1,660 research outputs found
Relativistic Jets from X-ray binaries
In this review I summarise the status of observational research into
relativistic jets from X-ray binaries, highlighting four areas in particular:
(i) How relativistic are the jets ?, (ii) The disc : jet coupling, (iii) the
nature of the underlying flat spectral component, and (iv) the relation between
jets from black holes and those from neutron stars. I have attempted to discuss
the extent of our (limited) physical understanding, and to point the way
towards relevant new observational tests of the various phenomena.Comment: Review article, to be published in `Astrophysics and Cosmology : A
collection of critical thoughts', Springer Lecture Notes in Physic
Soil organic matter and the extracellular microbial matrix show contrasting responses to C and N availability
AbstractAn emerging paradigm in soil science suggests microbes can perform ‘N mining’ from recalcitrant soil organic matter (SOM) in conditions of low N availability. However, this requires the production of extracellular structures rich in N (including enzymes and structural components) and thus defies stoichiometric expectation. We set out to extract newly synthesised peptides from the extracellular matrix in soil and compare the amino acid (AA) profiles, N incorporation and AA dynamics in response to labile inputs of contrasting C/N ratio. Glycerol was added both with and without an inorganic source of N (10% 15N labelled NH4NO3) to a soil already containing a large pool of refractory SOM and incubated for 10 days. The resulting total soil peptide (TSP) and extracellular pools were compared using colorimetric methods, gas chromatography, and isotope ratio mass spectrometry. N isotope compositions showed that the extracellular polymeric substance (EPS) contained a greater proportion of products formed de novo than did TSP, with hydrophobic EPS-AAs (leucine, isoleucine, phenylalanine, hydroxyproline and tyrosine) deriving substantially more N from the inorganic source provided. Quantitative comparison between extracts showed that the EPS contained greater relative proportions of alanine, glycine, proline, phenylalanine and tyrosine. The greatest increases in EPS-peptide and EPS-polysaccharide concentrations occurred at the highest C/N ratios. All EPS-AAs responded similarly to treatment whereas the responses of TSP were more complex. The results suggest that extracellular investment of N (as EPS peptides) is a microbial survival mechanism in conditions of low N/high C which, from an evolutionary perspective, must ultimately lead to the tendency for increased N returns to the microbial biomass. A conceptual model is proposed that describes the dynamics of the extracellular matrix in response to the C/N ratio of labile inputs
Columnar defects and vortex fluctuations in layered superconductors
We investigate fluctuations of Josephson-coupled pancake vortices in layered
superconductors in the presence of columnar defects. We study the
thermodynamics of a single pancake stack pinned by columnar defects and obtain
the temperature dependence of localization length, pinning energy and critical
current. We study the creep regime and compute the crossover current between
line-like creep and pancake-like creep motion. We find that columnar defects
effectively increase interlayer Josephson coupling by suppressing thermal
fluctuations of pancakes. This leads to an upward shift in the decoupling line
most pronounced around the matching field.Comment: 5 pages, REVTeX, no figure
Phase Separation Based on U(1) Slave-boson Functional Integral Approach to the t-J Model
We investigate the phase diagram of phase separation for the hole-doped two
dimensional system of antiferromagnetically correlated electrons based on the
U(1) slave-boson functional integral approach to the t-J model. We show that
the phase separation occurs for all values of J/t, that is, whether or with J, the Heisenberg coupling constant and t, the hopping
strength. This is consistent with other numerical studies of hole-doped two
dimensional antiferromagnets. The phase separation in the physically
interesting J region, is examined by introducing
hole-hole (holon-holon) repulsive interaction. We find from this study that
with high repulsive interaction between holes the phase separation boundary
tends to remain robust in this low region, while in the high J region, J/t
> 0.4, the phase separation boundary tends to disappear.Comment: 4 pages, 2 figures, submitted to Phys. Rev.
Controls on andesitic glaciovolcanism at ice-capped volcanoes from field and experimental studies
Glaciovolcanic deposits at Tongariro and Ruapehu volcanoes, New Zealand, represent diverse styles of interaction between wet-based glaciers and andesitic lava. There are ice-confined lavas, and also hydroclastic breccia and subaqueous pyroclastic deposits that formed during effusive and explosive eruptions into meltwater beneath the glacier; they are rare among globally reported products of andesitic glaciovolcanism. The apparent lack of hydrovolcanically fragmented andesite at ice-capped volcanoes has been attributed to a lack of meltwater at the interaction sites because either the thermal characteristics of andesite limit meltwater production or meltwater drains out through leaky glaciers and down steep volcano slopes. We used published field evidence and novel, dynamic andesite-ice experiments to show that, in some cases, meltwater accumulates under glaciers on andesitic volcanoes and that meltwater production rates increase as andesite pushes against an ice wall. We concur with models for eruptions beneath ice sheets showing that the glacial conditions and pre-eruption edifice morphology are more important controls on the style of glaciovolcanism and its products than magma composition and the thermal properties of magmas. Glaciovolcanic products can be useful proxies for paleoenvironment, and the range of andesitic products and the hydrological environments in which andesite erupts are greater than hitherto appreciated
The balance of power: accretion and feedback in stellar mass black holes
In this review we discuss the population of stellar-mass black holes in our
galaxy and beyond, which are the extreme endpoints of massive star evolution.
