14,016 research outputs found
Cuts for circular proofs: semantics and cut-elimination
One of the authors introduced in [Santocanale, FoSSaCS, 2002] a calculus of circular proofs for studying the computability arising from the following categorical operations: finite products, finite coproducts, initial algebras, final coalgebras. The calculus presented [Santocanale, FoSSaCS, 2002] is cut-free; even if sound and complete for provability, it lacked an important property for the semantics of proofs, namely fullness w.r.t. the class of intended categorical models (called mu-bicomplete categories in [Santocanale, ITA, 2002]).
In this paper we fix this problem by adding the cut rule to the calculus and by modifying accordingly the syntactical constraint ensuring soundness of proofs. The enhanced proof system fully represents arrows of the canonical model (a free mu-bicomplete category). We also describe a cut-elimination procedure as a a model of computation arising from the above mentioned categorical operations. The procedure constructs a cut-free proof-tree with possibly infinite branches out of a finite circular proof with cuts
Flux rope, hyperbolic flux tube, and late EUV phases in a non-eruptive circular-ribbon flare
We present a detailed study of a confined circular flare dynamics associated
with 3 UV late phases in order to understand more precisely which topological
elements are present and how they constrain the dynamics of the flare. We
perform a non-linear force free field extrapolation of the confined flare
observed with the HMI and AIA instruments onboard SDO. From the 3D magnetic
field we compute the squashing factor and we analyse its distribution.
Conjointly, we analyse the AIA EUV light curves and images in order to identify
the post-flare loops, their temporal and thermal evolution. By combining both
analysis we are able to propose a detailed scenario that explains the dynamics
of the flare. Our topological analysis shows that in addition to a null-point
topology with the fan separatrix, the spine lines and its surrounding
Quasi-Separatix Layers halo (typical for a circular flare), a flux rope and its
hyperbolic flux tube (HFT) are enclosed below the null. By comparing the
magnetic field topology and the EUV post-flare loops we obtain an almost
perfect match 1) between the footpoints of the separatrices and the EUV
1600~\AA{} ribbons and 2) between the HFT's field line footpoints and bright
spots observed inside the circular ribbons. We showed, for the first time in a
confined flare, that magnetic reconnection occured initially at the HFT, below
the flux rope. Reconnection at the null point between the flux rope and the
overlying field is only initiated in a second phase. In addition, we showed
that the EUV late phase observed after the main flare episode are caused by the
cooling loops of different length which have all reconnected at the null point
during the impulsive phase.Comment: Astronomy & Astrophysics, in pres
The chromosphere above a -sunspot in the presence of fan-shaped jets
-sunspots are known to be favourable locations for fast and energetic
events like flares and CMEs. The photosphere of this type of sunspots has been
thoroughly investigated in the past three decades. The atmospheric conditions
in the chromosphere are not so well known, however. his study is focused on the
chromosphere of a -sunspot that harbours a series of fan-shaped jets in
its penumbra . The aim of this study is to establish the magnetic field
topology and the temperature distribution in the presence of jets in the
photosphere and the chromosphere. We use data from the Swedish 1-m Solar
Telescope (SST) and the Solar Dynamics Observatory. We invert the
spectropolarimetric FeI 6302~\AA\ and CaII ~8542~\AA\ data from the SST using
the the non-LTE inversion code NICOLE to estimate the magnetic field
configuration, temperature and velocity structure in the chromosphere. A
loop-like magnetic structure is observed to emerge in the penumbra of the
sunspot. The jets are launched from the loop-like structure. Magnetic
reconnection between this emerging field and the pre-existing vertical field is
suggested by hot plasma patches on the interface between the two fields. The
height at which the reconnection takes place is located between and . The magnetic field vector and the
atmospheric temperature maps show a stationary configuration during the whole
observation.Comment: 12 pages, 15 figures. Recommended for publication in A&
A spatial likelihood analysis for MAGIC telescope data
Context. The increase in sensitivity of Imaging Atmospheric Cherenkov
Telescopes (IACTs) has lead to numerous detections of extended -ray
sources at TeV energies, sometimes of sizes comparable to the instrument's
field of view (FoV). This creates a demand for advanced and flexible data
analysis methods, able to extract source information by utilising the photon
counts in the entire FoV.
Aims. We present a new software package, "SkyPrism", aimed at performing 2D
(3D if energy is considered) fits of IACT data, possibly containing multiple
and extended sources, based on sky images binned in energy. Though the
development of this package was focused on the analysis of data collected with
the MAGIC telescopes, it can further be adapted to other instruments, such as
the future Cherenkov Telescope Array (CTA).
Methods. We have developed a set of tools that, apart from sky images (count
maps), compute the instrument response functions (IRFs) of MAGIC (effective
exposure throughout the FoV, point spread function (PSF), energy resolution and
background shape), based on the input data, Monte-Carlo simulations and the
pointing track of the telescopes. With this information, the presented package
can perform a simultaneous maximum likelihood fit of source models of arbitrary
morphology to the sky images providing energy spectra, detection significances,
and upper limits.
Results. We demonstrate that the SkyPrism tool accurately reconstructs the
MAGIC PSF, on and off-axis performance as well as the underlying background. We
further show that for a point source analysis with MAGIC's default
observational settings, SkyPrism gives results compatible with those of the
standard tools while being more flexible and widely applicable.Comment: 13 pages, 10 figure
Machine-learning identification of galaxies in the WISExSuperCOSMOS all-sky catalogue
The two currently largest all-sky photometric datasets, WISE and SuperCOSMOS,
were cross-matched by Bilicki et al. (2016) (B16) to construct a novel
photometric redshift catalogue on 70% of the sky. Galaxies were therein
separated from stars and quasars through colour cuts, which may leave
imperfections because of mixing different source types which overlap in colour
space. The aim of the present work is to identify galaxies in the
WISExSuperCOSMOS catalogue through an alternative approach of machine learning.
This allows us to define more complex separations in the multi-colour space
than possible with simple colour cuts, and should provide more reliable source
classification. For the automatised classification we use the support vector
machines learning algorithm, employing SDSS spectroscopic sources cross-matched
with WISExSuperCOSMOS as the training and verification set. We perform a number
of tests to examine the behaviour of the classifier (completeness, purity and
accuracy) as a function of source apparent magnitude and Galactic latitude. We
then apply the classifier to the full-sky data and analyse the resulting
catalogue of candidate galaxies. We also compare thus produced dataset with the
one presented in B16. The tests indicate very high accuracy, completeness and
purity (>95%) of the classifier at the bright end, deteriorating for the
faintest sources, but still retaining acceptable levels of 85%. No significant
variation of classification quality with Galactic latitude is observed.
Application of the classifier to all-sky WISExSuperCOSMOS data gives 15 million
galaxies after masking problematic areas. The resulting sample is purer than
the one in B16, at a price of lower completeness over the sky. The automatic
classification gives a successful alternative approach to defining a reliable
galaxy sample as compared to colour cuts.Comment: 12 pages, 15 figures, accepted for publication in A&A. Obtained
catalogue will be included in the public release of the WISExSuperCOSMOS
galaxy catalogue available from http://ssa.roe.ac.uk/WISExSCO
Quasi-Parallel Segments and Characterization of Unique Bichromatic Matchings
Given n red and n blue points in general position in the plane, it is
well-known that there is a perfect matching formed by non-crossing line
segments. We characterize the bichromatic point sets which admit exactly one
non-crossing matching. We give several geometric descriptions of such sets, and
find an O(nlogn) algorithm that checks whether a given bichromatic set has this
property.Comment: 31 pages, 24 figure
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