6,129 research outputs found
The X-ray spectrum of a disk illuminated by ions
The X-ray spectrum from a cool disk embedded in an ion supported torus is
computed. The interaction of the hot ions with the disk increases the hard
X-ray luminosity of the system}. A surface layer of the disk is heated by the
protons from the torus. The Comptonized spectrum produced by this layer has a
shape that depends only weakly on the incident energy flux and the distance
from the accreting compact object. It consists of a `blue bump' of
unComptonized soft photons and a flat high energy tail, reminiscent of the
observed spectra. The hard tail becomes flatter as the thermalization depth in
the cool disk is increased. Further evidence for ion illumination are the Li
abundance in the secondaries of low mass X-ray binaries and the 450 keV lines
sometimes seen in black-hole transient spectra.Comment: 7p, to appear in Monthly Notice
Perturbation Theory for the Rosenzweig-Porter Matrix Model
We study an ensemble of random matrices (the Rosenzweig-Porter model) which,
in contrast to the standard Gaussian ensemble, is not invariant under changes
of basis. We show that a rather complete understanding of its level
correlations can be obtained within the standard framework of diagrammatic
perturbation theory. The structure of the perturbation expansion allows for an
interpretation of the level structure on simple physical grounds, an aspect
that is missing in the exact analysis (T. Guhr, Phys. Rev. Lett. 76, 2258
(1996), T. Guhr and A. M\"uller-Groeling, cond-mat/9702113).Comment: to appear in PRE, 5 pages, REVTeX, 2 figures, postscrip
Evaluating spatial normalization methods for the human brain
Cortical stimulation mapping (CSM) studies have shown cortical locations for language function are highly variable from one subject to the next. If individual variation can be normalized, patterns of language organization may emerge that were heretofore hidden. In order to uncover this pattern, computer-aided spatial normalization to a common atlas is required. Our problem was how to determine which spatial normalization method was best for the given research application. We developed key metrics to measure accuracy of a surface-based (Caret) and volume-based (SPM2) method. We specified that the optimal method would i) minimize variation as measured by spread reduction between CSM language sites across subjects while also ii) preserving anatomical localization of all CSM sites. Eleven subject’s structural MR data and corresponding CSM site coordinates were registered to the colin27 human brain atlas using each method. Local analysis showed that mapping error rates for both methods were highest in morphological regions with the greatest difference between source and target. Also, SPM2 mapped significantly less type 2 errors. Although our experiment did not show statistically significant global differences between the methods, our methodology provided valuable insights into the pros and cons of each
Reduced Shear Power Spectrum
Measurements of ellipticities of background galaxies are sensitive to the
reduced shear, the cosmic shear divided by where is the
projected density field. We compute the difference between shear and reduced
shear both analytically and with simulations. The difference becomes more
important on smaller scales, and will impact cosmological parameter estimation
from upcoming experiments. A simple recipe is presented to carry out the
required correction.Comment: 8 pages, 5 figure
Comment on ``Critical Behavior in Disordered Quantum Systems Modified by Broken Time--Reversal Symmetry''
In a recent Letter [Phys. Rev. Lett. 80, 1003 (1998)] Hussein and Pato
employed the maximum entropy principle (MEP) in order to derive interpolating
ensembles between any pair of universality classes in random matrix theory.