In particular we focus on how we can attempt to balance the available accretion
energy with feedback to the environment via radiation, jets and winds,
considering also possible contributions to the energy balance from black hole
spin and advection. We review quantitatively the methods which are used to
estimate these quantities, regardless of the details of the astrophysics close
to the black hole. Once these methods have been outlined, we work through an
outburst of a black hole X-ray binary system, estimating the flow of mass and
energy through the different accretion rates and states. While we focus on
feedback from stellar mass black holes in X-ray binary systems, we also
consider the applicability of what we have learned to supermassive black holes
in active galactic nuclei. As an important control sample we also review the
coupling between accretion and feedback in neutron stars, and show that it is
very similar to that observed in black holes, which strongly constrains how
much of the astrophysics of feedback can be unique to black holes.Comment: To be published in Haardt et al. Astrophysical Black Holes. Lecture
Notes in Physics. Springer 201
Nanostructural characterization of geopolymers by advanced beamline techniques
This paper presents the outcomes of a series of beamline-based studies, the results of which are combined to provide a more detailed multiscale understanding of the structure and chemistry of geopolymer binders.
The range of beamline-based characterization techniques which have been applied to the study of geopolymer binders is increasing rapidly; although no single technique can provide a holistic view of binder structure across all the length scales which are of importance in determining strength development and durability, the synergy achievable through the combination of multiple beamline techniques is leading to rapid advances in knowledge in this area. Studies based around beamline infrared and X-ray fluorescence microscopy, in situ and ex situ neutron pair distribution function analysis, and nano- and micro-tomography, are combined to provide an understanding of geopolymer gel chemistry, nano- and microstructure in two and three dimensions, and the influences of seeded nucleation and precursor chemistry in these key areas.
The application of advanced characterization methods in recent years has brought the understanding of geopolymer chemistry from a point, not more than a decade ago, when the analysis of the detailed chemistry of the aluminosilicate binder gel was considered intractable due to its disordered (“X-ray amorphous”) nature, to the present day where the influence of key compositional parameters on nanostructure is well understood, and both gel structure and reaction kinetics can be manipulated through methods including seeding, temperature variation, and careful mix design.
This paper therefore provides a review outlining the value of nanotechnology – and particularly nanostructural characterization – in the development and optimization of a new class of environmentally beneficial cements and concretes. Key engineering parameters, in particularly strength development and permeability, are determined at a nanostructural level, and so it is essential that gel structures can be analyzed and manipulated at this level; beamline-based characterization techniques are critical in providing the ability to achieve this goal
Eruptions of Magnetic Ropes in Two Homologous Solar Events on 2002 June 1 and 2: a Key to Understanding of an Enigmatic Flare
The goal of this paper is to understand the drivers, configurations, and
scenarios of two similar eruptive events, which occurred in the same solar
active region 9973 on 2002 June 1 and 2. The June 2 event was previously
studied by Sui, Holman, and Dennis (2006, 2008), who concluded that it was
challenging for popular flare models. Using multi-spectral data, we analyze a
combination of the two events. Each of the events exhibited an evolving
cusp-like feature. We have revealed that these apparent ``cusps'' were most
likely mimicked by twisted magnetic flux ropes, but unlikely to be related to
the inverted Y-like magnetic configuration in the standard flare model. The
ropes originated inside a funnel-like magnetic domain whose base was bounded by
an EUV ring structure, and the top was associated with a coronal null point.
The ropes appear to be the major drivers for the events, but their rise was not
triggered by reconnection in the coronal null point. We propose a scenario and
a three-dimensional scheme for these events in which the filament eruptions and
flares were caused by interaction of the ropes.Comment: 22 pages, 11 figure
The one-dimensional Bose-Hubbard Model with nearest-neighbor interaction
We study the one-dimensional Bose-Hubbard model using the Density-Matrix
Renormalization Group (DMRG).For the cases of on-site interactions and
additional nearest-neighbor interactions the phase boundaries of the
Mott-insulators and charge density wave phases are determined. We find a direct
phase transition between the charge density wave phase and the superfluid
phase, and no supersolid or normal phases. In the presence of nearest-neighbor
interaction the charge density wave phase is completely surrounded by a region
in which the effective interactions in the superfluid phase are repulsive. It
is known from Luttinger liquid theory that a single impurity causes the system
to be insulating if the effective interactions are repulsive, and that an even
bigger region of the superfluid phase is driven into a Bose-glass phase by any
finite quenched disorder. We determine the boundaries of both regions in the
phase diagram. The ac-conductivity in the superfluid phase in the attractive
and the repulsive region is calculated, and a big superfluid stiffness is found
in the attractive as well as the repulsive region.Comment: 19 pages, 30 figure
Are bioresorbable polylactate devices comparable to titanium devices for stabilizing Le Fort I advancement?
AbstractThe purpose of this study was to evaluate whether skeletal and dental outcomes following Le Fort I surgery differed when stabilization was performed with polylactate bioresorbable devices or titanium devices. Fifty-seven patients with preoperative records and at least 1 year postoperative records were identified and grouped according to the stabilization method. All cephalometric X-rays were traced and digitized by a single operator. Analysis of covariance was used to compare the postsurgical change between the two stabilization methods. Twenty-seven patients received bioresorbable devices (group R), while 30 received titanium devices (group M). There were no statistically significant differences between the two groups with respect to gender, race/ethnicity, age, or dental and skeletal movements during surgery. Subtle postsurgical differences were noted, but were not statistically significant. Stabilization of Le Fort I advancement with polylactate bioresorbable and titanium devices produced similar clinical outcomes at 1 year following surgery
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