5,916 research outputs found
The Power of Posner: A Study of Prestige and Influence in the Federal Judiciary
Some judges have a disproportionate influence over the American judiciary; existing research has shown Judge Richard Posner is one of those judges. Our goal was to identify and determine how Judge Posner’s influence has changed over time. To measure and track his influence, we collected and compared citation and invocation data from three distinct time frames. While these measurements are imperfect, they can help illustrate the level of influence and prestige Judge Posner enjoys. The existing literature led us to expect Judge Posner’s early citation rates to be low. After several years on the bench, the citation rates for each opinion should rise dramatically. By contrast, Judge Posner’s citation rates are exceptionally high from the outset while more recent opinions actually have lower citation rates
Relativistic Winds from Compact Gamma-Ray Sources: II. Pair Loading and Radiative Acceleration in Gamma-ray Bursts
We consider the effects of rapid pair creation by an intense pulse of
gamma-rays propagating ahead of a relativistic shock. Side-scattered photons
colliding with the main gamma-ray beam amplify the density of scattering
charges. The acceleration rate of the pair-loaded medium is calculated, and its
limiting bulk Lorentz factor related to the spectrum and compactness of the
photon source. One obtains, as a result, a definite prediction for the relative
inertia in baryons and pairs. The deceleration of a relativistic shock in the
moving medium, and the resulting synchrotron emissivity, are compared with
existing calculations for a static medium. The radiative efficiency is
increased dramatically by pair loading. When the initial ambient density
exceeds a critical value, the scattering depth traversed by the main gamma-ray
pulse rises above unity, and the pulse is broadened. These considerations place
significant constraints on burst progenitors: a pre-burst mass loss rate
exceeding 10^{-5} M_\odot per year is difficult to reconcile with individual
pulses narrower than 10 s, unless the radiative efficiency is low. An
anisotropic gamma-ray flux (on an angular scale \Gamma^{-1} or larger) drives a
large velocity shear that greatly increases the energy in the seed magnetic
field forward of the propagating shock.Comment: 19 pp., LaTeX (aaspp4.sty), revised 12/23/99, Ap. J. in press;
summary section added and several minor improvements in presentatio
Diagnosing magnetars with transient cooling
Transient X-ray emission, with an approximate t^{-0.7} decay, was observed
from SGR 1900+14 over 40 days following the the giant flare of 27 Aug 1998. We
calculate in detail the diffusion of heat to the surface of a neutron star
through an intense 10^{14}-10^{15} G magnetic field, following the release of
magnetic energy in its outer layers. We show that the power law index, the
fraction of burst energy in the afterglow, and the return to persistent
emission can all be understood if the star is composed of normal baryonic
material.Comment: 9 pages, 1 eps figur
The analysis of iron-responsive transcriptional regulation in the intracellular human pathogen, Chlamydia trachomatis
The host-pathogen interaction is dictated by the competition for nutrients. Iron is a rare, but critical component for several conserved biochemical reactions, and as such, is at the forefront in the competition between host and pathogen. The obligate intracellular human pathogen, Chlamydia trachomatis, enters an alternative growth mode when iron is depleted within the host. Termed persistence, this growth mode is characterized by the arrest of development until suitable conditions arise within the host cell, and is likely an important evolutionary mechanism for chronic infection that has been linked to multiple sequelae of infection in vivo. Despite the relevance of persistence in human disease, the sensory and response mechanisms that trigger this alternative growth mode are not understood. The research presented in this report demonstrates a new, robust model for the iron-restriction of intracellular-dwelling C. trachomatis. The model was used to elicit uniform populations of persistent Chlamydiae in vitro, which allowed accurate analysis of differential transcription under iron-starvation conditions. From these studies, the gene ytgA, which is predicted to function as part of an ABC-metal transport complex, was elevated upon low-iron availability.
