826 research outputs found
The struggling infectious diseases fellow: Remediation challenges and opportunities
Remediation of struggling learners is a challenge faced by all educators. In recognition of this reality, and in light of contemporary challenges facing infectious diseases (ID) fellowship program directors, the Infectious Diseases Society of America Training Program Directors\u27 Committee focused the 2018 National Fellowship Program Directors\u27 Meeting at IDWeek on Remediation of the Struggling Fellow. Small group discussions addressed 7 core topics, including feedback and evaluations, performance management and remediation, knowledge deficits, fellow well-being, efficiency and time management, teaching skills, and career development. This manuscript synthesizes those discussions around a competency-based framework to provide program directors and other educators with a roadmap for addressing common contemporary remediation challenges
Mobility through Heterogeneous Networks in a 4G Environment
Serving and Managing users in a heterogeneous environment. 17th WWRF Meeting in Heidelberg, Germany, 15 - 17 November 2006. [Proceeding presented at WG3 - Co-operative and Ad-hoc Networks]The increase will of ubiquitous access of the users to the requested services points towards the integration of heterogeneous networks. In this sense, a user shall be able to access its services through different access technologies, such as WLAN, Wimax, UMTS and DVB technologies, from the same or different network operators, and to seamless move between different networks with active communications. In this paper we propose a mobility architecture able to support this users’ ubiquitous access and seamless movement, while simultaneously bringing a large flexibility to access network operators
Gluon fusion contribution to W+W- + jet production
We describe the computation of the process that contributes
to the production of two -bosons and a jet at the CERN Large Hadron Collider
(LHC). While formally of next-to-next-to-leading order (NNLO) in QCD, this
process can be evaluated separately from the bulk of NNLO QCD corrections
because it is finite and gauge-invariant. It is also enhanced by the large
gluon flux and by selection cuts employed in the Higgs boson searches in the
decay channel , as was first pointed out by Binoth {\it et al.}
in the context of production. For cuts employed by the ATLAS
collaboration, we find that the gluon fusion contribution to
enhances the background by about ten percent and can lead to moderate
distortions of kinematic distributions which are instrumental for the ongoing
Higgs boson searches at the LHC. We also release a public code to compute the
NLO QCD corrections to this process, in the form of an add-on to the package
{\tt MCFM}.Comment: 13 pages, 4 figures, 3 table
Measuring the Black Hole Spin in Sgr A*
The polarized mm/sub-mm radiation from Sgr A* is apparently produced by a
Keplerian structure whose peak emission occurs within several Schwarzschild
radii (r_S=2GM/c^2) of the black hole. The Chandra X-ray counterpart, if
confirmed, is presumably the self-Comptonized component from this region. In
this paper, we suggest that sub-mm timing observations could yield a signal
corresponding to the period P_0 of the marginally stable orbit, and therefore
point directly to the black hole's spin a. Sgr A*'s mass is now known to be
(2.6\pm 0.2)\times 10^6 M_\odot (an unusually accurate value for supermassive
black hole candidates), for which 2.7 min<P_0<36 min, depending on the value of
a and whether the Keplerian flow is prograde or retrograde. A Schwarzschild
black hole (a=0) should have P_0 ~ 20 min. The identification of the orbital
frequency with the innermost stable circular orbit is made feasible by the
transition from optically thick to thin emission at sub-mm wavelengths. With
stratification in the emitter, the peak of the sub-mm bump in Sgr A*'s spectrum
is thus produced at the smallest radius. We caution, however, that theoretical
uncertainties in the structure of the emission region may still produce some
ambiguity in the timing signal. Given that Sgr A*'s flux at mm is
several Jy, these periods should lie within the temporal-resolving capability
of sub-mm telescopes using bolometric detectors. A determination of P_0 should
provide not only a value of a, but it should also define the angular momentum
vector of the orbiting gas in relation to the black hole's spin axis. In
addition, since the X-ray flux detected by Chandra appears to be the
self-Comptonized mm to sub-mm component, these temporal fluctuations may also
be evident in the X-ray signal.Comment: 15 pages, 1 figures. Accepted for publication in ApJ Letter
The Role of Magnetic Field Dissipation in the Black Hole Candidate Sgr A*
The compact, nonthermal radio source Sgr A* at the Galactic Center appears to
be coincident with a 2.6 million solar mass point-like object. Its energy
source may be the release of gravitational energy as gas from the interstellar
medium descends into its potential well. Simple attempts at calculating the
spectrum and flux based on this picture have come close to the observations,
yet have had difficulty in accounting for the low efficiency in this source.
