2,507 research outputs found
Femtolens Imaging of a Quasar Central Engine Using a Dwarf Star Telescope
We show that it is possible to image the structure of a distant quasar on
scales of AU by constructing a telescope which uses a nearby dwarf
star as its ``primary lens'' together with a satellite-borne ``secondary''. The
image produced by the primary is magnified by in one direction but
is contracted by 0.5 in the other, and therefore contains highly degenerate
one-dimensional information about the two-dimensional source. We discuss
various methods for extracting information about the second dimension including
``femtolens interferometry'' where one measures the interference between
different parts of the one-dimensional image with each other. Assuming that the
satellite could be dispatched to a position along a star-quasar line of sight
at a distance from the Sun, the nearest available dwarf-star primary is
likely to be at \sim 15\,\pc\,(r/40\,\rm AU)^{-2}. The secondary should
consist of a one-dimensional array of mirrors extending m to
achieve 1 AU resolution, or m to achieve 4 AU resolution.Comment: 12 pages including 3 embedded figure
Quantifying Information Flow with Beliefs
To reason about information flow, a new model is developed that
describes how attacker beliefs change due to the attacker's observation of the execution of a probabilistic (or deterministic) program. The model enables compositional reasoning about information flow from attacks involving sequences of interactions. The model also supports a new metric for quantitative information flow that measures accuracy of an attacker's beliefs. Applying this new metric reveals inadequacies of traditional information flow metrics, which are based on reduction of uncertainty. However, the new metric is sufficiently general that it can be instantiated to measure either accuracy or uncertainty. The new metric can also be used to reason about misinformation; deterministic programs are shown to be incapable of producing misinformation. Additionally, programs in which nondeterministic choices are made by insiders, who collude with attackers, can be analyzed
Power-law decay of correlations after a global quench in the massive XXZ chain
We investigate the relaxation dynamics of equal-time correlations in the
antiferromagnetic phase of the XXZ spin-1/2 chain following a global quantum
quench of the anisotropy parameter. We focus, in particular, on the relaxation
dynamics starting from an initial N\'eel state. Using state-of-the-art
density-matrix renormalization group simulations, the exact solution of an
effective free-fermion model, and the quench-action approach within the
thermodynamic Bethe ansatz, we show that the late-time relaxation is
characterized by a power-law decay independent of anisotropy.
This is in contrast to the previously studied exponential decay of the
antiferromagnetic order parameter. Remarkably, the effective model describes
the numerical data extremely well even on a quantitative level if higher order
corrections to the leading asymptotic behavior are taken into account
JRIF: Reactive Information Flow Control for Java
A reactive information flow (RIF) automaton for a value v specifies (i) allowed uses for v and (ii) the RIF automaton for any value that might be directly or indirectly derived from v. RIF automata thus specify how transforming a value alters how the result might be used. Such labels are more expressive than existing approaches for controlling downgrading. We devised a type system around RIF automata and incorporated it into Jif, a dialect of Java that supports a classic form of labels for information flow. By implementing a compiler for the resulting JRIF language, we demonstrate how easy it is to replace a classic information-flow type system by a more expressive RIF-based type system. We programmed two example applications in JRIF, and we discuss insights they provide into the benefits of RIF-based security labels.Supported in part by AFOSR grants F9550-06-0019 and FA9550-11-1-0137, National Science Foundation grants 0430161, 0964409, and CCF-0424422 (TRUST), ONR grants N00014-01- 1-0968 and N00014-09-1-0652, and grants from Microsoft
Meralgia paresthetica after âall-in-oneâ appendectomy
AbstractMinimally invasive approaches have become standard for pediatric appendectomy. The laparoscopic assisted single port approach, also known as the âall-in-oneâ appendectomy, has gained recent popularity [1]. We describe a child who suffered meralgia paresthetica (a neuropathy in the distribution of the lateral femoral cutaneous nerve) after a laparoscopic assisted single port appendectomy, perhaps secondary to mobilization of the cecum
Criterion-related Validity of Forced-Choice Personality Measures: A Cautionary Note Regarding Thurstonian IRT versus Classical Test Theory Scoring
This study examined criterion-related validity for job-related composites of forced-choice personality scores against job performance using both Thurstonian Item Response Theory (TIRT) and Classical Test Theory (CTT) scoring methods. Correlations were computed across 11 different samples that differed in job or role within a job. A meta-analysis of the correlations (k = 11 and N = 613) found a higher average corrected correlation for CTT (mean Ï = .38) than for TIRT (mean Ï = .00). Implications and directions for future research are discussed
Spectral Decomposition of Broad-Line AGNs and Host Galaxies
Using an eigenspectrum decomposition technique, we separate the host galaxy
from the broad line active galactic nucleus (AGN) in a set of 4666 spectra from
the Sloan Digital Sky Survey (SDSS), from redshifts near zero up to about 0.75.
