8,215 research outputs found
Civic Engagement and Recent Immigrant Communities
Provides a step-by-step guide to developing strategies and planning efforts to strengthen immigrants' civic engagement , including suggested agendas, background materials, and discussion guides. Outlines considerations for planning and successful formats
Iron fluorescence from within the innermost stable orbit of black hole accretion disks
The fluorescent iron Ka line is a powerful observational probe of the inner
regions of black holes accretion disks. Previous studies have assumed that only
material outside the radius of marginal stability can contribute to the
observed line emission. Here, we show that fluorescence by material inside the
radius of marginal stability, which is in the process of spiralling towards the
event horizon, can have a observable influence on the iron line profile and
equivalent width. For concreteness, we consider the case of a geometrically
thin accretion disk, around a Schwarzschild black hole, in which fluorescence
is excited by an X-ray source placed at some height above the disk and on the
axis of the disk. Fully relativistic line profiles are presented for various
source heights and efficiencies. It is found that the extra line flux generally
emerges in the extreme red wing of the iron line, due to the large
gravitational redshift experienced by photons from the region within the radius
of marginal stability. We apply our models to the variable iron line seen in
the ASCA spectrum of the Seyfert nucleus MCG-6-30-15. It is found that the
change in the line profile, equivalent width, and continuum normalization, can
be well explained as being due to a change in the height of the source above
the disk. We discuss the implications of these results for distinguishing
rapidly-rotating black holes from slowly rotating holes using iron line
diagnostics.Comment: 20 pages, LaTeX. Accepted for publication in Astrophysical Journal.
Figures 3 to 7 replaced with corrected versions (previous figures affected by
calculational error). Some changes in the best fitting parameter
Wireless Handheld Computers in the Preclinical Undergraduate Curriculum
This report presents the results of a pilot project using wireless PDAs as teaching tools in an undergraduate medical curriculum. This technology was used to foster a transition from a passive to an interactive learning environment in the classroom and provided a solution for the implementation of computer-based exams for a large class. Wayne State Medical School recently provided model e570 Toshiba PocketPCs® (personal digital assistants or PDAs), network interface cards, and application software developed by CampusMobility® to 20 sophomore medical students. The pilot group of preclinical students used the PDAs to access web-based course content, for communication, scheduling, to participate in interactive teaching sessions, and to complete course evaluations. Another part of this pilot has been to utilize the PDAs for computer-based exams in a wireless environment. Server authentication that restricted access during the exams and a proctoring console to monitor and record the PDA screens will be described in this report. Results of a student satisfaction survey will be present
The Jets of AGN as giant co-axial cables
The currents carried by the jets of active galactic nuclei (AGNs) can be
probed using maps of the Faraday rotation measure (RM), since a jet current
will be accompanied by a toroidal magnetic (B) field, which will give rise to a
systematic change in the RM across the jet. The aim of this study is to
identify new AGNs displaying statistically significant transverse RM gradients
across their parsec-scale jets, and to look for overall patterns in the implied
directions for the toroidal B-field components and jet currents. We have
carried out new analyses of Faraday RM maps derived from previously published
8.1, 8.4, 12.1 and 15.3 GHz data obtained in 2006 on the NRAO VLBA. In a number
of important ways, our procedures were identical to those of the original
authors, but several other key aspects of the new imaging and analysis differ
from the original methods. Our new analysis has substantially increased the
number of AGNs known to display transverse RM gradients. The collected data on
parsec and kiloparsec scales indicate that the current typically flows inward
along the jet axis and outward in a more extended region surrounding the jet,
typical to the current structure of a coaxial cable, accompanied by a
self-consistent system of nested helical B fields, whose toroidal components
give rise to the observed transverse RM gradients. These new results make it
possible for the first time to conclusively demonstrate the existence of a
preferred direction for the toroidal B-field components - and therefore of the
currents - of AGN jets. Discerning the origin of this current-field system is
of cardinal importance for understanding the physical mechanisms leading to the
formation of the intrinsic jet B field, which likely plays an important role in
the propagation and collimation of the jets; one possibility is the action of a
"cosmic battery".Comment: 12 pages, 2 figures, 3 tables; accepted for publication in Astronomy
& Astrophysic
The iron K line complex in NGC1068: implications for X-ray reflection in the nucleus
We report a new analysis of ASCA data on the iron K line complex in NGC1068.
The line complex basically consists of three components, as previously
reported. A weak red wing of the 6.4 keV fluoresence iron K line is found. A
plausible explanation is Compton scattering in optically thick, cold matter
which can be identified with an obscuring torus or cold gas in the host galaxy.
We also show that this `Compton shoulder' should be observable with ASCA using
a simulated reflection spectrum. In order to explain the two higher energy
lines as well as the cold 6.4 keV line, we fit the ASCA data with a composite
model of cold and warm reflection. This shows that cold reflection dominates
the observed X-ray emission above 4 keV. The two higher energy lines have large
equivalent width with respect to the warm-scattered continuum, suggesting that
efficient resonant scattering operates. The line energies are systematically
lower than those expected from resonant lines for FeXXV and FeXXVI by 100 eV.
The redshifts may be due to either the ionized gas of the warm mirror receding
at a radial velocity of 4000-5000 km/s, or effects of Compton scattering in a
complicated geometry.Comment: 7 pages, 4 figures, to appear in MNRA
The XMM-Newton view of the relativistic spectral features in AXJ0447-0627
The XMM-Newton observation of the optically Type 1 AGN AXJ0447-0627 (z=0.214)
unambiguously reveals a complex, bright and prominent set of lines in the 4-8
keV rest frame energy range. Although, from a phenomenological point of view,
the observed properties can be described by a simple power law model plus 5
narrow Gaussian lines (at rest frame energies of nearly 4.49, 5.55, 6.39, 7.02
and 7.85 keV), we find that a model comprising a power law (Gamma of the order
of 2.2), a reflected relativistic continuum, a narrow Fe I Kalpha line from
neutral material as well as a broad Fe Kalpha relativistic line from a ionized
accretion disk represents a good physical description of the data. The ''double
horned'' profile of the relativistic line implies an inclination of the
accretion disk of the order of 45 degree, and an origin in a narrow region of
the disk, from R_in of the order of 19 GM/c^2 to R_out of the order of 30
GM/c^2. The narrow Fe I Kalpha line from neutral material is probably produced
far from the central black hole, most likely in the putative molecular torus.
Although some of these properties have been already found in other Type 1 AGN
and discussed in the literature, at odd with the objects reported so far we
measure high equivalent widths (EWs) of the observed lines: nearly 1.4 keV for
the ``double horned'' relativistic line and nearly 0.4 keV for the narrow line.Comment: 16 pages, 3 figures, Latex manuscript; accepted for publication in
Ap
An Analysis of Retail and Service Sector Count Data: Identification of Market Potential for Wisconsin Counties
The objective of this applied research project is to use Wisconsin county sales tax data to identify the strengths and weaknesses of selected retail and service sectors. Using "count" data on the number of businesses that report taxable sales we apply regression analysis to develop an estimate of the expected number of firms in the county. By comparing the observed and expected number of firms we can identify strengths and weaknesses. Through the regression analysis we can also identify which socioeconomic characteristics are associated with which types of retail and service firms. The method that we offer we refer to as Firm Count Analysis (FCA) and can be viewed as a complement to Trade Area Analysis (TAA) and the analysis of sales data.
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