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

Many-body dynamics of a Bose system with attractive interactions on a ring

We investigate the many-body dynamics of an effectively attractive one-dimensional Bose system confined in a toroidal trap. The mean-field theory predicts that a bright-soliton state will be formed when increasing the interparticle interaction over a critical point. The study of quantum many-body dynamics in this paper reveals that there is a modulation instability in a finite Bose system correspondingly. We show that Shannon entropy becomes irregular near and above the critical point due to quantum correlations. We also study the dynamical behavior of the instability by exploring the momentum distribution and the fringe visibility, which can be verified experimentally by releasing the trapComment: 6 pages,5 figure

Effects of grassland management on soil organic carbon density in agro-pastoral zone of Northern China

The objective of this study was to estimate the soil organic carbon (SOC) in grasslands with different management measures including: (1) uncontrolled or free grazing grassland (FG); (2) grassland enclosured, excluding grazing and mowing (EG); (3) grassland enclosured and mowed early in October every year (MG) and (4) grassland enclosured under controlled grazing (CG) by examining soil bulk density and SOC content from 0 to 50 cm soil depth in agro-pastoral ecotone, Northern China. The results showed that, by implementing CG, EG and MG practices, the grasslands in agro-pastoral ecotone of Northern China achieved higher SOC storage on decade scales when compared to FG field. CG field had the highest SOC density in 0 to 50 cm soil layer, while the least SOC density was displayed by FG. However, SOC density was similar between MG and EG plots. CG increased SOC concentration by 56.08% and SOC density by 4.96 kg/m2 when compared to FG practice. In addition, it was likely to give positive financial returns in providing livestock products when compared to EG practice. CG therefore was the most feasible and benign short-term grassland management option which could deposit even higher carbon dioxide in agro-pastoral ecotone in Northern China.Key words: Agro-pastoral zone, soil organic carbon density, grassland management, Northern China

Observing interventions: a logic for thinking about experiments

This paper makes a first step towards a logic of learning from experiments. For this, we investigate formal frameworks for modeling the interaction of causal and (qualitative) epistemic reasoning. Crucial for our approach is the idea that the notion of an intervention can be used as a formal expression of a (real or hypothetical) experiment (Pearl, 2009, Causality. Models, Reasoning, and Inference, 2nd edn. Cambridge University Press, Cambridge; Woodward, 2003, Making Things Happen, vol. 114 of Oxford Studies in the Philosophy of Science. Oxford University Press). In a first step we extend a causal model (Briggs, 2012, Philosophical Studies, 160, 139–166; Galles and Pearl, 1998, An axiomatic characterisation of causal counterfactuals. Foundations of Science, 3, 151–182; Halpern, 2000, Axiomatizing causal reasoning. Journal of Artificial Intelligence Research, 12, 317–337; Pearl, 2009, Causality. Models, Reasoning, and Inference, 2nd edn. Cambridge University Press, Cambridge) with a simple Hintikka-style representation of the epistemic state of an agent. In the resulting setting, one can talk about the knowledge of an agent and information update. The resulting logic can model reasoning about thought experiments. However, it is unable to account for learning from experiments, which is clearly brought out by the fact that it validates the principle of no learning for interventions. Therefore, in a second step, we implement a more complex notion of knowledge (Nozick, 1981, Philosophical Explanations. Harvard University Press, Cambridge, Massachusetts) that allows an agent to observe (measure) certain variables when an experiment is carried out. This extended system does allow for learning from experiments. For all the proposed logics, we provide a sound and complete axiomatization

Effects Of Attenuation And Thrombus Age On The Success Of Ultrasound And Microbubble-Mediated Thrombus Dissolution

The purpose of this study was to examine the effects of applied mechanical index, incident angle, attenuation and thrombus age on the ability of 2-D vs. 3-D diagnostic ultrasound and microbubbles to dissolve thrombi. A total of 180 occlusive porcine arterial thrombi of varying age (3 or 6 h) were examined in a flow system. A tissue-mimicking phantom of varying thickness (5 to 10 cm) was placed over the thrombosed vessel and the 2-D or 3-D diagnostic transducer aligned with the thrombosed vessel using a positioning system. Diluted lipid-encapsulated microbubbles were infused during ultrasound application. Percent thrombus dissolution (%TD) was calculated by comparison of clot mass before and after treatment. Both 2-D and 3-D-guided ultrasound increased %TD compared with microbubbles alone, but %TD achieved with 6-h-old thrombi was significantly less than 3-h-old thrombi. Thrombus dissolution was achieved at 10 cm tissue-mimicking depths, even without inertial cavitation. In conclusion, diagnostic 2-D or 3-D ultrasound can dissolve thrombi with intravenous nontargeted microbubbles, even at tissue attenuation distances of up to 10 cm. This treatment modality is less effective, however, for older aged thrombi. (E-mail: [email protected]) (C) 2011 World Federation for Ultrasound in Medicine & Biology

Anisotropies in insulating La2−x_{2-x}Srx_xCuO4_4: angular resolved photoemission and optical absorption

Due to the orthorhombic distortion of the lattice, the electronic hopping integrals along the $a$ and $b$ diagonals, the orthorhombic directions, are slightly different. We calculate their difference in the LDA and find $t_{a}^{\prime}-t_{b}^{\prime}\approx 8$meV. We argue that electron correlations in the insulating phase of La$_{2-x}$Sr$_{x}$CuO$_{4}$, i. e. at doping $x\leq 0.055,$ dramatically enhance the $(t_{a}^{\prime}-t_{b}^{\prime})$-splitting between the $a$- and $b$-hole valleys. In particular, we predict that the intensity of both angle-resolved photoemission and of optical absorption is very different for the $a$ and $b$ nodal points

Theory for high spin systems with orbital degeneracy

High-spin systems with orbital degeneracy are studied in the large spin limit. In the absence of Hund's coupling, the classical spin model is mapped onto disconnected orbital systems with spins up and down, respectively. The ground state of the isotropic model is an orbital valence bond state where each bond is an orbital singlet with parallel spins, and neighbouring bonds interact antiferromagnetically. The possible relevance to the transition metal oxides are discussed.Comment: 4 page, three figures, to appear in Phys. Rev. Let