\bf 72}, 035602(R) (2005), such that they support not only kinks but a
one-parameter set of exact static solutions. These solutions can be obtained
iteratively from a two-point nonlinear map whose role is played by the
discretized first integral of the static Klein-Gordon field, as suggested in J.
Phys. A {\bf 38}, 7617 (2005). We then discuss some discrete models
free of the Peierls-Nabarro barrier and identify for them the full space of
available static solutions, including those derived recently in Phys. Rev. E
{\bf 72} 036605 (2005) but not limited to them. These findings are also
relevant to standing wave solutions of discrete nonlinear Schr{\"o}dinger
models. We also study stability of the obtained solutions. As an interesting
aside, we derive the list of solutions to the continuum equation that
fill the entire two-dimensional space of parameters obtained as the continuum
limit of the corresponding space of the discrete models.Comment: Accepted for publication in PRE; the M/S has been revised in line
with the referee repor
Experimental generation of four-mode continuous-variable cluster states
Continuous-variable Gaussian cluster states are a potential resource for
universal quantum computation. They can be efficiently and unconditionally
built from sources of squeezed light using beam splitters. Here we report on
the generation of three different kinds of continuous-variable four-mode
cluster states. In our realization, the resulting cluster-type correlations are
such that no corrections other than simple phase-space displacements would be
needed when quantum information propagates through these states. At the same
time, the inevitable imperfections from the finitely squeezed resource states
and from additional thermal noise are minimized, as no antisqueezing components
are left in the cluster states.Comment: 5 pages, 4 figure
Gay-Straight Alliances as Settings to Discuss Health Topics: Individual and Group Factors Associated with Substance Use, Mental Health, and Sexual Health Discussions
Sexual minority (e.g. lesbian, gay, bisexual, questioning; LGBQ) and gender minority (e.g. transgender) youth experience myriad health risks. Gay-Straight Alliances (GSAs) are school-based settings where they may have opportunities to discuss substance use, mental health, and sexual health issues in ways that are safe and tailored to their experiences. Attention to these topics in GSAs could aid in developing programming for these settings. Among 295 youth from 33 Massachusetts high-school GSAs (69% LGBQ, 68% cisgender female, 68% White, Mage = 16.06), we examined how often youth discussed these topics within their GSA and identified factors associated with having more of these discussions. Youth and GSAs as a whole varied in their frequency of discussing these topics. Youth who accessed more information/resources in the GSA and did more advocacy more frequently engaged in discussions around substance use, mental health and sexual health. Youth who reported greater victimization more often discussed substance use and mental health, but not sexual health. Finally, GSAs whose members collectively reported greater victimization more frequently discussed these topics. These findings can assist the development of health programming to be delivered within GSAs
Rotation symmetry axes and the quality index in a 3D octahedral parallel robot manipulator system
The geometry of a 3D octahedral parallel robot manipulator system is specified in terms of two rigid octahedral structures (the fixed and moving platforms) and six actuation legs. The symmetry of the system is exploited to determine the behaviour of (a new version of) the quality index for various motions. The main results are presented graphically
Why is the condensed phase of DNA preferred at higher temperature? DNA compaction in the presence of a multivalent cation
Upon the addition of multivalent cations, a giant DNA chain exhibits a large
discrete transition from an elongated coil into a folded compact state. We
performed single-chain observation of long DNAs in the presence of a
tetravalent cation (spermine), at various temperatures and monovalent salt
concentrations. We confirmed that the compact state is preferred at higher
temperatures and at lower monovalent salt concentrations. This result is
interpreted in terms of an increase in the net translational entropy of small
ions due to ionic exchange between higher and lower valence ions.Comment: 4pages,3figure
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