There now appear to be two reasons for this low conversion rate: (1) the plasma
separates into two temperatures, with the protons attaining a significantly
higher temperature than that of the radiating electrons, and (2) the magnetic
field, B, is sub-equipartition, which reduces the magnetic bremsstrahlung
emissivity, and therefore the overall power of Sgr A*. We investigate the
latter with improvement over what has been attempted before: rather than
calculating B based on a presumed model, we instead infer its distribution with
radius empirically with the requirement that the resulting spectrum matches the
observations. Our ansatz for B(r) is motivated in part by earlier calculations
of the expected magnetic dissipation rate due to reconnection in a compressed
flow. We find reasonable agreement with the observed spectrum of Sgr A* as long
as its distribution consists of 3 primary components: an outer equipartition
field, a roughly constant field at intermediate radii (~1000 Schwarzschild
radii), and an inner dynamo (more or less within the last stable orbit for a
non-rotating black hole) which increases B to about 100 Gauss. The latter
component accounts for the observed sub-millimiter hump in this source.Comment: 33 pages including 2 figures; submitted to Ap
Viewing the Shadow of the Black Hole at the Galactic Center
In recent years, the evidence for the existence of an ultra-compact
concentration of dark mass associated with the radio source Sgr A* in the
Galactic Center has become very strong. However, an unambiguous proof that this
object is indeed a black hole is still lacking. A defining characteristic of a
black hole is the event horizon. To a distant observer, the event horizon casts
a relatively large ``shadow'' with an apparent diameter of ~10 gravitational
radii due to bending of light by the black hole, nearly independent of the
black hole spin or orientation. The predicted size (~30 micro-arcseconds) of
this shadow for Sgr A* approaches the resolution of current
radio-interferometers. If the black hole is maximally spinning and viewed
edge-on, then the shadow will be offset by ~8 micro-arcseconds from the center
of mass, and will be slightly flattened on one side. Taking into account
scatter-broadening of the image in the interstellar medium and the finite
achievable telescope resolution, we show that the shadow of Sgr A* may be
observable with very long-baseline interferometry at sub-millimeter
wavelengths, assuming that the accretion flow is optically thin in this region
of the spectrum. Hence, there exists a realistic expectation of imaging the
event horizon of a black hole within the next few years.Comment: 5 pages, 1 figure (color), (AAS)Tex, to appear in The Astrophysical
Journal Letters, Vol. 528, L13 (Jan 1, 2000 issue); also available at
http://www.mpifr-bonn.mpg.de/staff/hfalcke/publications.html#bhimag
Where have all the black holes gone?
We have calculated stationary models for accretion disks around super-massive
black holes in galactic nuclei. Our models show that below a critical mass flow
rate of ~3 10**-3 M_Edd advection will dominate the energy budget while above
that rate all the viscously liberated energy is radiated. The radiation
efficiency declines steeply below that critical rate. This leads to a clear
dichotomy between AGN and normal galaxies which is not so much given by
differences in the mass flow rate but by the radiation efficiency. At very low
mass accretion rates below 5 10**-5 M_Edd synchrotron emission and
Bremsstrahlung dominate the SED, while above 2 10**-4 M Edd the inverse Compton
radiation from synchrotron seed photons produce flat to inverted SEDs from the
radio to X-rays. Finally we discuss the implications of these findings for AGN
duty cycles and the long-term AGN evolution.Comment: 7 pages, 5 figures, accepted for publication in A&
What does the Sentence Structure component of the CELF-IV index, in monolinguals and bilinguals?
