8,481 research outputs found
The origins of Causality Violations in Force Free Simulations of Black Hole Magnetospheres
Recent simulations of force-free, degenerate (ffde) black hole magnetospheres
indicate that the fast mode radiated from (or near) the event horizon can
modify the global potential difference in the poloidal direction orthogonal to
the magnetic field, V, in a black hole magnetosphere. There is a fundamental
contradiction in a wave that alters V coming from near the horizon. The
background fields in ffde satisfy the ``ingoing wave condition'' near the
horizon (that arises from the requirement that all matter is ingoing at the
event horizon), yet outgoing waves are radiated from this region in the
simulation. Studying the properties of the waves in the simulations are useful
tools to this end. It is shown that regularity of the stress-energy tensor in a
freely falling frame requires that the outgoing (as viewed globally) waves near
the event horizon are redshifted away and are ineffectual at changing V. It is
also concluded that waves in massless MHD (ffde) are extremely inaccurate
depictions of waves in a tenuous MHD plasma, near the event horizon, as a
consequence black hole gravity. Any analysis based on ffde near the event
horizon is seriously flawed.Comment: 9 pages to appear in ApJ Letter
In Situ Scanning Tunneling Microscopy of DNA-Modified Gold Surfaces: Bias and Mismatch Dependence
In situ scanning tunneling microscopy has been performed on DNA-modified gold surfaces under physiological conditions. The STM images of DNA-modified gold surfaces are strongly dependent on the applied potential and percentage of DNA duplexes containing a single base mismatch. At negative surface potentials we observe reproducible features that are attributed to DNA agglomerates where the DNA duplexes are in the upright orientation; at positive potentials, when DNA molecules lie down on the surface, the film is transparent, and only the gold surface is distinguishable. These observations indicate that DNA possesses a non-negligible local density of states which can be probed when the DNA duplex is in the upright orientation. By varying the percentage of DNA duplexes containing a single base mismatch, we have observed a dramatic change in the image contrast as a result of the perturbation induced by the mismatch on the electronic pathway inside the DNA. These results emphasize the central role of the integrity of the π-stack for DNA charge transport. Duplex DNA is a promising candidate in molecular electronics, but only in arrangements where the orbitals can efficiently overlap with the electronic states of the electrodes and the environment does not constrain the DNA in non-native, poorly stacked conformations
Analysis of the effect of geometrical and operational parameters on solute transport in hollow fiber dialyzers with a two-dimensional transport model
Limitations of Radar Coordinates
The construction of a radar coordinate system about the world line of an
observer is discussed. Radar coordinates for a hyperbolic observer as well as a
uniformly rotating observer are described in detail. The utility of the notion
of radar distance and the admissibility of radar coordinates are investigated.
Our results provide a critical assessment of the physical significance of radar
coordinates.Comment: 12 pages, revtex and pictex macros, 3 pictex figures, 1 eps figure.
Expanded versio
A constant dark matter halo surface density in galaxies
We confirm and extend the recent finding that the central surface density
r_0*rho_0 galaxy dark matter halos, where r_0 and rho_0 are the halo core
radius and central density, is nearly constant and independent of galaxy
luminosity. Based on the co-added rotation curves of about 1000 spiral
galaxies, mass models of individual dwarf irregular and spiral galaxies of late
and early types with high-quality rotation curves and, galaxy-galaxy weak
lensing signals from a sample of spiral and elliptical galaxies, we find that
log(r_0*rho_0) = 2.15 +- 0.2, in units of log(Msol/pc^2). We also show that the
observed kinematics of Local Group dwarf spheroidal galaxies are consistent
with this value. Our results are obtained for galactic systems spanning over 14
magnitudes, belonging to different Hubble Types, and whose mass profiles have
been determined by several independent methods. In the same objects, the
approximate constancy of rho_0*r_0 is in sharp contrast to the systematical
variations, by several orders of magnitude, of galaxy properties, including
rho_0 and central stellar surface density.Comment: Accepted for publication in MNRAS. 9 pages, 4 figure
Personality Traits and Performance Contracts: Evidence From A Field Experiment Among Maternity Care Providers In India
We study how agents respond to performance incentives according to key personality traits (conscientiousness and neuroticism) through a field experiment offering financial incentives for improving maternal and neonatal health outcomes to rural Indian doctors. More conscientious providers performed better--but improved less--under performance incentives. The effect of the performance incentives was also smaller for providers with higher levels of neuroticism. Our results contribute to a growing body of empirical research on heterogeneous responses to incentives and have implications for worker selection
