37,107 research outputs found
The role of nonthermal electrons in the optical continuum of stellar flares
We explore the possibility that the continuum emission in stellar flares is
powered by nonthermal electrons accelerated during the flares. We compute the
continuum spectra from an atmospheric model for a dMe star, AD Leo, at its
quiescent state, when considering the nonthermal excitation and ionisation
effects by precipitating electron beams. The results show that if the electron
beam has an energy flux large enough, the U band brightening and, in
particular, the U-B colour are roughly comparable with observed values for a
typical large flare. Moreover, for electron beams with a moderate energy flux,
a decrease of the emission at the Paschen continuum appears. This can explain
at least partly the continuum dimming observed in some stellar flares. Adopting
an atmospheric model for the flaring state can further raise the continuum flux
but it yields a spectral colour incomparable with observations. This implies
that the nonthermal effects may play the chief role in powering the continuum
emission in some stellar flares.Comment: 6 pages, 4 figures, LaTeX (psfigs.sty), to appear in MNRA
H-Alpha and Hard X-Ray Observations of a Two-Ribbon Flare Associated with a Filament Eruption
We perform a multi-wavelength study of a two-ribbon flare on 2002 September
29 and its associated filament eruption, observed simultaneously in the H-alpha
line by a ground-based imaging spectrograph and in hard X-rays by RHESSI. The
flare ribbons contain several H-alpha bright kernels that show different
evolutional behaviors. In particular, we find two kernels that may be the
footpoints of a loop. A single hard X-ray source appears to cover these two
kernels and to move across the magnetic neutral line. We explain this as a
result of the merging of two footpoint sources that show gradually asymmetric
emission owing to an asymmetric magnetic topology of the newly reconnected
loops. In one of the H-alpha kernels, we detect a continuum enhancement at the
visible wavelength. By checking its spatial and temporal relationship with the
hard X-ray emission, we ascribe it as being caused by electron beam
precipitation. In addition, we derive the line-of-sight velocity of the
filament plasma based on the Doppler shift of the filament-caused absorption in
the H-alpha blue wing. The filament shows rapid acceleration during the
impulsive phase. These observational features are in principal consistent with
the general scenario of the canonical two-ribbon flare model.Comment: 15 pages, 5 figures, accepted for publication in Ap
Robust Face Recognition via Multimodal Deep Face Representation
© 2015 IEEE. Face images appearing in multimedia applications, e.g., social networks and digital entertainment, usually exhibit dramatic pose, illumination, and expression variations, resulting in considerable performance degradation for traditional face recognition algorithms. This paper proposes a comprehensive deep learning framework to jointly learn face representation using multimodal information. The proposed deep learning structure is composed of a set of elaborately designed convolutional neural networks (CNNs) and a three-layer stacked auto-encoder (SAE). The set of CNNs extracts complementary facial features from multimodal data. Then, the extracted features are concatenated to form a high-dimensional feature vector, whose dimension is compressed by SAE. All of the CNNs are trained using a subset of 9,000 subjects from the publicly available CASIA-WebFace database, which ensures the reproducibility of this work. Using the proposed single CNN architecture and limited training data, 98.43% verification rate is achieved on the LFW database. Benefitting from the complementary information contained in multimodal data, our small ensemble system achieves higher than 99.0% recognition rate on LFW using publicly available training set
A comprehensive survey on Pose-Invariant Face Recognition
© 2016 ACM. The capacity to recognize faces under varied poses is a fundamental human ability that presents a unique challenge for computer vision systems. Compared to frontal face recognition, which has been intensively studied and has gradually matured in the past few decades, Pose-Invariant Face Recognition (PIFR) remains a largely unsolved problem. However, PIFR is crucial to realizing the full potential of face recognition for real-world applications, since face recognition is intrinsically a passive biometric technology for recognizing uncooperative subjects. In this article, we discuss the inherent difficulties in PIFR and present a comprehensive review of established techniques. Existing PIFR methods can be grouped into four categories, that is, pose-robust feature extraction approaches, multiview subspace learning approaches, face synthesis approaches, and hybrid approaches. The motivations, strategies, pros/cons, and performance of representative approaches are described and compared. Moreover, promising directions for future research are discussed
Metastability of Asymptotically Well-Behaved Potential Games
One of the main criticisms to game theory concerns the assumption of full
rationality. Logit dynamics is a decentralized algorithm in which a level of
irrationality (a.k.a. "noise") is introduced in players' behavior. In this
context, the solution concept of interest becomes the logit equilibrium, as
opposed to Nash equilibria. Logit equilibria are distributions over strategy
profiles that possess several nice properties, including existence and
uniqueness. However, there are games in which their computation may take time
exponential in the number of players. We therefore look at an approximate
version of logit equilibria, called metastable distributions, introduced by
Auletta et al. [SODA 2012]. These are distributions that remain stable (i.e.,
players do not go too far from it) for a super-polynomial number of steps
(rather than forever, as for logit equilibria). The hope is that these
distributions exist and can be reached quickly by logit dynamics.
We identify a class of potential games, called asymptotically well-behaved,
for which the behavior of the logit dynamics is not chaotic as the number of
players increases so to guarantee meaningful asymptotic results. We prove that
any such game admits distributions which are metastable no matter the level of
noise present in the system, and the starting profile of the dynamics. These
distributions can be quickly reached if the rationality level is not too big
when compared to the inverse of the maximum difference in potential. Our proofs
build on results which may be of independent interest, including some spectral
characterizations of the transition matrix defined by logit dynamics for
generic games and the relationship of several convergence measures for Markov
chains
Physical implementation of holonomic quantum computation in decoherence-free subspaces with trapped ions
We propose a feasible scheme to achieve holonomic quantum computation in a
decoherence-free subspace (DFS) with trapped ions. By the application of
appropriate bichromatic laser fields on the designated ions, we are able to
construct two noncommutable single-qubit gates and one controlled-phase gate
using the holonomic scenario in the encoded DFS.Comment: 4 pages, 3 figures. To appear in Phys. Rev. A 74 (2006
NMR Determination of an Incommensurate Helical Antiferromagnetic Structure in EuCo2As2
We report Eu, As and Co nuclear magnetic resonance
(NMR) results on EuCoAs single crystal. Observations of Eu and
As NMR spectra in zero magnetic field at 4.3 K below an
antiferromagnetic (AFM) ordering temperature = 45 K and its
external magnetic field dependence clearly evidence an incommensurate helical
AFM structure in EuCoAs. Furthermore, based on Co NMR data in
both the paramagnetic and the incommensurate AFM states, we have determined the
model-independent value of the AFM propagation vector = (0, 0, 0.73
0.07)2/ where is the lattice parameter. Thus the
incommensurate helical AFM state was characterized by only NMR data with
model-independent analyses, showing NMR to be a unique tool for determination
of the spin structure in incommensurate helical AFMs.Comment: 6 pages, 4 figures, accepted for publication in Phys.Rev.
The Impossibility Of Secure Two-Party Classical Computation
We present attacks that show that unconditionally secure two-party classical
computation is impossible for many classes of function. Our analysis applies to
both quantum and relativistic protocols. We illustrate our results by showing
the impossibility of oblivious transfer.Comment: 10 page
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