2,402 research outputs found
Black-hole jets without large-scale net magnetic flux
We propose a scenario for launching relativistic jets from rotating black
holes, in which small-scale magnetic flux loops, sustained by disc turbulence,
are forced to inflate and open by differential rotation between the black hole
and the accretion flow. This mechanism does not require a large-scale net
magnetic flux in the accreting plasma. Estimates suggest that the process could
operate effectively in many systems, and particularly naturally and efficiently
when the accretion flow is retrograde. We present the results of
general-relativistic force-free electrodynamic simulations demonstrating the
time evolution of the black hole's magnetosphere, the cyclic formation of jets,
and the effect of magnetic reconnection. The jets are highly variable on
timescales ~ 10-10^3 r_ g/c, where r_g is the black hole's gravitational
radius. The reconnecting current sheets observed in the simulations may be
responsible for the hard X-ray emission from accreting black holes.Comment: 5 pages, 2 figures. Accepted for publication in MNRAS Letter
Particle acceleration in axisymmetric pulsar current sheets
The equatorial current sheet in pulsar magnetospheres is often regarded as an
ideal site for particle acceleration via relativistic reconnection. Using 2D
spherical particle-in-cell simulations, we investigate particle acceleration in
the axisymmetric pulsar magnetosphere as a function of the injected plasma
multiplicity and magnetization. We observe a clear transition from a highly
charge-separated magnetosphere for low plasma injection with little current and
spin-down power, to a nearly force-free solution for high plasma multiplicity
characterized by a prominent equatorial current sheet and high spin-down power.
We find significant magnetic dissipation in the current sheet, up to 30% within
5 light-cylinder radii in the high-multiplicity regime. The simulations
unambiguously demonstrate that the dissipated Poynting flux is efficiently
channeled to the particles in the sheet, close to the Y-point within about 1-2
light cylinder radii from the star. The mean particle energy in the sheet is
given by the upstream plasma magnetization at the light cylinder. The study of
particle orbits shows that all energetic particles originate from the boundary
layer between the open and the closed field lines. Energetic positrons always
stream outward, while high-energy electrons precipitate back towards the star
through the sheet and along the separatrices, which may result in auroral-like
emission. Our results suggest that the current sheet and the separatrices may
be the main source of high-energy radiation in young pulsars.Comment: 16 pages, 17 figures, Accepted for publication in MNRA
The Evolution of Bias - Generalized
Fry (1996) showed that galaxy bias has the tendency to evolve towards unity,
i.e. in the long run, the galaxy distribution tends to trace that of matter.
Generalizing slightly Fry's reasoning, we show that his conclusion remains
valid in theories of modified gravity (or equivalently, complex clustered dark
energy). This is not surprising: as long as both galaxies and matter are
subject to the same force, dynamics would drive them towards tracing each
other. This holds, for instance, in theories where both galaxies and matter
move on geodesics. This relaxation of bias towards unity is tempered by cosmic
acceleration, however: the bias tends towards unity but does not quite make it,
unless the formation bias were close to unity. Our argument is extended in a
straightforward manner to the case of a stochastic or nonlinear bias. An
important corollary is that dynamical evolution could imprint a scale
dependence on the large scale galaxy bias. This is especially pronounced if
non-standard gravity introduces new scales to the problem: the bias at
different scales relaxes at different rates, the larger scales generally more
slowly and retaining a longer memory of the initial bias. A consistency test of
the current (general relativity + uniform dark energy) paradigm is therefore to
look for departure from a scale independent bias on large scales. A simple way
is to measure the relative bias of different populations of galaxies which are
at different stages of bias relaxation. Lastly, we comment on the possibility
of directly testing the Poisson equation on cosmological scales, as opposed to
indirectly through the growth factor.Comment: 8 pages, 2 figures. References added. Accepted for publication in
Physical Review
Dynamic Genomes of Eukaryotes and the Maintenance of Genomic Integrity
Many biologists assume that eukaryotic genomes are transmitted stably between generations with only minor variations. Yet, this presumed constancy is at odds with data indicating that eukaryotic genomes are dynamic, varying extensively in content among many different lineages. Thus, rather than being constant, genomes vary considerably within individuals during their lifetimes
Genome Dynamics are Influenced by Food Source in \u3ci\u3eAllogromia laticollaris\u3c/i\u3e Strain CSH (Foraminifera)
Across the eukaryotic tree of life, genomes vary within populations and within individuals during their life cycle. Understanding intraspecific genome variation in diverse eukaryotes is key to elucidating the factors that underlie this variation. Here, we characterize genome dynamics during the life cycle of Allogromia laticollaris strain CSH, a member of the Foraminifera, using fluorescence microscopy and reveal extensive variation in nuclear size and DNA content. Both nuclear size and DNA content are tightly correlated across a 700-fold range in cell volume. In contrast to models in yeast where nuclear size is determined solely by cell size, the relationship in A. laticollaris CSH differs according to both life cycle stage and food source. Feeding A. laticollaris CSH a diet that includes algae results in a 2-fold increase in DNA content in reproductive cells compared with a diet of bacteria alone. This difference in DNA content likely corresponds to increased fecundity, as reproduction occurs through segregation of the polyploid nucleus into numerous daughter nuclei. Environmentally mediated variation in DNA content may be a widespread phenomenon, as it has been previously reported in the plant flax and the flagellate Euglena. We hypothesize that DNA content is influenced by food in other single-celled eukaryotes with ploidy cycles and that this genome flexibility may enable these eukaryotes to maximize fitness across changing environmental conditions
Development of a Consensus Statement for the Definition, Diagnosis, and Treatment of Acute Exacerbations of Idiopathic Pulmonary Fibrosis Using the Delphi Technique.
© 2015, The Author(s).Introduction: There is a lack of agreed and established guidelines for the treatment of acute exacerbations of idiopathic pulmonary fibrosis (AE-IPF). This reflects, in part, the limited evidence-base underpinning the management of AE-IPF. In the absence of high-quality evidence, the aim of this research was to develop a clinician-led consensus statement for the definition, diagnosis and treatment of AE-IPF. Methods: A literature review was conducted to obtain published material on the definition and treatment of AE-IPF. The results of this review were circulated to an online panel of clinicians for review. Statements were then shared with ten expert respiratory clinicians who regularly treat patients with IPF. A Delphi technique was then used to develop a consensus statement for the definition, diagnosis and treatment of AE-IPF. During the first round of review, clinicians rated the clarity of each statement, the extent to which the statement should be included and provided comments. In two subsequent rounds of review, clinicians were provided with the group median inclusion rating for each statement, and any revised wording of statements to aid clarity. Clinicians were asked to repeat the clarity and inclusion ratings for the revised statements. Results: The literature review, online panel discussion, and face-to-face meeting generated 65 statements covering the definition, diagnosis, and management of AE-IPF. Following three rounds of blind review, 90% of clinicians agreed 39 final statements. These final statements included a definition of AE-IPF, approach to diagnosis, and treatment options, specifically: supportive measures, use of anti-microbials, immunosuppressants, anti-coagulants, anti-fibrotic therapy, escalation, transplant management, and long-term management including discharge planning. Conclusion: This clinician-led consensus statement establishes the ‘best practice’ for the management and treatment of AE-IPF based on current knowledge, evidence, and available treatments. Funding: Boehringer Ingelheim Ltd., Bracknell, West Berkshire, UK
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