The decomposition technique uses separate sets of galaxy and quasar
eigenspectra to efficiently and reliably separate the AGN and host
spectroscopic components. The technique accurately reproduces the host galaxy
spectrum, its contributing fraction, and its classification. We show how the
accuracy of the decomposition depends upon S/N, host galaxy fraction, and the
galaxy class. Based on the eigencoefficients, the sample of SDSS broad-line AGN
host galaxies spans a wide range of spectral types, but the distribution
differs significantly from inactive galaxies. In particular, post-starburst
activity appears to be much more common among AGN host galaxies. The
luminosities of the hosts are much higher than expected for normal early-type
galaxies, and their colors become increasingly bluer than early-type galaxies
with increasing host luminosity. Most of the AGNs with detected hosts are
emitting at between 1% and 10% of their estimated Eddington luminosities, but
the sensitivity of the technique usually does not extend to the Eddington
limit. There are mild correlations among the AGN and host galaxy
eigencoefficients, possibly indicating a link between recent star formation and
the onset of AGN activity. The catalog of spectral reconstruction parameters is
available as an electronic table.Comment: 18 pages; accepted for publication in A
A new short-faced archosauriform from the Upper Triassic Placerias/Downsâ quarry complex, Arizona, USA, expands the morphological diversity of the Triassic archosauriform radiation
The Placerias/Downsâ Quarry complex in eastern Arizona, USA, is the most diverse Upper Triassic vertebrate locality known. We report a new short-faced archosauriform, Syntomiprosopus sucherorum gen. et sp. nov., represented by four incomplete mandibles, that expands that diversity with a morphology unique among Late Triassic archosauriforms. The most distinctive feature of Syntomiprosopus gen. nov. is its anteroposteriorly short, robust mandible with 3â4 anterior, a larger caniniform, and 1â3 âpostcanineâ alveoli. The size and shape of the alveoli and the preserved tips of replacement teeth preclude assignment to any taxon known only from teeth. Additional autapomorphies of S. sucherorum gen. et sp. nov. include a large fossa associated with the mandibular fenestra, an interdigitating suture of the surangular with the dentary, fine texture ornamenting the medial surface of the splenial, and a surangular ridge that completes a 90° arc. The external surfaces of the mandibles bear shallow, densely packed, irregular, fine pits and narrow, arcuate grooves. This combination of character states allows an archosauriform assignment; however, an associated and similarly sized braincase indicates that Syntomiprosopus n. gen. may represent previously unsampled disparity in early-diverging crocodylomorphs. The Placerias Quarry is Adamanian (Norian, maximum depositional age ~219 Ma), and this specimen appears to be an early example of shortening of the skull, which occurs later in diverse archosaur lineages, including the Late Cretaceous crocodyliform Simosuchus. This is another case where Triassic archosauriforms occupied morphospace converged upon by other archosaurs later in the Mesozoic and further demonstrates that even well-sampled localities can yield new taxa
Spectrophotometric Resolution of Stellar Atmospheres with Microlensing
Microlensing is a powerful tool for studying stellar atmospheres because as
the source crosses regions of formally infinite magnification (caustics) the
surfaceof the star is resolved, thereby allowing one to measure the radial
intensity profile, both photometrically and spectroscopically. However, caustic
crossing events are relatively rare, and monitoring them requires intensive
application of telescope resources. It is therefore essential that the
observational parameters needed to accurately measure the intensity profile are
quantified. We calculate the expected errors in the recovered radial intensity
profile as a function of the unlensed flux, source radius, spatial resolution
the recovered intensity profile, and caustic crossing time for the two
principle types of caustics: point-mass and binary lenses. We demonstrate that
for both cases there exist simple scaling relations between these parameters
and the resultant errors. We find that the error as a function of the spatial
resolution of the recovered profile, parameterized by the number of radial
bins, increases as , considerably faster than the naive
expectation. Finally, we discuss the relative advantages of binary
caustic-crossing events and point-lens events. Binary events are more common,
easier to plan for, and provide more homogeneous information about the stellar
atmosphere. However, a sub-class of point-mass events with low impact
parameters can provide dramatically more information provided that they can be
recognized in time to initiate observations.Comment: 20 pages, 5 figures, submitted to the Astrophysical Journa
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