The long-term optical behavior of MRK421

All data available in B band for the BL Lac object MRK421 from 22 publications are used to construct a historical light curve, dating back to 1900. It is found that the light curve is very complicated and consists of a set of outbursts with very large duration. The brightness of MRK421 varies from 11.6 magnitude to more than 16 magnitude. Analyses with Jurkevich method of computing period of cyclic phenomena reveal in the light curve two kinds of behaviors. The first one is non-periodic with rapid, violent variations in intensity on time scales of hours to days. The second one is periodic with a possible period of $23.1\pm 1.1$ years. Another possible period of $15.3\pm 0.7$ years is not very significant. We have tested the robustness of the Jurkevich method. The period of about one year found in the light curves of MRK421 and of other objects is a spurious period due to the method and the observing window. We try to explain the period of $23.1 \pm1.1$ years under the thermal instability of a slim accretion disk around a massive black hole of mass of $2 *10^6 M_\odot$.Comment: Tex, 14 pages, 5 Postscript figures. Accepted for publication in A&A Supplement Serie

Extraordinary variability and sharp transitions in a maximally frustrated dynamic network

Using Monte Carlo and analytic techniques, we study a minimal dynamic network involving two populations of nodes, characterized by different preferred degrees. Reminiscent of introverts and extroverts in a population, one set of nodes, labeled \textit{introverts} ($I$), prefers fewer contacts (a lower degree) than the other, labeled \textit{extroverts} ($E$). As a starting point, we consider an \textit{extreme} case, in which an $I$ simply cuts one of its links at random when chosen for updating, while an $E$ adds a link to a random unconnected individual (node). The model has only two control parameters, namely, the number of nodes in each group, $N_{I}$ and $N_{E}$). In the steady state, only the number of crosslinks between the two groups fluctuates, with remarkable properties: Its average ($X$) remains very close to 0 for all $N_{I}>N_{E}$ or near its maximum ($\mathcal{N}\equiv N_{I}N_{E}$) if $N_{I}<N_{E}$. At the transition ($N_{I}=N_{E}$), the fraction $X/\mathcal{N}$ wanders across a substantial part of $[0,1]$, much like a pure random walk. Mapping this system to an Ising model with spin-flip dynamics and unusual long-range interactions, we note that such fluctuations are far greater than those displayed in either first or second order transitions of the latter. Thus, we refer to the case here as an `extraordinary transition.' Thanks to the restoration of detailed balance and the existence of a `Hamiltonian,' several qualitative aspects of these remarkable phenomena can be understood analytically.Comment: 6 pages, 3 figures, accepted for publication in EP

Identification of photons in double beta-decay experiments using segmented germanium detectors - studies with a GERDA Phase II prototype detector

The sensitivity of experiments searching for neutrinoless double beta-decay of germanium was so far limited by the background induced by external gamma-radiation. Segmented germanium detectors can be used to identify photons and thus reduce this background component. The GERmanium Detector Array, GERDA, will use highly segmented germanium detectors in its second phase. The identification of photonic events is investigated using a prototype detector. The results are compared with Monte Carlo data.Comment: 20 pages, 7 figures, to be submitted to NIM-

Extreme Thouless effect in a minimal model of dynamic social networks

In common descriptions of phase transitions, first order transitions are characterized by discontinuous jumps in the order parameter and normal fluctuations, while second order transitions are associated with no jumps and anomalous fluctuations. Outside this paradigm are systems exhibiting `mixed order transitions' displaying a mixture of these characteristics. When the jump is maximal and the fluctuations range over the entire range of allowed values, the behavior has been coined an `extreme Thouless effect'. Here, we report findings of such a phenomenon, in the context of dynamic, social networks. Defined by minimal rules of evolution, it describes a population of extreme introverts and extroverts, who prefer to have contacts with, respectively, no one or everyone. From the dynamics, we derive an exact distribution of microstates in the stationary state. With only two control parameters, $N_{I,E}$ (the number of each subgroup), we study collective variables of interest, e.g., $X$, the total number of $I$-$E$ links and the degree distributions. Using simulations and mean-field theory, we provide evidence that this system displays an extreme Thouless effect. Specifically, the fraction $X/\left( N_{I}N_{E}\right)$ jumps from $0$ to $1$ (in the thermodynamic limit) when $N_{I}$ crosses $N_{E}$, while all values appear with equal probability at $N_{I}=N_{E}$.Comment: arXiv admin note: substantial text overlap with arXiv:1408.542

First principles theoretical studies of half-metallic ferromagnetism in CrTe

Using full-potential linear augmented plane wave method (FP-LAPW) and the density functional theory, we have carried out a systematic investigation of the electronic, magnetic, and cohesive properties of the chalcogenide CrTe in three competing structures: rock-salt (RS), zinc blende (ZB) and the NiAs-type (NA) hexagonal. Although the ground state is of NA structure, RS and ZB are interesting in that these fcc-based structures, which can possibly be grown on many semiconductor substrates, exhibit half-metallic phases above some critical values of the lattice parameter. We find that the NA structure is not half-metallic at its equilibrium volume, while both ZB and RS structures are. The RS structure is more stable than the ZB, with an energy that is lower by 0.25 eV/atom. While confirming previous results on the half-metallic phase in ZB structure, we provide hitherto unreported results on the half-metallic RS phase, with a gap in the minority channel and a magnetic moment of 4.0 $\mu_{B}$ per formula unit. A comparison of total energies for the ferromagnetic (FM), non-magnetic (NM), and antiferromagnetic (AFM) configurations shows the lowest energy configuration to be FM for CrTe in all the three structures. The FP-LAPW calculations are supplemented by linear muffin-tin orbital (LMTO) calculations using both local density approximation (LDA) and LDA+U method. The exchange interactions and the Curie temperatures calculated via the linear response method in ZB and RS CrTe are compared over a wide range of the lattice parameter. The calculated Curie temperatures for the RS phase are consistently higher than those for the ZB phase.Comment: 11 pages, 14 figure

Semantic levels of domain-independent commonsense knowledgebase for visual indexing and retrieval applications

Building intelligent tools for searching, indexing and retrieval applications is needed to congregate the rapidly increasing amount of visual data. This raised the need for building and maintaining ontologies and knowledgebases to support textual semantic representation of visual contents, which is an important block in these applications. This paper proposes a commonsense knowledgebase that forms the link between the visual world and its semantic textual representation. This domain-independent knowledge is provided at different levels of semantics by a fully automated engine that analyses, fuses and integrates previous commonsense knowledgebases. This knowledgebase satisfies the levels of semantic by adding two new levels: temporal event scenarios and psycholinguistic understanding. Statistical properties and an experiment evaluation, show coherency and effectiveness of the proposed knowledgebase in providing the knowledge needed for wide-domain visual applications