545 research outputs found
X-ray Emission Diagnostics from the M87 Jet
We use Chandra, HST and VLA observations of M87 to investigate the physics of
X-ray emission from AGN jets. We find that X-ray hotspots in the M87 jet occur
primarily in regions with hard optical-to-X-ray spectra and lower than average
polarization. Particle injection appears to be required both continuously in
the jet sheath as well as locally at X-ray hotspots.Comment: 4 pages, 2 figures, submitted to "The Physics of Relativistic Jets in
the CHANDRA and XMM Era", Bologna 200
CHANDRA Observations of X-ray Jet Structure on kpc to Mpc Scales
With its exquisite spatial resolution of better than 0.5 arcsecond, the
Chandra observatory is uniquely capable of resolving and studying the spatial
structure of extragalactic X-ray jets on scales of a few to a few hundred
kilo-parsec. Our analyses of four recent Chandra images of quasar jets
interpret the X-ray emission as inverse Compton scattering of high energy
electrons on the cosmic microwave background. We infer that these jets are in
bulk relativistic motion, carrying kinetic powers upwards of 10^46 ergs/s to
distances of hundreds of kpc, with very high efficiency.Comment: 4 pages, 3 figures, to be published in the proceedings of the Bologna
jet workshop, "The Physics of Relativistic Jets in the CHANDRA and XMM Era.
Microplastics on the menu: Plastics pollute Indonesian Manta Ray and Whale Shark feeding grounds
The implications of plastic pollution, including microplastics, on marine ecosystems and species are increasingly seen as an environmental disaster. Yet few reports focus on filter-feeding megafauna in regions heavily impacted by plastic pollution, such as Indonesia in the Coral Triangle, a global marine biodiversity hotspot. Here, we evaluate plastic abundance and characterize debris from feeding grounds for manta rays Mobula alfredi and whale sharks Rhincodon typus in three coastal locations in Indonesia: Nusa Penida Marine Protected Area, Komodo National Park, and Pantai Bentar, East Java. A 200 μm plankton net was used to sample the top 0.5 m of the water column (‘trawl survey’) and floating plastics were assessed along ∼440 m long transects (‘visual survey’) during the Indonesian north-west (wet) and south-east (dry) monsoon seasons during 2016–2018. Microplastics were identified visually, measured and categorized from trawl samples, and larger floating plastics were counted and categorized visually from boats. Plastic abundance ranged widely from 0.04 to 0.90 pieces m–3 (trawl survey) and 210 to 40,844 pieces km–2 (visual survey). Results from linear models showed significant seasonal and location differences in estimated plastic abundance for trawl and visual surveys in Nusa Penida and Komodo. Plastic abundance was up to ∼ 44 times higher in the wet than the dry season, with the largest seasonal effect observed in Nusa Penida. Overall, small pieces 50% combined) were the most prevalent plastics. Theoretical plastic ingestion rates were calculated using estimated filtration volumes of manta rays and whale sharks and the mean plastic abundance in their feeding grounds. Upper plastic ingestion estimates for manta rays were ∼63 and 25 pieces h–1 for Nusa Penida and Komodo locations, respectively, and ∼137 pieces h–1 for whale sharks in Java. Analysis of manta ray egested material confirmed plastic ingestion, the consequences of which might include exposure to toxic plastic additives and adhered persistent organic pollutants. Communicating this information to communities who stand to benefit from healthy megafauna populations might help local governments as they work toward reducing plastics in the marine environment
Postmodern String Theory: Stochastic Formulation
In this paper we study the dynamics of a statistical ensemble of strings,
building on a recently proposed gauge theory of the string geodesic field. We
show that this stochastic approach is equivalent to the Carath\'eodory
formulation of the Nambu-Goto action, supplemented by an averaging procedure
over the family of classical string world-sheets which are solutions of the
equation of motion. In this new framework, the string geodesic field is
reinterpreted as the Gibbs current density associated with the string
statistical ensemble. Next, we show that the classical field equations derived
from the string gauge action, can be obtained as the semi-classical limit of
the string functional wave equation. For closed strings, the wave equation
itself is completely analogous to the Wheeler-DeWitt equation used in quantum
