3,968 research outputs found
Adversarial attacks hidden in plain sight
Convolutional neural networks have been used to achieve a string of successes
during recent years, but their lack of interpretability remains a serious
issue. Adversarial examples are designed to deliberately fool neural networks
into making any desired incorrect classification, potentially with very high
certainty. Several defensive approaches increase robustness against adversarial
attacks, demanding attacks of greater magnitude, which lead to visible
artifacts. By considering human visual perception, we compose a technique that
allows to hide such adversarial attacks in regions of high complexity, such
that they are imperceptible even to an astute observer. We carry out a user
study on classifying adversarially modified images to validate the perceptual
quality of our approach and find significant evidence for its concealment with
regards to human visual perception
The Radio Jet Associated with the Multiple V380 Ori System
The giant Herbig-Haro object 222 extends over 6 in the plane of the
sky, with a bow shock morphology. The identification of its exciting source has
remained uncertain over the years. A non-thermal radio source located at the
core of the shock structure was proposed to be the exciting source. However,
Very Large Array studies showed that the radio source has a clear morphology of
radio galaxy and a lack of flux variations or proper motions, favoring an
extragalactic origin. Recently, an optical-IR study proposed that this giant HH
object is driven by the multiple stellar system V380 Ori, located about 23
to the SE of HH 222. The exciting sources of HH systems are usually detected as
weak free-free emitters at centimeter wavelengths. Here we report the detection
of an elongated radio source associated with the Herbig Be star or with its
close infrared companion in the multiple V380 Ori system. This radio source has
the characteristics of a thermal radio jet and is aligned with the direction of
the giant outflow defined by HH~222 and its suggested counterpart to the SE,
HH~1041. We propose that this radio jet traces the origin of the large scale HH
outflow. Assuming that the jet arises from the Herbig Be star, the radio
luminosity is a few times smaller than the value expected from the
radio-bolometric correlation for radio jets, confirming that this is a more
evolved object than those used to establish the correlation.Comment: 13 pages, 3 figure
Momentum-driven outflow emission from an O-type YSO: Comparing the radio jet with the molecular outflow
Aims: We want to study the physical properties of the ionized jet emission in
the vicinity of an O-type young stellar object (YSO), and estimate how
efficient is the transfer of energy and momentum from small- to large-scale
outflows. Methods: We conducted Karl G. Jansky Very Large Array (VLA)
observations, at both 22 and 45 GHz, of the compact and faint radio continuum
emission in the high-mass star-forming region G023.01-00.41, with an angular
resolution between 0.3" and 0.1", and a thermal rms of the order of 10
uJy/beam. Results: We discovered a collimated thermal (bremsstrahlung) jet
emission, with a radio luminosity (L_rad) of 24 mJy kpc^2 at 45 GHz, in the
inner 1000 AU from an O-type YSO. The radio thermal jet has an opening angle of
44 degrees and brings a momentum rate of 8 10^-3 M_sun yr^-1 km/s. By combining
the new data with previous observations of the molecular outflow and water
maser shocks, we can trace the outflow emission from its driving source through
the molecular clump, across more than two order of magnitude in length (500
AU-0.2 pc). We find that the momentum-transfer efficiency, between the inner
jet emission and the extended outflow of entrained ambient gas, is near unity.
