760 research outputs found
Three-Body Encounters of Black Holes in Globular Clusters
Evidence has been mounting for the existence of black holes with masses from
10^2 to 10^4 M_Solar associated with stellar clusters. Such intermediate-mass
black holes (IMBHs) will encounter other black holes in the dense cores of
these clusters. The binaries produced in these interactions will be perturbed
by other objects as well thus changing the orbital characteristics of the
binaries. These binaries and their subsequent mergers due to gravitational
radiation are important sources of gravitational waves. We present the results
of numerical simulations of high mass ratio encounters, which help clarify the
interactions of intermediate-mass black holes in globular clusters and help
determine what types of detectable gravitational wave signatures are likely.Comment: 4 pages, 3 figures to appear in the proceedings of The Astrophysics
of Gravitational Wave Sources, College Park, MD, 24-26 April 200
Swift/UVOT grism monitoring of NGC 5548 in 2013: an attempt at MgII reverberation mapping
Reverberation-mapping-based scaling relations are often used to estimate the
masses of black holes from single-epoch spectra of AGN. While the
radius-luminosity relation that is the basis of these scaling relations is
determined using reverberation mapping of the H line in nearby AGN, the
scaling relations are often extended to use other broad emission lines, such as
MgII, in order to get black hole masses at higher redshifts when H is
redshifted out of the optical waveband. However, there is no radius-luminosity
relation determined directly from MgII. Here, we present an attempt to perform
reverberation mapping using MgII in the well-studied nearby Seyfert 1, NGC
5548. We used Swift to obtain UV grism spectra of NGC 5548 once every two days
from April to September 2013. Concurrent photometric UV monitoring with Swift
provides a well determined continuum lightcurve that shows strong variability.
The MgII emission line, however, is not strongly correlated with the continuum
variability, and there is no significant lag between the two. We discuss these
results in the context of using MgII scaling relations to estimate
high-redshift black hole masses.Comment: 8 pages, 7 figures, accepted for publication in Ap
X-ray Spectral and Variability Properties of Low-Mass AGN
We study the X-ray properties of a sample of 14 optically-selected low-mass
AGN whose masses lie within the range 1E5 -2E6 M(solar) with XMM-Newton. Only
six of these low-mass AGN have previously been studied with sufficient quality
X-ray data, thus, we more than double the number of low-mass AGN observed by
XMM-Newton with the addition of our sample. We analyze their X-ray spectral
properties and variability and compare the results to their more massive
counterparts. The presence of a soft X-ray excess is detectable in all five
objects which were not background dominated at 2-3 keV. Combined with previous
studies, this gives a total of 8 low-mass AGN with a soft excess. The low-mass
AGN exhibit rapid, short-term variability (hundreds to thousands of seconds) as
well as long-term variability (months to years). There is a well-known
anti-correlation between black hole mass and variability amplitude (normalized
excess variance). Comparing our sample of low-mass AGN with this relation we
find that all of our sample lie below an extrapolation of the linear relation.
Such a flattening of the relation at low masses (below about 1E6 M(solar)) is
expected if the variability in all AGN follows the same shape power spectrum
with a break frequency that is dependent on mass. Finally, we also found two
objects that show significant absorption in their X-ray spectrum, indicative of
type 2 objects, although they are classified as type 1 AGN based on optical
spectra.Comment: 12 pages, 5 figures, 7 tables, accepted for publication in MNRA
Three-Body Dynamics with Gravitational Wave Emission
We present numerical three-body experiments that include the effects of
gravitational radiation reaction by using equations of motion that include the
2.5-order post-Newtonian force terms, which are the leading order terms of
energy loss from gravitational waves. We simulate binary-single interactions
and show that close approach cross sections for three 1 solar mass objects are
unchanged from the purely Newtonian dynamics except for close approaches
smaller than 1.0e-5 times the initial semimajor axis of the binary. We also
present cross sections for mergers resulting from gravitational radiation
during three-body encounters for a range of binary semimajor axes and mass
ratios including those of interest for intermediate-mass black holes (IMBHs).
