51,305 research outputs found
Survival mediation analysis with the death-truncated mediator: The completeness of the survival mediation parameter
In medical research, the development of mediation analysis with a survival outcome has facilitated investigation into causal mechanisms. However, studies have not discussed the death-truncation problem for mediators, the problem being that conventional mediation parameters cannot be well-defined in the presence of a truncated mediator. In the present study, we systematically defined the completeness of causal effects to uncover the gap, in conventional causal definitions, between the survival and nonsurvival settings. We proposed three approaches to redefining the natural direct and indirect effects, which are generalized forms of the conventional causal effects for survival outcomes. Furthermore, we developed three statistical methods for the binary outcome of the survival status and formulated a Cox model for survival time. We performed simulations to demonstrate that the proposed methods are unbiased and robust. We also applied the proposed method to explore the effect of hepatitis C virus infection on mortality, as mediated through hepatitis B viral load
The Halo Occupation Distribution of X-ray-Bright Active Galactic Nuclei: A Comparison with Luminous Quasars
We perform halo occupation distribution (HOD) modeling of the projected
two-point correlation function (2PCF) of high-redshift (z~1.2) X-ray-bright
active galactic nuclei (AGN) in the XMM-COSMOS field measured by Allevato et
al. The HOD parameterization is based on low-luminosity AGN in cosmological
simulations. At the median redshift of z~1.2, we derive a median mass of
(1.02+0.21/-0.23)x10^{13} Msun/h for halos hosting central AGN and an upper
limit of ~10% on the AGN satellite fraction. Our modeling results indicate (at
the 2.5-sigma level) that X-ray AGN reside in more massive halos compared to
more bolometrically luminous, optically-selected quasars at similar redshift.
The modeling also yields constraints on the duty cycle of the X-ray AGN, and we
find that at z~1.2 the average duration of the X-ray AGN phase is two orders of
magnitude longer than that of the quasar phase. Our inferred mean occupation
function of X-ray AGN is similar to recent empirical measurements with a group
catalog and suggests that AGN halo occupancy increases with increasing halo
mass. We project the XMM-COSMOS 2PCF measurements to forecast the required
survey parameters needed in future AGN clustering studies to enable higher
precision HOD constraints and determinations of key physical parameters like
the satellite fraction and duty cycle. We find that N^{2}/A~5x10^{6} deg^{-2}
(with N the number of AGN in a survey area of A deg^{2}) is sufficient to
constrain the HOD parameters at the 10% level, which is easily achievable by
upcoming and proposed X-ray surveys.Comment: 11 pages, 4 figures, accepted in Ap
The Most Massive Black Holes in the Universe: Effects of Mergers in Massive Galaxy Clusters
Recent observations support the idea that nuclear black holes grew by gas
accretion while shining as luminous quasars at high redshift, and they
establish a relation of the black hole mass with the host galaxy's spheroidal
stellar system. We develop an analytic model to calculate the expected impact
of mergers on the masses of black holes in massive clusters of galaxies. We use
the extended Press-Schechter formalism to generate Monte Carlo merger histories
of halos with a mass 10^{15} h^{-1} Msun. We assume that the black hole mass
function at z=2 is similar to that inferred from observations at z=0 (since
quasar activity declines markedly at z<2), and we assign black holes to the
progenitor halos assuming a monotonic relation between halo mass and black hole
mass. We follow the dynamical evolution of subhalos within larger halos,
allowing for tidal stripping, the loss of orbital energy by dynamical friction,
and random orbital perturbations in gravitational encounters with subhalos, and
we assume that mergers of subhalos are followed by mergers of their central
black holes. Our analytic model reproduces numerical estimates of the subhalo
mass function. We find that the most massive black holes in massive clusters
typically grow by a factor ~ 2 by mergers after gas accretion has stopped. In
our ten realizations of 10^{15} h^{-1} Msun clusters, the highest initial (z=2)
black hole masses are 5-7 x 10^9 Msun, but four of the clusters contain black
holes in the range 1-1.5 x 10^{10} Msun at z=0. Satellite galaxies may host
black holes whose mass is comparable to, or even greater than, that of the
central galaxy. Thus, black hole mergers can significantly extend the very high
end of the black hole mass function.Comment: 13 pages, 7 figures, accepted for publication in The Astrophysical
Journa
New Approach on the General Shape Equation of Axisymmetric Vesicles
The general Helfrich shape equation determined by minimizing the curvature
free energy describes the equilibrium shapes of the axisymmetric lipid bilayer
vesicles in different conditions. It is a non-linear differential equation with
variable coefficients. In this letter, by analyzing the unique property of the
solution, we change this shape equation into a system of the two differential
equations. One of them is a linear differential equation. This equation system
contains all of the known rigorous solutions of the general shape equation. And
the more general constraint conditions are found for the solution of the
general shape equation.Comment: 8 pages, LaTex, submit to Mod. Phys. Lett.
Thermodynamic evidence for pressure-induced bulk superconductivity in the Fe-As pnictide superconductor CaFe2As2
We report specific-heat and resistivity experiments performed in parallel in
a Bridgman-type of pressure cell in order to investigate the nature of
pressure-induced superconductivity in the iron pnictide compound CaFe2As2. The
presence of a pronounced specific-heat anomaly at Tc reveals a bulk nature of
the superconducting state. The thermodynamic transition temperature differs
dramatically from the onset of the resistive transition. Our data indicates
that superconductivity occurs in the vicinity of a crystallographic phase
transition. We discuss the discrepancy between the two methods as caused by
strain-induced superconducting precursors formed above the bulk thermodynamic
transition due to the vicinity of the structural instability
Critical Behaviour of One-particle Spectral Weights in the Transverse Ising Model
We investigate the critical behaviour of the spectral weight of a single
quasiparticle, one of the key observables in experiment, for the particular
case of the transverse Ising model.Series expansions are calculated for the
linear chain and the square and simple cubic lattices. For the chain model, a
conjectured exact result is discovered. For the square and simple cubic
lattices, series analyses are used to estimate the critical exponents. The
results agree with the general predictions of Sachdev.Comment: 4 pages, 3 figure
Using Ontology Fingerprints to evaluate genome-wide association study results
We describe an approach to characterize genes or phenotypes via ontology fingerprints which are composed of Gene Ontology (GO) terms overrepresented among those PubMed abstracts linked to the genes or phenotypes. We then quantify the biological relevance between genes and phenotypes by comparing their ontology fingerprints to calculate a similarity score. We validated this approach by correctly identifying genes belong to their biological pathways with high accuracy, and applied this approach to evaluate GWA study by ranking genes associated with the lipid concentrations in plasma as well as to prioritize genes within linkage disequilibrium (LD) block. We found that the genes with highest scores were: ABCA1, LPL, and CETP for HDL; LDLR, APOE and APOB for LDL; and LPL, APOA1 and APOB for triglyceride. In addition, we identified some top ranked genes linking to lipid metabolism from the literature even in cases where such knowledge was not reflected in current annotation of these genes. These results demonstrate that ontology fingerprints can be used effectively to prioritize genes from GWA studies for experimental validation
- …