119 research outputs found
General Relativistic Simulations of Jet Formation in a Rapidly Rotating Black Hole Magnetosphere
To investigate the formation mechanism of relativistic jets in active
galactic nuclei and micro-quasars, we have developed a new general relativistic
magnetohydrodynamic code in Kerr geometry. Here we report on the first
numerical simulation of jet formation in a rapidly-rotating (a=0.95) Kerr black
hole magnetosphere. We study cases in which the Keplerian accretion disk is
both co-rotating and counter-rotating with respect to the black hole rotation.
In the co-rotating disk case, our results are almost the same as those in
Schwarzschild black hole cases: a gas pressure-driven jet is formed by a shock
in the disk, and a weaker magnetically-driven jet is also generated outside the
gas pressure-driven jet. On the other hand, in the counter-rotating disk case,
a new powerful magnetically-driven jet is formed inside the gas pressure-driven
jet. The newly found magnetically-driven jet in the latter case is accelerated
by a strong magnetic field created by frame dragging in the ergosphere. Through
this process, the magnetic field extracts the energy of the black hole
rotation.Comment: Co-rotating and counter-rotating disks; 8 pages; submitted to ApJ
letter
A Photometric Method for Quantifying Asymmetries in Disk Galaxies
A photometric method for quantifying deviations from axisymmetry in optical
images of disk galaxies is applied to a sample of 32 face-on and nearly face-on
spirals. The method involves comparing the relative fluxes contained within
trapezoidal sectors arranged symmetrically about the galaxy center of light,
excluding the bulge and/or barred regions. Such a method has several advantages
over others, especially when quantifying asymmetry in flocculent galaxies.
Specifically, the averaging of large regions improves the signal-to-noise in
the measurements; the method is not strongly affected by the presence of spiral
arms; and it identifies the kinds of asymmetry that are likely to be
dynamically important. Application of this "method of sectors" to R-band images
of 32 disk galaxies indicates that about 30% of spirals show deviations from
axisymmetry at the 5-sigma level.Comment: 17 pages, 2 tables and 6 figures, uses psfig and AAS LaTex; to appear
in A
Departures From Axisymmetric Morphology and Dynamics in Spiral Galaxies
New HI synthesis data have been obtained for six face-on galaxies with the
Very Large Array. These data and reanalyses of three additional data sets make
up a sample of nine face-on galaxies analyzed for deviations from axisymmetry
in morphology and dynamics. This sample represents a subsample of galaxies
already analyzed for morphological symmetry properties in the R-band. Four
quantitative measures of dynamical nonaxisymmetry are compared to one another
and to the quantitative measures of morphological asymmetry in HI and R-band to
investigate the relationships between nonaxisymmetric morphology and dynamics.
We find no significant relationship between asymmetric morphology and most of
the dynamical measures in our sample. A possible relationship is found,
however, between morphology and dynamical position angle differences between
approaching and receding sides of the galaxy.Comment: 24 pages, 19 figures, AASTeX, accepted for publication in AJ,
postscript figures available at
ftp://culebra.tn.cornell.edu/pub/david/figures.tar.g
Momentum flow in black-hole binaries: II. Numerical simulations of equal-mass, head-on mergers with antiparallel spins
Research on extracting science from binary-black-hole (BBH) simulations has
often adopted a "scattering matrix" perspective: given the binary's initial
parameters, what are the final hole's parameters and the emitted gravitational
waveform? In contrast, we are using BBH simulations to explore the nonlinear
dynamics of curved spacetime. Focusing on the head-on plunge, merger, and
ringdown of a BBH with transverse, antiparallel spins, we explore numerically
the momentum flow between the holes and the surrounding spacetime. We use the
Landau-Lifshitz field-theory-in-flat-spacetime formulation of general
relativity to define and compute the density of field energy and field momentum
outside horizons and the energy and momentum contained within horizons, and we
define the effective velocity of each apparent and event horizon as the ratio
of its enclosed momentum to its enclosed mass-energy. We find surprisingly good
agreement between the horizons' effective and coordinate velocities. To
investigate the gauge dependence of our results, we compare pseudospectral and
moving-puncture evolutions of physically similar initial data; although
spectral and puncture simulations use different gauge conditions, we find
remarkably good agreement for our results in these two cases. We also compare
our simulations with the post-Newtonian trajectories and near-field
energy-momentum. [Abstract abbreviated; full abstract also mentions additional
results.]Comment: Submitted to Phys. Rev.
