1,874 research outputs found
Frame-dragging effects on magnetic fields near a rotating black hole
We discuss the role of general relativity frame dragging acting on magnetic
field lines near a rotating (Kerr) black hole. Near ergosphere the magnetic
structure becomes strongly influenced and magnetic null points can develop. We
consider aligned magnetic fields as well as fields inclined with respect to the
rotation axis, and the two cases are shown to behave in profoundly different
ways. Further, we construct surfaces of equal values of local electric and
magnetic intensities, which have not yet been discussed in the full generality
of a boosted rotating black hole.Comment: to appear in the proceedings of "The Central Kiloparsec in Galactic
Nuclei (AHAR 2011)", Journal of Physics: Conference Series (JPCS), IOP
Publishin
Laboratory surveillance of Shigella dysenteriae type 1 in Kwazulu-natal
Objective. To collect data on the antimicrobial susceptibility of Shigella dysenteriae type 1 in KwaZulu-Natal, including the testing of newer therapeutic agents, and to evaluate the ability of laboratories to participate in a provincial surveillance programme.Design. Prospective descriptive study.Setting. Hospital laboratories in KwaZulu-Natal, including peripheral laboratories and the medical microbiology laboratory of the University of Natal.Main outcome measures. Antimicrobial susceptibility pattern of surveillance strains and evaluation of the ability of provincial laboratories to isolate Shigella.Results. All 354 strains tested were resistant to ampicillin, chloramphenicol and tetracycline. Co-trimoxazole resistance was found in 99.2% of strains, and 0.8% of strains were resistant to nalidixic acid. All strains were susceptible to ceftriaxone, ciprofloxacin, ofloxacin, pivmecillinam, azithromycin, loracarbef and fosfomycin. Of the 29 laboratories surveyed, 18 (62.1%) were able to isolate and identify S, dysenteriae correctly, and 9 (32%) were able to serotype it further to S, dysenteriae type 1. Twenty-seven (93.1%) had appropriate culture media and 26 (89.7%) had antisera for Shigella identification.Conclusions. There is little variation among strains of S. dysenteriae type 1 in KwaZulu-Natal with regard to their antimicrobial susceptibility pattern. Nalidixic acid should remain the antimicrobial of choice for treatment of dysentery in our region as resistance to it is low. The majority of KwaZulu-Natallaboratories.have the expertise and equipment to perform the isolation and identification of Shigella species
What do near-term observations tell us about long-term developments in greenhouse gas emissions? A letter
Long-term scenarios developed by integrated assessment models are used in climate research to provide an indication of plausible long-term emissions of greenhouse gases and other radiatively active substances based on developments in the global energy system, land-use and the emissions associated with these systems The phenomena that determine these long-term developments (several decades or even centuries) are very different than those that operate on a shorter time-scales (a few years) Nevertheless, in the literature, we still often find direct comparisons between short-term observations and long-term developments that do not take into account the differing dynamics over these time scales In this letter, we discuss some of the differences between the factors that operate in the short term and those that operate in the long term We use long-term historical emissions trends to show that short-term observations are very poor indicators of long-term future emissions developments Based on this, we conclude that the performance of long-term scenarios should be evaluated against the appropriate, corresponding long-term variables and trends The research community may facilitate this by developing appropriate data sets and protocols that can be used to test the performance of long-term scenarios and the models that produce the
The Galactic centre mini-spiral in the mm-regime
The mini-spiral is a feature of the interstellar medium in the central ~2 pc
of the Galactic center. It is composed of several streamers of dust and ionised
and atomic gas with temperatures between a few 100 K to 10^4 K. There is
evidence that these streamers are related to the so-called circumnuclear disk
of molecular gas and are ionized by photons from massive, hot stars in the
central parsec. We attempt to constrain the emission mechanisms and physical
properties of the ionized gas and dust of the mini-spiral region with the help
of our multiwavelength data sets. Our observations were carried out at 1.3 mm
and 3 mm with the mm interferometric array CARMA in California in March and
April 2009, with the MIR instrument VISIR at ESO's VLT in June 2006, and the
NIR Br-gamma with VLT NACO in August 2009. We present high resolution maps of
the mini-spiral, and obtain a spectral index of 0.5 for Sgr A*, indicating an
inverted synchrotron spectrum. We find electron densities within the range
0.8-1.5x10^4 cm-3 for the mini-spiral from the radio continuum maps, along with
a dust mass contribution of ~0.25 solar masses from the MIR dust continuum, and
extinctions ranging from 1.8-3 at 2.16 micron in the Br-gamma line. We observe
a mixture of negative and positive spectral indices in our 1.3 mm and 3 mm
observations of the extended emission of the mini-spiral, which we interpret as
evidence that there are a range of contributions to the thermal free-free
emission by the ionized gas emission and by dust at 1.3 mm.Comment: 9 pages, 11 figures, accepted to A&
An extended scheme for fitting X-ray data with accretion disk spectra in the strong gravity regime
Accreting black holes are believed to emit X-rays which then mediate
information about strong gravity in the vicinity of the emission region. We
report on a set of new routines for the Xspec package for analysing X-ray
spectra of black-hole accretion disks. The new computational tool significantly
extends the capabilities of the currently available fitting procedures that
include the effects of strong gravity, and allows one to systematically explore
the constraints on more model parameters than previously possible (for example
black-hole angular momentum). Moreover, axial symmetry of the disk intrinsic
emissivity is not assumed, although it can be imposed to speed up the
computations. The new routines can be used also as a stand-alone and flexible
code with the capability of handling time-resolved spectra in the regime of
strong gravity. We have used the new code to analyse the mean X-ray spectrum
from the long XMM--Newton 2001 campaign of the Seyfert 1 galaxy MCG--6-30-15.
Consistent with previous findings, we obtained a good fit to the broad Fe K
line profile for a radial line intrinsic emissivity law in the disk which is
not a simple power law, and for near maximal value of black hole angular
momentum. However, equally good fits can be obtained also for small values of
the black hole angular momentum. The code has been developed with the aim of
allowing precise modelling of relativistic effects. Although we find that
current data cannot constrain the parameters of black-hole/accretion disk
system well, the approach allows, for a given source or situation, detailed
investigations of what features of the data future studies should be focused on
in order to achieve the goal of uniquely isolating the parameters of such
systems.Comment: Accepted for publication in ApJ S
A survey of spinning test particle orbits in Kerr spacetime
We investigate the dynamics of the Papapetrou equations in Kerr spacetime.
These equations provide a model for the motion of a relativistic spinning test
particle orbiting a rotating (Kerr) black hole. We perform a thorough parameter
space search for signs of chaotic dynamics by calculating the Lyapunov
exponents for a large variety of initial conditions. We find that the
Papapetrou equations admit many chaotic solutions, with the strongest chaos
occurring in the case of eccentric orbits with pericenters close to the limit
of stability against plunge into a maximally spinning Kerr black hole. Despite
the presence of these chaotic solutions, we show that physically realistic
solutions to the Papapetrou equations are not chaotic; in all cases, the
chaotic solutions either do not correspond to realistic astrophysical systems,
or involve a breakdown of the test-particle approximation leading to the
Papapetrou equations (or both). As a result, the gravitational radiation from
bodies spiraling into much more massive black holes (as detectable, for
example, by LISA, the Laser Interferometer Space Antenna) should not exhibit
any signs of chaos.Comment: Submitted to Phys. Rev. D. Follow-up to gr-qc/0210042. Figures are
low-resolution in order to satisfy archive size constraints; a
high-resolution version is available at http://www.michaelhartl.com/papers
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