229 research outputs found
Relativistic Binary Pulsars with Black-Hole Companions
Binaries containing a stellar-mass black hole and a recycled radio pulsar
have so far eluded detection. We present a focused investigation of the
formation and evolution of these systems in the Galactic disk, highlighting the
factors that limit their numbers and the reasons why they may be extremely
rare. We surmise that the birthrate of black-hole/recycled-pulsar binaries in
the Galactic disk is probably no higher than ~10^{-7}/yr, and may be much less.
Simple arguments regarding common-envelope evolution suggest that these
binaries should have orbital periods less than 10 hr and an average lifetime of
<~10^8 yr before coalescence due to the emission of gravitational radiation. We
expect that fewer than ~10 of these compact, relativistic binaries currently
reside in the Galactic disk, less than 0.1--1% of the number of double neutron
stars. The discovery of two or more black-hole/recycled-pulsar binaries using
current radio telescopes would tightly constrain certain ideas regarding the
evolution of massive stars, dynamical mass transfer, and black-hole formation.Comment: 10 pages, 6 figures, accepted by ApJ, minor change
Formation of Short-Period Binary Pulsars in Globular Clusters
We present a new dynamical scenario for the formation of short-period binary
millisecond pulsars in globular clusters. Our work is motivated by the recent
observations of 20 radio pulsars in 47 Tuc. In a dense cluster such as 47 Tuc,
most neutron stars acquire binary companions through exchange interactions with
primordial binaries. The resulting systems have semimajor axes in the range
\~0.1-1 AU and neutron star companion masses ~1-3 Msun. For many of these
systems we find that, when the companion evolves off the main sequence and
fills its Roche lobe, the subsequent mass transfer is dynamically unstable.
This leads to a common envelope phase and the formation of short-period neutron
star - white dwarf binaries. For a significant fraction of these binaries, the
decay of the orbit due to gravitational radiation will be followed by a period
of stable mass transfer driven by a combination of gravitational radiation and
tidal heating of the companion. The properties of the resulting short-period
binaries match well those of observed binary pulsars in 47 Tuc.Comment: To appear in ApJ Letters, slightly abbreviated version with only
minor change
Formation of Compact Binaries in Globular Clusters
We report here on two complementary population synthesis studies which relate
directly to the formation and evolution of neutron star binaries in globular
clusters. In the first, we compute the probability of retaining neutron stars
in globular clusters, and quantitatively confirm the idea that the retention
fraction for neutron stars born in binary systems is greatly enhanced over
those born in isolated stars. However, the retention fraction may well be
insufficient to explain the current population of neutron star binaries. In the
second study, we follow a large population of primordial binaries and neutron
stars throughout the lifetime of a globular cluster whose properties may be
similar to 47 Tuc. We directly compute all 3-body interactions among binary
systems, neutron stars, and isolated field stars throughout the history of the
cluster. The evolution of certain types of neutron star binaries is followed up
to the current epoch. The numbers of close, recycled, binary radio pulsars are
evaluated and compared with the results of radio observations.Comment: 14 pages; to appear in Evolution of Binary and Multiple Star Systems,
a Meeting in Celebration of Peter Eggleton's 60th Birthday, Bormio, Italy,
ASP Conference Series, eds. P. Podsiadlowski et a
Binaries with Compact Components: Theoretical and Observational Challenges
We report on recent progress in our theoretical understanding of X-ray
binaries, which has largely been driven by new observations, and illustrate the
interplay between theory and observations considering as examples
intermediate-mass X-ray binaries, irradiation-driven evolution, ultraluminous
X-ray sources and neutron stars with low-velocity kicks.Comment: 4 pages, 2 figures, Proc. of La Paz Meeting, IAUC 194: "Compact
Binaries in the Galaxy and Beyond
New Constraints on Quantum Gravity from X-ray and Gamma-Ray Observations
One aspect of the quantum nature of spacetime is its "foaminess" at very
small scales. Many models for spacetime foam are defined by the accumulation
