22,077 research outputs found
Solitons in Affine and Permutation Orbifolds
We consider properties of solitons in general orbifolds in the algebraic
quantum field theory framework and constructions of solitons in affine and
permutation orbifolds. Under general conditions we show that our construction
gives all the twisted representations of the fixed point subnet. This allows us
to prove a number of conjectures: in the affine orbifold case we clarify the
issue of ``fixed point resolutions''; in the permutation orbifold case we
determine all irreducible representations of the orbifold, and we also
determine the fusion rules in a nontrivial case, which imply an integral
property of chiral data for any completely rational conformal net.Comment: Latex, 48 pages, minor style correction
Neural Nets and Star/Galaxy Separation in Wide Field Astronomical Images
One of the most relevant problems in the extraction of scientifically useful
information from wide field astronomical images (both photographic plates and
CCD frames) is the recognition of the objects against a noisy background and
their classification in unresolved (star-like) and resolved (galaxies) sources.
In this paper we present a neural network based method capable to perform both
tasks and discuss in detail the performance of object detection in a
representative celestial field. The performance of our method is compared to
that of other methodologies often used within the astronomical community.Comment: 6 pages, to appear in the proceedings of IJCNN 99, IEEE Press, 199
A fireworks model for Gamma-Ray Bursts
The energetics of the long duration GRB phenomenon is compared with models of
a rotating Black Hole (BH) in a strong magnetic field generated by an accreting
torus. A rough estimate of the energy extracted from a rotating BH with the
Blandford-Znajek mechanism is obtained with a very simple assumption: an
inelastic collision between the rotating BH and the torus. The GRB energy
emission is attributed to an high magnetic field that breaks down the vacuum
around the BH and gives origin to a e+- fireball. Its subsequent evolution is
hypothesized, in analogy with the in-flight decay of an elementary particle, to
evolve in two distinct phases. The first one occurs close to the engine and is
responsible of energizing and collimating the shells. The second one consists
of a radiation dominated expansion, which correspondingly accelerates the
relativistic photon--particle fluid and ends at the transparency time. This
mechanism simply predicts that the observed Lorentz factor is determined by the
product of the Lorentz factor of the shell close to the engine and the Lorentz
factor derived by the expansion. An anisotropy in the fireball propagation is
thus naturally produced, whose degree depends on the bulk Lorentz factor at the
end of the collimation phase.Comment: Accepted for publication in MNRA
A kinematically decoupled component in NGC4778
We present a kinematical and photometrical study of a member, NGC4778, of the
nearest (z=0.0137) compact group: Hickson 62. Our analysis reveals that Hickson
62a, also known as NGC4778, is an S0 galaxy with kinematical and morphological
peculiarities, both in its central regions (r < 5'') and in the outer halo. In
the central regions, the rotation curve shows the existence of a kinematically
decoupled stellar component, offset with respect to the photometric center. In
the outer halo we find an asymmetric rotation curve and a velocity dispersion
profile showing a rise on the SW side, in direction of the galaxy NGC4776.Comment: Proceedings of the first workshop of astronomy and astrophysics for
student
Data Driven Discovery in Astrophysics
We review some aspects of the current state of data-intensive astronomy, its
methods, and some outstanding data analysis challenges. Astronomy is at the
forefront of "big data" science, with exponentially growing data volumes and
data rates, and an ever-increasing complexity, now entering the Petascale
regime. Telescopes and observatories from both ground and space, covering a
full range of wavelengths, feed the data via processing pipelines into
dedicated archives, where they can be accessed for scientific analysis. Most of
the large archives are connected through the Virtual Observatory framework,
that provides interoperability standards and services, and effectively
constitutes a global data grid of astronomy. Making discoveries in this
overabundance of data requires applications of novel, machine learning tools.
We describe some of the recent examples of such applications.Comment: Keynote talk in the proceedings of ESA-ESRIN Conference: Big Data
from Space 2014, Frascati, Italy, November 12-14, 2014, 8 pages, 2 figure
Star Formation Rates for photometric samples of galaxies using machine learning methods
Star Formation Rates or SFRs are crucial to constrain theories of galaxy
formation and evolution. SFRs are usually estimated via spectroscopic
observations requiring large amounts of telescope time. We explore an
alternative approach based on the photometric estimation of global SFRs for
large samples of galaxies, by using methods such as automatic parameter space
optimisation, and supervised Machine Learning models. We demonstrate that, with
such approach, accurate multi-band photometry allows to estimate reliable SFRs.
We also investigate how the use of photometric rather than spectroscopic
redshifts, affects the accuracy of derived global SFRs. Finally, we provide a
publicly available catalogue of SFRs for more than 27 million galaxies
extracted from the Sloan Digital Sky survey Data Release 7. The catalogue is
available through the Vizier facility at the following link
ftp://cdsarc.u-strasbg.fr/pub/cats/J/MNRAS/486/1377
The footprint of large scale cosmic structure on the ultra-high energy cosmic ray distribution
Current experiments collecting high statistics in ultra-high energy cosmic
rays (UHECRs) are opening a new window on the universe. In this work we discuss
a large scale structure model for the UHECR origin which evaluates the expected
anisotropy in the UHECR arrival distribution starting from a given astronomical
catalogue of the local universe. The model takes into account the main
selection effects in the catalogue and the UHECR propagation effects. By
applying this method to the IRAS PSCz catalogue, we derive the minimum
statistics needed to significatively reject the hypothesis that UHECRs trace
the baryonic distribution in the universe, in particular providing a forecast
for the Auger experiment.Comment: 21 pages, 14 figures. Reference added, minor changes, matches
published versio
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