227 research outputs found
Current Status of Simulations
As the title suggests, the purpose of this chapter is to review the current
status of numerical simulations of black hole accretion disks. This chapter
focuses exclusively on global simulations of the accretion process within a few
tens of gravitational radii of the black hole. Most of the simulations
discussed are performed using general relativistic magnetohydrodynamic (MHD)
schemes, although some mention is made of Newtonian radiation MHD simulations
and smoothed particle hydrodynamics. The goal is to convey some of the exciting
work that has been going on in the past few years and provide some speculation
on future directions.Comment: 15 pages, 14 figures, to appear in the proceedings of the ISSI-Bern
workshop on "The Physics of Accretion onto Black Holes" (8-12 October 2012
On the Potts model partition function in an external field
We study the partition function of Potts model in an external (magnetic)
field, and its connections with the zero-field Potts model partition function.
Using a deletion-contraction formulation for the partition function Z for this
model, we show that it can be expanded in terms of the zero-field partition
function. We also show that Z can be written as a sum over the spanning trees,
and the spanning forests, of a graph G. Our results extend to Z the well-known
spanning tree expansion for the zero-field partition function that arises
though its connections with the Tutte polynomial
Charge-Symmetry Breaking and the Two-Pion-Exchange Two-Nucleon Interaction
Charge-symmetry breaking in the nucleon-nucleon force is investigated within
an effective field theory, using a classification of isospin-violating
interactions based on power-counting arguments. The relevant
charge-symmetry-breaking interactions corresponding to the first two orders in
the power counting are discussed, including their effects on the 3He-3H
binding-energy difference. The static charge-symmetry-breaking potential linear
in the nucleon-mass difference is constructed using chiral perturbation theory.
Explicit formulae in momentum and configuration spaces are presented. The
present work completes previously obtained results.Comment: 15 pages, 2 figure
Use of viral pathogens and indicators to differentiate between human and non-human fecal contamination in a microbial source tracking comparison study
Assays for the detection and typing of adenoviruses, enteroviruses and F+ specific coliphages were performed on samples created as part of a national microbial source tracking methods comparison study. The samples were created blind to the researchers, and were inoculated with a variety of types of fecal contamination source (human, sewage, dog, seagull and cow) and mixtures of sources. Viral tracer and pathogen assays demonstrated a general ability to discriminate human from non-human fecal contamination. For example, samples inoculated with sewage were correctly identified as containing human fecal contamination because they contained human adenovirus or human enterovirus. In samples containing fecal material from individual humans, human pathogen analysis yielded negative results probably because the stool samples were taken from healthy individuals. False positive rates for the virus-based methods (0-8%) were among the lowest observed during the methods comparison study. It is suggested that virus-based source tracking methods are useful for identification of sewage contamination, and that these methods may also be useful as an indication of the public health risk associated with viral pathogens. Overall, virus-based source tracking methods are an important approach to include in the microbial source tracking 'toolbox'
Black Hole Spin via Continuum Fitting and the Role of Spin in Powering Transient Jets
The spins of ten stellar black holes have been measured using the
continuum-fitting method. These black holes are located in two distinct classes
of X-ray binary systems, one that is persistently X-ray bright and another that
is transient. Both the persistent and transient black holes remain for long
periods in a state where their spectra are dominated by a thermal accretion
disk component. The spin of a black hole of known mass and distance can be
measured by fitting this thermal continuum spectrum to the thin-disk model of
Novikov and Thorne; the key fit parameter is the radius of the inner edge of
the black hole's accretion disk. Strong observational and theoretical evidence
links the inner-disk radius to the radius of the innermost stable circular
orbit, which is trivially related to the dimensionless spin parameter a_* of
the black hole (|a_*| < 1). The ten spins that have so far been measured by
this continuum-fitting method range widely from a_* \approx 0 to a_* > 0.95.