The identification of the trans- and cis-regulatory factors responsible for the modulation of ytgA transcription was attempted. Our research indicates that the ytgC, which exists in an operon with ytgA, encodes a genetic fusion between a
predicted membrane channel forming polypeptide (N-terminus) and a metal-dependent DNA binding polypeptide (C-terminus), which we have termed C-YtgC. C-YtgC recognized the IGR upstream of ytgA in an iron-dependent manner in vitro, providing a molecular mechanism for the regulation of ytgA under iron-limiting conditions. Two regions containing cis-regulatory elements within the IGR of ytgA were identified for this binding. Additional preliminary evidence suggests a mechanism for the liberation of the C-YtgC polypeptide from the membrane channel portion of the protein. In summary, the research presented in this report has gained significant advances in the field of Chlamydia iron-biology. The search for a robust iron-restriction model has implicated that the chlamydial iron source may be cytosolic, rather than endocytic. Furthermore, the characterization of a novel trans-regulatory factor suggests that Chlamydia may employ a sophisticated regulatory network for maintenance of iron-homeostasis. Continued research into the mechanisms of iron acquisition is warranted, considering the strong link between low-iron availability and chronic infection
Hard Burst Emission from the Soft Gamma Repeater SGR 1900+14
We present evidence for burst emission from SGR 1900+14 with a power-law high
energy spectrum extending beyond 500 keV. Unlike previous detections of high
energy photons during bursts from SGRs, these emissions are not associated with
high-luminosity burst intervals. Not only is the emission hard, but the spectra
are better fit by Band's GRB function rather than by the traditional
optically-thin thermal bremsstrahlung model. We find that the spectral
evolution within these hard events obeys a hardness/intensity anti-correlation.
Temporally, these events are distinct from typical SGR burst emissions in that
they are longer (~ 1 s) and have relatively smooth profiles. Despite a
difference in peak luminosity of > 1E+11 between these bursts from SGR 1900+14
and cosmological GRBs, there are striking temporal and spectral similarities
between the two kinds of bursts, aside from spectral evolution. We outline an
interpretation of these events in the context of the magnetar model.Comment: 11 pages (text and figures), submitted to ApJ Letters, corrected
erroneous hardness ratio
Electrodynamics of Magnetars IV: Self-Consistent Model of the Inner Accelerator, with Implications for Pulsed Radio Emission
We consider the voltage structure in the open-field circuit and outer
magnetosphere of a magnetar. The standard polar-cap model for radio pulsars is
modified significantly when the polar magnetic field exceeds 1.8x10^{14} G.
Pairs are created by accelerated particles via resonant scattering of thermal
X-rays, followed by the nearly instantaneous conversion of the scattered photon
to a pair. A surface gap is then efficiently screened by e+- creation, which
regulates the voltage in the inner part of the circuit to ~10^9 V. We also
examine the electrostatic gap structure that can form when the magnetic field
is somewhat weaker, and deduce a voltage 10-30 times larger over a range of
surface temperatures. We examine carefully how the flow of charge back to the
star above the gap depends on the magnitude of the current that is extracted
from the surface of the star, on the curvature of the magnetic field lines, and
on resonant drag. The rates of different channels of pair creation are
determined self-consistently, including the non-resonant scattering of X-rays,
and collisions between gamma rays and X-rays. We find that the electrostatic
gap solution has too small a voltage to sustain the observed pulsed radio
output of magnetars unless i) the magnetic axis is nearly aligned with the
rotation axis and the light of sight; or ii) the gap is present on the closed
as well as the open magnetic field lines. Several properties of the radio
magnetars -- their rapid variability, broad pulses, and unusually hard radio
spectra -- are consistent with a third possibility, that the current in the
outer magnetosphere is strongly variable, and a very high rate of pair creation
is sustained by a turbulent cascade.Comment: 32 pages, submitted to the Astrophysical Journa
Diverging volumetric trajectories following pediatric traumatic brain injury.