cosmology. Thus, in the string case, the wave function has support on the space
of all possible spatial loop configurations. Finally, we show that the string
distribution induces a multi-phase, or {\it cellular} structure on the
spacetime manifold characterized by domains with a purely Riemannian geometry
separated by domain walls over which there exists a predominantly Weyl
geometry.Comment: 24pages, ReVTe
The acquisition of Sign Language: The impact of phonetic complexity on phonology
Research into the effect of phonetic complexity on phonological acquisition has a long history in spoken languages. This paper considers the effect of phonetics on phonological development in a signed language. We report on an experiment in which nonword-repetition methodology was adapted so as to examine in a systematic way how phonetic complexity in two phonological parameters of signed languages — handshape and movement — affects the perception and articulation of signs. Ninety-one Deaf children aged 3–11 acquiring British Sign Language (BSL) and 46 hearing nonsigners aged 6–11 repeated a set of 40 nonsense signs. For Deaf children, repetition accuracy improved with age, correlated with wider BSL abilities, and was lowest for signs that were phonetically complex. Repetition accuracy was correlated with fine motor skills for the youngest children. Despite their lower repetition accuracy, the hearing group were similarly affected by phonetic complexity, suggesting that common visual and motoric factors are at play when processing linguistic information in the visuo-gestural modality
Analytic models of plausible gravitational lens potentials
Gravitational lenses on galaxy scales are plausibly modelled as having
ellipsoidal symmetry and a universal dark matter density profile, with a Sersic
profile to describe the distribution of baryonic matter. Predicting all lensing
effects requires knowledge of the total lens potential: in this work we give
analytic forms for that of the above hybrid model. Emphasising that complex
lens potentials can be constructed from simpler components in linear
combination, we provide a recipe for attaining elliptical symmetry in either
projected mass or lens potential. We also provide analytic formulae for the
lens potentials of Sersic profiles for integer and half-integer index. We then
present formulae describing the gravitational lensing effects due to
smoothly-truncated universal density profiles in cold dark matter model. For
our isolated haloes the density profile falls off as radius to the minus fifth
or seventh power beyond the tidal radius, functional forms that allow all
orders of lens potential derivatives to be calculated analytically, while
ensuring a non-divergent total mass. We show how the observables predicted by
this profile differ from that of the original infinite-mass NFW profile.
Expressions for the gravitational flexion are highlighted. We show how
decreasing the tidal radius allows stripped haloes to be modelled, providing a
framework for a fuller investigation of dark matter substructure in galaxies
and clusters. Finally we remark on the need for finite mass halo profiles when
doing cosmological ray-tracing simulations, and the need for readily-calculable
higher order derivatives of the lens potential when studying catastrophes in
strong lenses.Comment: 24 pages, 10 figures, matches published versio
A Multi-band Study of the Remarkable Jet in Quasar 4C+19.44
We present arcsecond-resolution data in the radio, IR, optical, and X-ray for 4C+19.44 (=PKS 1354+195), the longest and straightest quasar jet with deep X-ray observations. We report results from radio images with half to one arcsecond angular resolution at three frequencies, plus Hubble Space Telescope and Spitzer data. The Chandra data allow us to measure the X-ray spectral index in 10 distinct regions along the 18'' jet and compare with the radio index. The radio and X-ray spectral indices of the jet regions are consistent with a value of throughout the jet, to within uncertainties. The X-ray jet structure to the south extends beyond the prominent radio jet and connects to the southern radio lobe, and there is extended X-ray emission, both in the direction of the unseen counter-jet, and also coincident with the northern radio lobe. This jet is remarkable because its straight appearance over a large distance allows the geometry factors to be taken as fixed along the jet. Using the model of inverse Compton scattering of the cosmic microwave background (iC/CMB) by relativistic electrons, we find that the magnetic field strengths and Doppler factors are relatively constant along the jet. If instead the X-rays are synchrotron emission, they must arise from a population of electrons distinct from the particles producing the radio synchrotron spectrum