Novel Class 1 Integron Harboring Antibiotic Resistance Genes in Wastewater-Derived Bacteria as Revealed by Functional Metagenomics
Combatting antibiotic resistance is critical to our ability to treat infectious diseases. Here, we identified and characterized diverse antimicrobial resistance genes, including potentially mobile elements, from synthetic wastewater treatment microcosms exposed to the antibacterial agent triclosan. After seven weeks of exposure, the microcosms were subjected to functional metagenomic selection across 13 antimicrobials. This was achieved by cloning the combined genetic material from the microcosms, introducing this genetic library into E. coli, and selecting for clones that grew on media supplemented with one of the 13 antimicrobials. We recovered resistant clones capable of growth on media supplemented with a single antimicrobial, yielding 13 clones conferring resistance to at least one antimicrobial agent. Antibiotic susceptibility analysis revealed resistance ranging from 4 to \u3e50 fold more resistant, while one clone showed resistance to multiple antibiotics. Using both Sanger and SMRT sequencing, we identified the predicted active gene(s) on each clone. One clone that conferred resistance to tetracycline contained a gene encoding a novel tetA-type efflux pump that was named TetA(62). Three clones contained predicted active genes on class 1 integrons. One integron had a previously unreported genetic arrangement and was named In1875. This study demonstrated the diversity and potential for spread of resistance genes present in human-impacted environments
Group selection models in prebiotic evolution
The evolution of enzyme production is studied analytically using ideas of the
group selection theory for the evolution of altruistic behavior. In particular,
we argue that the mathematical formulation of Wilson's structured deme model
({\it The Evolution of Populations and Communities}, Benjamin/Cumings, Menlo
Park, 1980) is a mean-field approach in which the actual environment that a
particular individual experiences is replaced by an {\it average} environment.
That formalism is further developed so as to avoid the mean-field approximation
and then applied to the problem of enzyme production in the prebiotic context,
where the enzyme producer molecules play the altruists role while the molecules
that benefit from the catalyst without paying its production cost play the
non-altruists role. The effects of synergism (i.e., division of labor) as well
as of mutations are also considered and the results of the equilibrium analysis
are summarized in phase diagrams showing the regions of the space of parameters
where the altruistic, non-altruistic and the coexistence regimes are stable. In
general, those regions are delimitated by discontinuous transition lines which
end at critical points.Comment: 22 pages, 10 figure
Different strokes for different folks? Experimental evidence on the effectiveness of input and output incentive contracts for health care providers with varying skills
A central issue in designing incentive contracts is the decision to reward agents' input use versus outputs. The trade-off between risk and return to innovation in production can also lead agents with varying skill levels to perform differentially under different contracts. We study this issue experimentally, observing and verifying inputs and outputs in Indian maternity care. We find that both contract types achieve comparable reductions in postpartum hemorrhage rates, but payments for outputs were four times that of inputs. Providers with varying qualifications performed equivalently under input incentives, while providers with advanced qualifications may have performed better under output contracts
Chandra X-ray Observations of the Two Brightest Unidentified High Galactic Latitude Fermi-LAT gamma-ray Sources
We present Chandra ACIS-I X-ray observations of 0FGL J1311.9-3419 and 0FGL
J1653.4-0200, the two brightest high Galactic latitude (|b|>10 deg) gamma-ray
sources from the 3 month Fermi-LAT bright source list that are still
unidentified. Both were also detected previously by EGRET, and despite
dedicated multi-wavelength follow-up, they are still not associated with
established classes of gamma-ray emitters like pulsars or radio-loud active
galactic nuclei. X-ray sources found in the ACIS-I fields of view are
catalogued, and their basic properties are determined. These are discussed as
candidate counterparts to 0FGL J1311.9-3419 and 0FGL J1653.4-0200, with
particular emphasis on the brightest of the 9 and 13 Chandra sources detected
within their respective Fermi-LAT 95% confidence regions. Further follow-up
studies, including optical photometric and spectroscopic observations are
necessary to identify these X-ray candidate counterparts in order to ultimately
reveal the nature of these enigmatic gamma-ray objects.Comment: ApJ, accepted. 18 pages, 5 figure
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