power , which parameterizes the rate at which Planck-scale spatial
uncertainties (and thephase shifts they produce) may accumulate over large
path-lengths. Here is defined by theexpression for the path-length
fluctuations, , of a source at distance , wherein , with being the Planck
length. We reassess previous proposals to use astronomical observations
ofdistant quasars and AGN to test models of spacetime foam. We show explicitly
how wavefront distortions on small scales cause the image intensity to decay to
the point where distant objects become undetectable when the path-length
fluctuations become comparable to the wavelength of the radiation. We use X-ray
observations from {\em Chandra} to set the constraint ,
which rules out the random walk model (with ). Much firmer
constraints canbe set utilizing detections of quasars at GeV energies with {\em
Fermi}, and at TeV energies with ground-based Cherenkovtelescopes: and , respectively. These limits on
seem to rule out , the model of some physical interest.Comment: 11 pages, 9 figures, ApJ, in pres
Incorporation of Genetic Pathway Information into Analysis of Multivariate Gene Expression Data
Abstract: Multivariate microarray gene expression data are commonly collected to study the genomic responses under ordered conditions such as over increasing/decreasing dose levels or over time during biological processes. One important question from such multivariate gene expression experiments is to identify genes that show different expression patterns over treatment dosages or over time and pathways that are perturbed during a given biological process. In this paper, we develop a hidden Markov random field model for multivariate expression data in order to identify genes and subnetworks that are related to biological processes, where the dependency of the differential expression patterns of genes on the networks are modeled by a Markov random field. Simulation studies indicated that the method is quite effective in identifying genes and the modified subnetworks and has higher sensitivity than the commonly used procedures that do not use the pathway information, with similar observed false discovery rates. We applied the proposed methods for analysis of a microarray time course gene expression study of TrkA- and TrkB-transfected neuroblastoma cell lines and identified genes and subnetworks on MAPK, focal adhesion and prion disease pathways that may explain cell differentiation in TrkA-transfected cell lines
Subsurface Sensing of Buried Objects Under a Randomly Rough Surface Using Scattered Electromagnetic Field Data
Identification of a novel locus on 2q for autosomal dominant high-grade myopia.
PURPOSE. Myopia, or nearsightedness, is a visual disorder of high and growing prevalence in the United States and in other countries. Pathologic high myopia, or myopia of ≤-6.00 D, predisposes individuals to retinal detachment, macular degeneration, cataracts, and glaucoma. Autosomal dominant (AD) nonsyndromic high-grade myopia has been mapped to loci on 18p11.31, 12q21-q23, 17q21-q23, and 7q36. This is the report of significant linkage to a novel locus on the long arm of chromosome 2 in a large, multigenerational family with AD high-grade myopia. METHODS. The family contains 31 participating members (14 affected). The average spherical refractive error for affected individuals was -14.46 D (range, -7.25 to -27.00). Before a genome screening was undertaken, linkage to intragenic or flanking markers for the myopic genetic syndromes of Stickler syndrome types I, II, and III; Marfan syndrome; and juvenile glaucoma were ruled out. In addition, no linkage was found to the known AD high-grade myopia loci listed above. A full genome screen of the family was performed with 382 microsatellite markers with an average intermarker distance of 10 cM. SimWalk2 software was used for multipoint linkage analysis based on an AD model with a penetrance of 90% and a disease allele frequency of 0.01. RESULTS. Fine-point mapping with an additional nine custommade and five commercial markers yielded a maximum two-point lod score of 5.67 at marker D2S2348. Results of multipoint analysis indicate that the 1-unit support intervals for this new locus spans approximately 9.1 cM from (238.7 to 247.8 cM) on the chromosome 2 genetic map at q37.1. CONCLUSIONS. A novel locus for AD high-grade myopia has been determined, providing further evidence of genetic heterogeneity for this disorder
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