Active Galactic Nuclei at the Crossroads of Astrophysics
Over the last five decades, AGN studies have produced a number of spectacular
examples of synergies and multifaceted approaches in astrophysics. The field of
AGN research now spans the entire spectral range and covers more than twelve
orders of magnitude in the spatial and temporal domains. The next generation of
astrophysical facilities will open up new possibilities for AGN studies,
especially in the areas of high-resolution and high-fidelity imaging and
spectroscopy of nuclear regions in the X-ray, optical, and radio bands. These
studies will address in detail a number of critical issues in AGN research such
as processes in the immediate vicinity of supermassive black holes, physical
conditions of broad-line and narrow-line regions, formation and evolution of
accretion disks and relativistic outflows, and the connection between nuclear
activity and galaxy evolution.Comment: 16 pages, 5 figures; review contribution; "Exploring the Cosmic
Frontier: Astrophysical Instruments for the 21st Century", ESO Astrophysical
Symposia Serie
Radio emission and jets from microquasars
To some extent, all Galactic binary systems hosting a compact object are
potential `microquasars', so much as all galactic nuclei may have been quasars,
once upon a time. The necessary ingredients for a compact object of stellar
mass to qualify as a microquasar seem to be: accretion, rotation and magnetic
field. The presence of a black hole may help, but is not strictly required,
since neutron star X-ray binaries and dwarf novae can be powerful jet sources
as well. The above issues are broadly discussed throughout this Chapter, with a
a rather trivial question in mind: why do we care? In other words: are jets a
negligible phenomenon in terms of accretion power, or do they contribute
significantly to dissipating gravitational potential energy? How do they
influence their surroundings? The latter point is especially relevant in a
broader context, as there is mounting evidence that outflows powered by
super-massive black holes in external galaxies may play a crucial role in
regulating the evolution of cosmic structures. Microquasars can also be thought
of as a form of quasars for the impatient: what makes them appealing, despite
their low number statistics with respect to quasars, are the fast variability
time-scales. In the first approximation, the physics of the jet-accretion
coupling in the innermost regions should be set by the mass/size of the
accretor: stellar mass objects vary on 10^5-10^8 times shorter time-scales,
making it possible to study variable accretion modes and related ejection
phenomena over average Ph.D. time-scales. [Abridged]Comment: 28 pages, 13 figures, To appear in Belloni, T. (ed.): The Jet
Paradigm - From Microquasars to Quasars, Lect. Notes Phys. 794 (2009
Towards Machine Wald
The past century has seen a steady increase in the need of estimating and
predicting complex systems and making (possibly critical) decisions with
limited information. Although computers have made possible the numerical
evaluation of sophisticated statistical models, these models are still designed
\emph{by humans} because there is currently no known recipe or algorithm for
dividing the design of a statistical model into a sequence of arithmetic
operations. Indeed enabling computers to \emph{think} as \emph{humans} have the
ability to do when faced with uncertainty is challenging in several major ways:
(1) Finding optimal statistical models remains to be formulated as a well posed
problem when information on the system of interest is incomplete and comes in
the form of a complex combination of sample data, partial knowledge of
constitutive relations and a limited description of the distribution of input
random variables. (2) The space of admissible scenarios along with the space of
relevant information, assumptions, and/or beliefs, tend to be infinite
dimensional, whereas calculus on a computer is necessarily discrete and finite.
With this purpose, this paper explores the foundations of a rigorous framework
for the scientific computation of optimal statistical estimators/models and
reviews their connections with Decision Theory, Machine Learning, Bayesian
Inference, Stochastic Optimization, Robust Optimization, Optimal Uncertainty
Quantification and Information Based Complexity.Comment: 37 page
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