Traumatic brain injury (TBI) is a significant public health concern, and can be especially disruptive in children, derailing on-going neuronal maturation in periods critical for cognitive development. There is considerable heterogeneity in post-injury outcomes, only partially explained by injury severity. Understanding the time course of recovery, and what factors may delay or promote recovery, will aid clinicians in decision-making and provide avenues for future mechanism-based therapeutics. We examined regional changes in brain volume in a pediatric/adolescent moderate-severe TBI (msTBI) cohort, assessed at two time points. Children were first assessed 2-5 months post-injury, and again 12 months later. We used tensor-based morphometry (TBM) to localize longitudinal volume expansion and reduction. We studied 21 msTBI patients (5 F, 8-18 years old) and 26 well-matched healthy control children, also assessed twice over the same interval. In a prior paper, we identified a subgroup of msTBI patients, based on interhemispheric transfer time (IHTT), with significant structural disruption of the white matter (WM) at 2-5 months post injury. We investigated how this subgroup (TBI-slow, N = 11) differed in longitudinal regional volume changes from msTBI patients (TBI-normal, N = 10) with normal WM structure and function. The TBI-slow group had longitudinal decreases in brain volume in several WM clusters, including the corpus callosum and hypothalamus, while the TBI-normal group showed increased volume in WM areas. Our results show prolonged atrophy of the WM over the first 18 months post-injury in the TBI-slow group. The TBI-normal group shows a different pattern that could indicate a return to a healthy trajectory
Statistical properties of SGR 1806-20 bursts
We present statistics of SGR 1806-20 bursts, combining 290 events detected
with RXTE/PCA, 111 events detected with BATSE and 134 events detected with ICE.
We find that the fluence distribution of bursts observed with each instrument
are well described by power laws with indices 1.43, 1.76 and 1.67,
respectively. The distribution of time intervals between successive bursts from
SGR 1806-20 is described by a lognormal function with a peak at 103 s. There is
no correlation between the burst intensity and either the waiting times till
the next burst or the time elapsed since the previous burst. In all these
statistical properties, SGR 1806-20 bursts resemble a self-organized critical
system, similar to earthquakes and solar flares. Our results thus support the
hypothesis that the energy source for SGR bursts is crustquakes due to the
evolving, strong magnetic field of the neutron star, rather than any accretion
or nuclear power.Comment: 11 pages, 4 figures, To appear in ApJ Letter
Intense Electromagnetic Outbursts from Collapsing Hypermassive Neutron Stars
We study the gravitational collapse of a magnetized neutron star using a
novel numerical approach able to capture both the dynamics of the star and the
behavior of the surrounding plasma. In this approach, a fully general
relativistic magnetohydrodynamics implementation models the collapse of the
star and provides appropriate boundary conditions to a force-free model which
describes the stellar exterior. We validate this strategy by comparing with
known results for the rotating monopole and aligned rotator solutions and then
apply it to study both rotating and non-rotating stellar collapse scenarios,
and contrast the behavior with what is obtained when employing the
electrovacuum approximation outside the star. The non-rotating electrovacuum
collapse is shown to agree qualitatively with a Newtonian model of the
electromagnetic field outside a collapsing star. We illustrate and discuss a
fundamental difference between the force-free and electrovacuum solutions,
involving the appearance of large zones of electric-dominated field in the
vacuum case. This provides a clear demonstration of how dissipative
singularities appear generically in the non-linear time-evolution of force-free
fluids. In both the rotating and non-rotating cases, our simulations indicate
that the collapse induces a strong electromagnetic transient. In the case of
sub-millisecond rotation, the magnetic field experiences strong winding and the
transient carries much more energy. This result has important implications for
models of gamma-ray bursts.Comment: 28 pages, 20 figures (quality lowered to reduce sizes). Improved
initial data and matching condition results in a lower, but still important,
energy emission. Added appendix with a discussion on effects of transition
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