The robustness of the method is demonstrated by the dozens or hundreds of
independent and consistent measurements of spin that have been obtained for
several black holes, and through careful consideration of many sources of
systematic error. Among the results discussed is a dichotomy between the
transient and persistent black holes; the latter have higher spins and larger
masses. Also discussed is recently discovered evidence in the transient sources
for a correlation between the power of ballistic jets and black hole spin.Comment: 30 pages. Accepted for publication in Space Science Reviews. Also to
appear in hard cover in the Space Sciences Series of ISSI "The Physics of
Accretion onto Black Holes" (Springer Publisher). Changes to Sections 5.2,
6.1 and 7.4. Section 7.4 responds to Russell et al. 2013 (MNRAS, 431, 405)
who find no evidence for a correlation between the power of ballistic jets
and black hole spi
Massive binary black holes in galactic nuclei and their path to coalescence
Massive binary black holes form at the centre of galaxies that experience a
merger episode. They are expected to coalesce into a larger black hole,
following the emission of gravitational waves. Coalescing massive binary black
holes are among the loudest sources of gravitational waves in the Universe, and
the detection of these events is at the frontier of contemporary astrophysics.
Understanding the black hole binary formation path and dynamics in galaxy
mergers is therefore mandatory. A key question poses: during a merger, will the
black holes descend over time on closer orbits, form a Keplerian binary and
coalesce shortly after? Here we review progress on the fate of black holes in
both major and minor mergers of galaxies, either gas-free or gas-rich, in
smooth and clumpy circum-nuclear discs after a galactic merger, and in
circum-binary discs present on the smallest scales inside the relic nucleus.Comment: Accepted for publication in Space Science Reviews. To appear in hard
cover in the Space Sciences Series of ISSI "The Physics of Accretion onto
Black Holes" (Springer Publisher
Menus for Feeding Black Holes
Black holes are the ultimate prisons of the Universe, regions of spacetime
where the enormous gravity prohibits matter or even light to escape to
infinity. Yet, matter falling toward the black holes may shine spectacularly,
generating the strongest source of radiation. These sources provide us with
astrophysical laboratories of extreme physical conditions that cannot be
realized on Earth. This chapter offers a review of the basic menus for feeding
matter onto black holes and discusses their observational implications.Comment: 27 pages. Accepted for publication in Space Science Reviews. Also to
appear in hard cover in the Space Sciences Series of ISSI "The Physics of
Accretion onto Black Holes" (Springer Publisher
Supermassive Black Hole Binaries: The Search Continues
Gravitationally bound supermassive black hole binaries (SBHBs) are thought to
be a natural product of galactic mergers and growth of the large scale
structure in the universe. They however remain observationally elusive, thus
raising a question about characteristic observational signatures associated
with these systems. In this conference proceeding I discuss current theoretical
understanding and latest advances and prospects in observational searches for
SBHBs.Comment: 17 pages, 4 figures. To appear in the Proceedings of 2014 Sant Cugat
Forum on Astrophysics. Astrophysics and Space Science Proceedings, ed.
C.Sopuerta (Berlin: Springer-Verlag
A six-gene phylogeny provides new insights into choanoflagellate evolution
Recent studies have shown that molecular phylogenies of the choanoflagellates (Class Choanoflagellatea)
are in disagreement with their traditional taxonomy, based on morphology, and that Choanoflagellatea
requires considerable taxonomic revision. Furthermore, phylogenies suggest that the morphological
and ecological evolution of the group is more complex than has previously been recognized. Here we
address the taxonomy of the major choanoflagellate order Craspedida, by erecting four new genera.
The new genera are shown to be morphologically, ecologically and phylogenetically distinct from other
choanoflagellate taxa. Furthermore, we name five novel craspedid species, as well as formally describe
ten species that have been shown to be either misidentified or require taxonomic revision. Our revised
phylogeny, including 18 new species and sequence data for two additional genes, provides insights into
the morphological and ecological evolution of the choanoflagellates. We examine the distribution within
choanoflagellates of these two additional genes, EF-1A and EFL, closely related translation GTPases which
are required for protein synthesis. Mapping the presence and absenc
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