A decades-long fast-rise-exponential-decay flare in low-luminosity AGN NGC 7213

We analysed the four-decades-long X-ray light curve of the low-luminosity active galactic nucleus (LLAGN) NGC 7213 and discovered a fast-rise-exponential-decay (FRED) pattern, i.e. the X-ray luminosity increased by a factor of $\approx 4$ within 200d, and then decreased exponentially with an $e$-folding time $\approx 8116$d ($\approx 22.2$ yr). For the theoretical understanding of the observations, we examined three variability models proposed in the literature: the thermal-viscous disc instability model, the radiation pressure instability model, and the tidal disruption event (TDE) model. We find that a delayed tidal disruption of a main-sequence star is most favourable; either the thermal-viscous disk instability model or radiation pressure instability model fails to explain some key properties observed, thus we argue them unlikely.Comment: Accepted for publication in MNRAS, updated version after proof correction

Topological Band Theory for Non-Hermitian Hamiltonians

We develop the topological band theory for systems described by non-Hermitian Hamiltonians, whose energy spectra are generally complex. After generalizing the notion of gapped band structures to the non-Hermitian case, we classify "gapped" bands in one and two dimensions by explicitly finding their topological invariants. We find nontrivial generalizations of the Chern number in two dimensions, and a new classification in one dimension, whose topology is determined by the energy dispersion rather than the energy eigenstates. We then study the bulk-edge correspondence and the topological phase transition in two dimensions. Different from the Hermitian case, the transition generically involves an extended intermediate phase with complex-energy band degeneracies at isolated "exceptional points" in momentum space. We also systematically classify all types of band degeneracies.Comment: 6 pages, 3 figures + 6 pages of supplemental materia

Semantic Graph for Zero-Shot Learning

Zero-shot learning aims to classify visual objects without any training data via knowledge transfer between seen and unseen classes. This is typically achieved by exploring a semantic embedding space where the seen and unseen classes can be related. Previous works differ in what embedding space is used and how different classes and a test image can be related. In this paper, we utilize the annotation-free semantic word space for the former and focus on solving the latter issue of modeling relatedness. Specifically, in contrast to previous work which ignores the semantic relationships between seen classes and focus merely on those between seen and unseen classes, in this paper a novel approach based on a semantic graph is proposed to represent the relationships between all the seen and unseen class in a semantic word space. Based on this semantic graph, we design a special absorbing Markov chain process, in which each unseen class is viewed as an absorbing state. After incorporating one test image into the semantic graph, the absorbing probabilities from the test data to each unseen class can be effectively computed; and zero-shot classification can be achieved by finding the class label with the highest absorbing probability. The proposed model has a closed-form solution which is linear with respect to the number of test images. We demonstrate the effectiveness and computational efficiency of the proposed method over the state-of-the-arts on the AwA (animals with attributes) dataset.Comment: 9 pages, 5 figure

Superfluidity and effective mass of magnetoexcitons in topological insulator bilayers: Effect of inter-Landau-level Coulomb interaction

The effective mass and superfluidity-normal phase transition temperature of magnetoexcitons in topological insulator bilayers are theoretically investigated. The intra-Landau-level Coulomb interaction is treated perturbatively, from which the effective magnetoexciton mass is analytically discussed. The inclusion of inter-Landau-level Coulomb interaction by more exact numerical diagonalization of the Hamiltonian brings out important modifications to magnetoexciton properties, which are specially characterized by prominent reduction in the magnetoexciton effective mass and promotion in the superfluidity-normal phase transition temperature at a wide range of external parameters.Comment: 5.6 EPL pages, 4 figure