They apply their formalism also to the transition from random matrix to Poisson
statistics of spectra that is observed for the case of the Anderson-type
metal-insulator transition. We point out the problems with the latter
procedure.Comment: 1 page in PS, to appear in PRL Sept. 2
Death or survival from invasive pneumococcal disease in Scotland: associations with serogroups and multilocus sequence types
We describe associations between death from invasive pneumococcal disease (IPD) and particular serogroups and sequence types (STs) determined by multilocus sequence typing (MLST) using data from Scotland. All IPD episodes where blood or cerebrospinal fluid (CSF) culture isolates were referred to the Scottish Haemophilus, Legionella, Meningococcal and Pneumococcal Reference Laboratory (SHLMPRL) from January 1992 to February 2007 were matched to death certification records by the General Register Office for Scotland. This represented 5959 patients. The median number of IPD cases in Scotland each year was 292. Deaths, from any cause, within 30 days of pneumococcal culture from blood or CSF were considered to have IPD as a contributing factor. Eight hundred and thirty-three patients died within 30 days of culture of Streptococcus pneumoniae from blood or CSF [13.95%; 95% confidence interval (13.10, 14.80)]. The highest death rates were in patients over the age of 75. Serotyping data exist for all years but MLST data were only available from 2001 onward. The risk ratio of dying from infection due to particular serogroups or STs compared to dying from IPD due to all other serogroups or STs was calculated. Fisher's exact test with Bonferroni adjustment for multiple testing was used. Age adjustment was accomplished using the Cochran-Mantel-Haenszel test and 95% confidence intervals were reported. Serogroups 3, 11 and 16 have increased probability of causing fatal IPD in Scotland while serogroup 1 IPD has a reduced probability of causing death. None of the 20 most common STs were significantly associated with death within 30 days of pneumococcal culture, after age adjustment. We conclude that there is a stronger association between a fatal outcome and pneumococcal capsular serogroup than there is between a fatal outcome and ST
Population Genomics of Early Events in the Ecological Differentiation of Bacteria
Genetic exchange is common among bacteria, but its effect on population diversity during ecological differentiation remains controversial. A fundamental question is whether advantageous mutations lead to selection of clonal genomes or, as in sexual eukaryotes, sweep through populations on their own. Here, we show that in two recently diverged populations of ocean bacteria, ecological differentiation has occurred akin to a sexual mechanism: A few genome regions have swept through subpopulations in a habitat-specific manner, accompanied by gradual separation of gene pools as evidenced by increased habitat specificity of the most recent recombinations. These findings reconcile previous, seemingly contradictory empirical observations of the genetic structure of bacterial populations and point to a more unified process of differentiation in bacteria and sexual eukaryotes than previously thought.National Science Foundation (U.S.) (Grant DEB-0918333)Woods Hole Center for Oceans & Human HealthGordon and Betty Moore FoundationUnited States. Dept. of Energy. Genomes To Lif
The SINS/zC-SINF survey of z~2 galaxy kinematics: Outflow properties
Based on SINFONI Ha, [NII] and [SII] AO data of 30 z \sim 2 star-forming
galaxies (SFGs) from the SINS and zcSINF surveys, we find a strong correlation
of the Ha broad flux fraction with the star formation surface density of the
galaxy, with an apparent threshold for strong outflows occurring at 1 Msun
yr^-1 kpc^-2. Above this threshold, we find that SFGs with logm_\ast>10 have
similar or perhaps greater wind mass loading factors (eta = Mdotout/SFR) and
faster outflow velocities than lower mass SFGs. This trend suggests that the
majority of outflowing gas at z \sim 2 may derive from high-mass SFGs, and that
the z \sim 2 mass-metallicity relation is driven more by dilution of enriched
gas in the galaxy gas reservoir than by the efficiency of outflows. The mass
loading factor is also correlated with the SFR and inclination, such that more
star-forming and face-on galaxies launch more powerful outflows. For galaxies
that have evidence for strong outflows, we find that the broad emission is
spatially extended to at least the half-light radius (\sim a few kpc). We
propose that the observed threshold for strong outflows and the observed mass
loading of these winds can be explained by a simple model wherein break-out of
winds is governed by pressure balance in the disk. Using the ratio of the [SII]
doublet in a broad and narrow component, we find that outflowing gas has a
density of \sim10-100 cm^-3, significantly less than that of the star forming
gas (600 cm^-3).Comment: 7 pages, 3 figures, accepted by Ap
Draft genome sequences of gammaproteobacterial methanotrophs isolated from marine ecosystems
The genome sequences of Methylobacter marinus A45, Methylobacter sp. strain BBA5.1, and Methylomarinum vadi IT-4 were obtained. These aerobic methanotrophs are typical members of coastal and hydrothermal vent marine ecosystems
- …