Building on previous work, we simulate sequences of high-mass-ratio three-body
encounters that include the effects of gravitational radiation. The simulations
show that the binaries merge with extremely high eccentricity such that when
the gravitational waves are detectable by LISA, most of the binaries will have
eccentricities e > 0.9 though all will have circularized by the time they are
detectable by LIGO. We also investigate the implications for the formation and
growth of IMBHs and find that the inclusion of gravitational waves during the
encounter results in roughly half as many black holes ejected from the host
cluster for each black hole accreted onto the growing IMBH.Comment: 34 pages, 14 figures, minor corrections to match version accepted by
Ap
Pure cycles in flexible robotic cells
Cataloged from PDF version of article.In this study, an m-machine flexible robotic manufacturing cell consisting of CNC machines is considered. The flexibility of the
machines leads to a new class of robot move cycles called the pure cycles. We first model the problem of determining the best pure
cycle in an m-machine cell as a special travelling salesman problem in which the distance matrix consists of decision variables as
well as parameters.We focus on two specific cycles among the huge class of pure cycles.We prove that, in most of the regions, either
one of these two cycles is optimal. For the remaining regions we derive worst case performances of these cycles.We also prove that
the set of pure cycles dominates the flowshop-type robot move cycles considered in the literature. As a design problem, we consider
the number of machines in a cell as a decision variable. We determine the optimal number of machines that minimizes the cycle
time for given cell parameters such as the processing times, robot travel times and the loading/unloading times of the machines.
2007 Elsevier Ltd. All rights reserved
Using Sociograms to Enhance Power and Voice in Focus Groups
ObjectiveTo discuss the use of sociograms in our focus groups with homeless sheltered mothers and to assess facilitator influence and the distribution of power influence.Design and SampleAn exploratory, descriptive qualitative design that utilizes both focus groups and sociograms. Two focus groups were conducted in December 2009 (N = 7) and January 2010 (N = 4). Data analysis included a content analysis and a process analysis using sociograms to graphically represent group participant dynamics.ResultsUse of the sociogram provided a means to assess the influence of the facilitator as well as quantify the degree to which group participants' voices are included.ConclusionUsing sociograms provides a viable mechanism to complement content analysis and increase the methodological rigor of focus groups in health care research.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/113728/1/phn12199.pd
Scheduling in a three-machine robotic flexible manufacturing cell
Cataloged from PDF version of article.In this study, we consider a flexible manufacturing cell (FMC) processing identical parts on which the loading and unloading
of machines are made by a robot. The machines used in FMCs are predominantly CNC machines and these machines are flexible
enough for performing several operations provided that the required tools are stored in their tool magazines. Traditional research
in this area considers a flowshop type system. The current study relaxes this flowshop assumption which unnecessarily limits the
number of alternatives. In traditional robotic cell scheduling literature, the processing time of each part on each machine is a known
parameter. However, in this study the processing times of the parts on the machines are decision variables. Therefore, we investigated
the productivity gain attained by the additional flexibility introduced by the FMCs. We propose new lower bounds for the 1-unit
and 2-unit robot move cycles (for which we present a completely new procedure to derive the activity sequences of 2-unit cycles
in a three-machine robotic cell) under the new problem domain for the flowshop type robot move cycles. We also propose a new
robot move cycle which is a direct consequence of process and operational flexibility of CNC machines.We prove that this proposed
cycle dominates all 2-unit robot move cycles and present the regions where the proposed cycle dominates all 1-unit cycles.We also
present a worst case performance bound of using this proposed cycle.
2005 Elsevier Ltd. All rights reserved
What is on Tap? The Role of Spin in Compact Objects and Relativistic Jets
We examine the role of spin in launching jets from compact objects across the
mass scale. Our work includes a total of 37 Seyferts, 11 stellar-mass black
holes, and 13 neutron stars. We find that when the Seyfert reflection lines are
modeled with Gaussian line features (a crude proxy for inner disk radius and
therefore spin), only a slight inverse correlation is found between the
Doppler-corrected radio luminosity at 5 GHz (a proxy for jet power) and line
width. When the Seyfert reflection features are fit with
relativistically-blurred disk reflection models that measure spin, there is a
tentative positive correlation between the Doppler-corrected radio luminosity
and the spin measurement. Further, when we include stellar-mass black holes in
the sample, to examine the effects across the mass scale, we find a slightly
stronger correlation with radio luminosity per unit mass and spin, at a
marginal significance (2.3 sigma confidence level). Finally, when we include
neutron stars, in order to probe lower spin values, we find a positive
correlation (3.3 sigma confidence level) between radio luminosity per unit mass
and spin. Although tentative, these results suggest that spin may have a role
in determining the jet luminosity. In addition, we find a slightly more
significant correlation (4.4 sigma confidence level) between radio luminosity
per Bolometric luminosity and spin, using our entire sample of black holes and
neutrons stars. Again, although tentative, these relations point to the
possibility that the mass accretion rate, i.e. Bolometric luminosity, is also
important in determining the jet luminosity, in addition to spin. Our analysis
suggests that mass accretion rate and disk or coronal magnetic field strength
may be the "throttle" in these compact systems, to which the Eddington limit
and spin may set the maximum jet luminosity that can be achieved.Comment: 14 pages, 13 Figures, ApJ Accepte
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