Description, Utilisation and Results from a Telehealth Primary Care Weight Management Intervention for Adults with Obesity in South Carolina
Introduction: In the US, obesity rates are higher in rural areas than in urban areas. Rural access to treatment of obesity is limited by a lack of qualified clinicians and by transportation and financial barriers. We describe a telemedicine weight management programme, Wellness Connect, developed through a partnership of academic clinicians and rural primary care providers in South Carolina, and present utilisation and weight outcomes from seven patient cohorts. Methods: Eight bi-weekly sessions were provided via telemedicine videoconferencing for groups of patients at these rural primary care clinics. Protocol-based sessions were led by registered dietitians, exercise physiologists and clinical psychologists at a central urban location. Results: Of 138 patients who started the programme, 62% (N ¼ 86) of patients met the criteria for completion. Completers lost an average of 3.5% (standard deviation (SD) ¼ 3.9%) body weight, which was statistically significant (p \u3c.001) and corresponded with an average loss of 3.8 kg (SD ¼ 4.5 kg). There were no differences in weight change among clinics (p ¼.972). Overall, patients and providers reported satisfaction with the programme and identified several challenges to sustainability. Discussion: The use of innovative telemedicine interventions continues to be necessary to alleviate barriers to accessing evidence-based services to reduce chronic diseases and decrease obesity rates among rural populations
Low energy effective string cosmology
We give the general analytic solutions derived from the low energy string
effective action for four dimensional Friedmann-Robertson-Walker models with
dilaton and antisymmetric tensor field, considering both long and short
wavelength modes of the -field. The presence of a homogeneous -field
significantly modifies the evolution of the scale factor and dilaton. In
particular it places a lower bound on the allowed value of the dilaton. The
scale factor also has a lower bound but our solutions remain singular as they
all contain regions where the spacetime curvature diverges signalling a
breakdown in the validity of the effective action. We extend our results to the
simplest Bianchi I metric in higher dimensions with only two scale factors. We
again give the general analytic solutions for long and short wavelength modes
for the field restricted to the three dimensional space, which produces an
anisotropic expansion. In the case of field radiation (wavelengths within
the Hubble length) we obtain the usual four dimensional radiation dominated FRW
model as the unique late time attractor.Comment: 22 pages, LaTeX, SUSX-TH-94/37, SUSSEX-AST-94/6-2. (Some terminology
and figure captions corrected, references added.
Building better Sex Robots: Lessons from Feminist Pornography
How should we react to the development of sexbot technology? Taking their cue from anti-porn feminism, several academic critics lament the development of sexbot technology, arguing that it objectifies and subordinates women, is likely to promote misogynistic attitudes toward sex, and may need to be banned or restricted. In this chapter I argue for an alternative response. Taking my cue from the sex positive ‘feminist porn’ movement, I argue that the best response to the development of ‘bad’ sexbots is to make better ones. This will require changes to the content, process and context of sexbot development. Doing so will acknowledge the valuable role that technology can play in human sexuality, and allow us to challenge gendered norms and assumptions about male and female sexual desire. This will not be a panacea to the social problems that could arise from sexbot development, but it offers a more realistic and hopeful vision for the future of this technology in a pluralistic and progressive society
Comprehensive analysis of the chromatin landscape in Drosophila melanogaster.
Chromatin is composed of DNA and a variety of modified histones and non-histone proteins, which have an impact on cell differentiation, gene regulation and other key cellular processes. Here we present a genome-wide chromatin landscape for Drosophila melanogaster based on eighteen histone modifications, summarized by nine prevalent combinatorial patterns. Integrative analysis with other data (non-histone chromatin proteins, DNase I hypersensitivity, GRO-Seq reads produced by engaged polymerase, short/long RNA products) reveals discrete characteristics of chromosomes, genes, regulatory elements and other functional domains. We find that active genes display distinct chromatin signatures that are correlated with disparate gene lengths, exon patterns, regulatory functions and genomic contexts. We also demonstrate a diversity of signatures among Polycomb targets that include a subset with paused polymerase. This systematic profiling and integrative analysis of chromatin signatures provides insights into how genomic elements are regulated, and will serve as a resource for future experimental investigations of genome structure and function
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