The Sentence Structure sub-test (SST) of the Clinical Evaluation of Language Fundamentals (CELF) aims to “measure the acquisition of grammatical (structural) rules at the sentence level”. Although originally designed for clinical practice with monolingual children, components of the CELF, such as the SST, are often used to inform psycholinguistic research. Raw scores are also commonly used to estimate the English proficiency of bilingual children. This study queries the reliability of the SST as an index of children's ability to deal with structural complexity in sentence comprehension, and demonstrates that cognitive complexity induces a considerable confound in the task, affecting 5- to 7-year-old monolinguals (n = 87) and bilinguals (n = 87) alike
The Formation of Broad Line Clouds in the Accretion Shocks of Active Galactic Nuclei
Recent work on the gas dynamics in the Galactic Center has improved our
understanding of the accretion processes in galactic nuclei, particularly with
regard to properties such as the specific angular momentum distribution,
density, and temperature of the inflowing plasma. This information can be
valuable in trying to determine the origin of the Broad Line Region (BLR) in
Active Galactic Nuclei (AGNs). In this paper, we explore various scenarios for
the cloud formation based on the underlying principle that the source of plasma
is ultimately that portion of the gas trapped by the central black hole from
the interstellar medium. Based on what we know about the Galactic Center, it is
likely that in highly dynamic environments such as this, the supply of matter
is due mostly to stellar winds from the central cluster. Winds accreting onto a
central black hole are subjected to several disturbances capable of producing
shocks, including a Bondi-Hoyle flow, stellar wind-wind collisions, and
turbulence. Shocked gas is initially compressed and heated out of thermal
equilibrium with the ambient radiation field; a cooling instability sets in as
the gas is cooled via inverse-Compton and bremsstrahlung processes. If the
cooling time is less than the dynamical flow time through the shock region, the
gas may clump to form the clouds responsible for broad line emission seen in
many AGN spectra. Clouds produced by this process display the correct range of
densities and velocity fields seen in broad emission lines. Very importantly,
the cloud distribution agrees with the results of reverberation studies, in
which it is seen that the central line peak responds slower to continuum
changes than the line wings.Comment: 22 pages, 5 figure
Neutrinos and Gamma Rays from Galaxy Clusters
The next generation of neutrino and gamma-ray detectors should provide new
insights into the creation and propagation of high-energy protons within galaxy
clusters, probing both the particle physics of cosmic rays interacting with the
background medium and the mechanisms for high-energy particle production within
the cluster. In this paper we examine the possible detection of gamma-rays (via
the GLAST satellite) and neutrinos (via the ICECUBE and Auger experiments) from
the Coma cluster of galaxies, as well as for the gamma-ray bright clusters
Abell 85, 1758, and 1914. These three were selected from their possible
association with unidentified EGRET sources, so it is not yet entirely certain
that their gamma-rays are indeed produced diffusively within the intracluster
medium, as opposed to AGNs. It is not obvious why these inconspicuous
Abell-clusters should be the first to be seen in gamma-rays, but a possible
reason is that all of them show direct evidence of recent or ongoing mergers.
Their identification with the EGRET gamma-ray sources is also supported by the
close correlation between their radio and (purported) gamma-ray fluxes. Under
favorable conditions (including a proton spectral index of 2.5 in the case of
Abell 85, and sim 2.3 for Coma, and Abell 1758 and 1914), we expect ICECUBE to
make as many as 0.3 neutrino detections per year from the Coma cluster of
galaxies, and as many as a few per year from the Abell clusters 85, 1758, and
1914. Also, Auger may detect as many as 2 events per decade at ~ EeV energies
from these gamma-ray bright clusters.Comment: Accepted for publication in Ap
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