This result suggests that the large-scale flow is swept-up by the mechanical
force of the radio jet emission, which originates in the inner 1000 AU from the
high-mass YSO.Comment: 5 pages, 2 figures, 2 tables, accepted by Astronomy & Astrophysic
Magnetospheres of black hole-neutron star binaries
We perform force-free simulations for a neutron star orbiting a black hole, aiming at clarifying the main magnetosphere properties of such binaries towards their innermost stable circular orbits. Several configurations are explored, varying the orbital separation, the individual spins and misalignment angle among the magnetic and orbital axes. We find significant electromagnetic luminosities, (depending on the specific setting), primarily powered by the orbital kinetic energy, being about one order of magnitude higher than those expected from unipolar induction. The systems typically develop current sheets that extend to long distances following a spiral arm structure. The intense curvature of the black hole produces extreme bending on a particular set of magnetic field lines as it moves along the orbit, leading to magnetic reconnections in the vicinity of the horizon. For the most symmetric scenario (aligned cases), these reconnection events can release large-scale plasmoids that carry the majority of the Poynting fluxes. On the other hand, for misaligned cases, a larger fraction of the luminosity is instead carried outwards by large-amplitude Alfv{\'e}n waves disturbances. We estimate possible precursor electromagnetic emissions based on our numerical solutions, finding radio signals as the most promising candidates to be detectable within distances of \,Mpc by forthcoming facilities like the Square Kilometer Array
MPI Collective Operations over IP Multicast
Many common implementations of Message Passing Inter- face (MPI) implement collective operations over point-to-point operations. This work examines IP multicast as a framework for collective operations. IP multicast is not reliable. If a receiver is not ready when a message is sent via IP multicast, the message is lost. Two techniques for ensuring that a message is not lost due to a slow receiving process are examined. The techniques are implemented and compared experimentally over both a shared and a switched Fast Ethernet. The average performance of collective operations is improved as a function of the number of participating processes and message size for both networks
Skylight: a new code for general-relativistic ray-tracing and radiative transfer in arbitrary spacetimes
To reproduce the observed spectra and light curves originated in the neighborhood of compact objects requires accurate relativistic ray-tracing codes. In this work, we present Skylight, a new numerical code for general-relativistic ray tracing and radiative transfer in arbitrary space-time geometries and coordinate systems. The code is capable of producing images, spectra, and light curves from astrophysical models of compact objects as seen by distant observers. We incorporate two different schemes, namely Monte Carlo radiative transfer, integrating geodesics from the astrophysical region to distant observers, and camera techniques with backwards integration from the observer to the emission region. The code is validated by successfully passing several test cases, among them: thin accretion disks and neutron star hot spot emission
Magnetism of NaFePO4 and related polyanionic compounds
Magnetic properties of maricite (m) and triphlyte (t) polymorphs of NaFePO4 are investigated by combining ab initio density functional theory with a model Hamiltonian approach, where a realistic Hubbard-type model for magnetic Fe 3d states in NaFePO4 is constructed entirely from first-principles calculations. For these purposes, we perform a comparative study based on the pseudopotential and linear muffin-tin orbital methods while tackling the problem of parasitic non-sphericity of the exchange-correlation potential. Upon calculating the model parameters, magnetic properties are studied by applying the mean-field Hartree-Fock approximation and the theory of superexchange interactions to extract the corresponding interatomic exchange parameters. Despite some differences, the two methods provide a consistent description of the magnetic properties of NaFePO4. On the one hand, our calculations reproduce the correct magnetic ordering for t-NaFePO4 allowing for magnetoelectric effect, and the theoretical values of Néel and Curie-Weiss temperatures are in fair agreement with reported experimental data. Furthermore, we investigate the effect of chemical pressure on magnetic properties by substituting Na with Li and, in turn, we explain how a noncollinear magnetic alignment induced by an external magnetic field leads to magnetoelectric effect in NaFePO4 and other transition-metal phosphates. However, the origin of a magnetic superstructure with q = (1/2, 0, 1/2) observed experimentally in m-NaFePO4 remains puzzling. Instead, we predict that competing exchange interactions can lead to the formation of magnetic superstructures along the shortest orthorhombic c axis of m-NaFePO4, similar to multiferroic manganites. © 2018 the Owner Societies.Oier Arcelus and Javier Carrasco acknowledge the financial support of the Ministerio de Economía y Competitividad of Spain through the project ENE2016-81020-R. The SGI/IZO-SGIker UPV/EHU (Arina cluster), the i2BASQUE academic network, and the Barcelona Supercomputer Center are acknowledged for computational resources. Oier Arcelus acknowledges support by the Basque Government through a PhD grant (Reference No. PRE-2016-1-0044)
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