THE FOLIAR NEMATODE APHELENCHOIDES FRAGARIAE: STUDIES ON FACULTATIVE FEEDING AND DESICCATION TOLERANCE

The foliar nematode Aphelenchoides fragariae is an economically important and frequent pest of ornamental crops. In nurseries and landscapes in the United States, foliar nematodes can be a serious problem affecting hostas (Hosta spp.), a common perennial ornamental plant that thrives in shady environments. Fundamental research on the biology and ecology of this nematode provides the basis for development of sound management strategies. The general objective of this dissertation was to study two behaviors that make foliar nematodes very successful as plant pests: desiccation tolerance and alternative feeding on fungi (or facultative feeding on the plant). To study the molecular basis of these behaviors, I used a transcriptomic approach complemented with in vitro and greenhouse experiments. As part of the greenhouse studies, I developed a standard protocol for the assessment of resistance to Aphelenchoides fragariae on hosta cultivars. I evaluated the effects of inoculum type (nematodes maintained on fungus vs. maintained on plants), inoculation method (with injury vs. without injury), and harvesting method to select the best procedures, and estimated the correlations between nematode reproduction and symptom severity. The variability of the correlation between symptoms and nematode reproduction highlighted the importance of measuring both parameters when evaluating cultivar resistance. More importantly, I demonstrated that there is a loss of virulence for inoculum maintained and increased on fungus cultures. For the molecular studies, I used Illumina technology and de novo assembly to complete a transcriptome of the nematode (a non-model organism lacking a reference genome) to generate data corresponding to nematode mRNAs from mixed life stages under four different treatments (fungus diet, plant diet, diet changed from plant to fungus, and desiccation stress). This transcriptome is the first comprehensive sequence resource available for foliar nematodes. I present an annotated overview of the Aphelenchoides fragariae transcriptome and provide information on gene families, gene structure, potential gene functions, and gene pathways (molecular, cellular and biological). I identified and characterized a ß-1,4-endoglucanase, which I named Afr-ENG-1, that is differentially expressed when the nematode feeds on fungi or plants. When individuals from hosta plants were transferred to a fungus culture, expression of the enzyme decreased 1,812- fold after five generations on the fungus diet. Diet was also associated with changes in nematode body size and in the severity of symptoms caused on hosta leaves. Plant-diet nematodes caused larger lesions and were longer and thinner than fungus diet nematodes. Full length sequences of Afr-eng-1 were obtained and found to encode a glycosyl hydrolase family 5 (GHF5) protein. This is the first ß-1,4-endoglucanase and plant-parasitism related gene described in the genus Aphelenchoides. For the desiccation tolerance experiments, Aphelenchus avenae, a model nematode for anhydrobiosis studies, was used as a reference. Aphelenchoides fragariae showed higher survival rates than A. avenae under desiccation and osmotic stress conditions. Two glutaredoxin (glx) and three trehalose phosphate synthase (tps) genes were identified in the A. fragariae transcriptome of nematodes subjected to desiccation conditions. Phylogenetic analysis showed that the Af-TPS was closely related to TPS of A. avenae. Expression of Af-tps and Af-glx suggests participation of anti-oxidation and cellular membrane protection mechanisms in the desiccation tolerance mechanisms of foliar nematodes. These observations on the feeding behavior and desiccation tolerance of the foliar nematode improve our knowledge of the basic biology of this nematode and will enhance our ability to increase the arsenal of weapons to fight damage to plants from these parasitic nematodes

Probing crossover from analogous weak antilocalization to localization by an Aharonov-Bohm interferometer on topological insulator surface

We propose a scanning tunneling microscopy Aharonov-Bohm (AB) interferometer on the surface of a topological insulator (TI) to probe the crossover from analogous weak antilocalization (WAL) to weak localization (WL) phenomenon via the AB oscillations in spin-resolved local density of states (LDOS). Based on our analytical and numerical results, we show that with increasing the energy gap of TI surface states, the $\Phi_{0}/2$=$hc/2e$ periodic AB oscillations in spin-resolved LDOS gradually transit into the $\Phi_{0}$ periodic oscillations.Comment: 4.2 APL pages, 2 figure