Particle acceleration close to the supermassive black hole horizon: the case of M87

The radio galaxy M87 has recently been found to be a rapidly variable TeV emitting source. We analyze the implications of the observed TeV characteristics and show that it proves challenging to account for them within conventional acceleration and emission models. We discuss a new pulsar-type scenario for the origin of variable, very high energy (VHE) emission close to the central supermassive black hole and show that magneto-centrifugally accelerated electrons could efficiently Compton upscatter sub-mm ADAF disk photons to the TeV regime, leading to VHE characteristics close to the observed ones. This suggests, conversely, that VHE observations of highly under-luminous AGNs could provide an important diagnostic tool for probing the conditions prevalent in the inner accretion disk of these sources.Comment: 5 pages, one figure (typos corrected); based on presentation at "High Energy Phenomena in Relativistic Outflows", Dublin, Sept. 2007; accepted for publication in International Journal of Modern Physics

Counterstreaming beams in magnetised Vlasov plasma

In this paper, we investigate nonrelativistic, kinetic, linear phase of the filamentation instability when an external magnetic field is present in the direction of the counterstreaming electron beams using Vlasov simulations in 1D-3V space.We first investigate the growth rate of instability. In the linear growth regime, our results correspond to the previous conclusions that with the increase in strength of the ambient magnetic field, there is a suppression of instability. Interestingly, we established that at a critical/threshold magnetic field, Vlasov simulations and particlein-cell (PIC) simulations differ in their instability behaviour. At this particular magnetic field, there is a complete suppression of the growth of instability in Vlasov results compared to PIC simulations, where a strong growth of instability is shown. It is believed that thermal noise in the PIC leads to the growth. However, Vlasov simulations show wave–wave coupling which stabilises the modes. In this work, our focus is to demonstrate the difference in this behaviour and to thoroughly analyse the spectra and wave generation for the same

Beyond pairs: definition and interpretation of third-order structure in spatial point patterns

Spatial distributions of biological species are an important source of information for understanding local interactions at the scale of individuals. Technological advances have made it easier to measure these distributions as spatial point patterns, specifying the locations of individuals. Extensive attention has been devoted to analyzing the second-order structure of such point patterns by focusing on pairs of individuals, and it is well known that the local crowdedness of individuals can thus be quantified. Statistical measures such as a point pattern's pair correlation function or Ripley's K function show whether a given point pattern is clustered (excess of short-distance pairs) or overdispersed (shortage of short-distance pairs). These notions are naturally defined in comparison with control patterns exhibiting complete spatial randomness, i.e., an absence of any spatial structure. However, here is no rational reason why the analysis of point patterns should stop at the second order. In this paper, we focus on triplets of individuals in an attempt to quantify and interpret the third-order structure of a point pattern. We demonstrate that point patterns with 'bandedness', in which individuals are primarily distributed within bands, can be detected by an excess of thinner triplets at a characteristic spatial scale linked to the band's width. In this context, we show how the generation of control patterns as a reference for gauging a test pattern's triplet frequencies is critical for defining and interpreting the third-order structure of point patterns. Since perfect information on a point pattern's second-order structure typically suffices for its unique reconstruction (up to translation, rotation, and reflection), we conjecture that it is essential to minimally coarse-grain such second-order information before using it to generate control patterns for identifying a point pattern's third-order structure. We recommend the further exploration of this conjecture for future studies

The Adaptive Dynamics of Altruism in Spatially Heterogeneous Populations

We study the spatial adaptive dynamics of a continuous trait that measures individual investment in altruism. Our study is based on an ecological model of a spatially heterogeneous population from which we derive an appropriate measure of fitness. The analysis of this fitness measure uncovers three different selective processes controlling the evolution of altruism: the direct physiological cost, the indirect genetic benefits of cooperative interactions, and the indirect genetic costs of competition for space. In contrast with earliest suggestions, we find that the cost of competing for space with relatives exerts a negligible selective pressure against altruism. Our study yields a classification of adaptive patterns of altruism according to how the costs of altruism vary with an individuals investment in altruism (we distinguish between decelerating, linear, and accelerating dependence). The invasion of altruism occurs readily in species with accelerating costs, but large mutations are critical for altruism to evolve in selfish species with decelerating costs. Strict selfishness is maintained by natural selection only under very restricted conditions. In species with rapidly accelerating costs, adaptation leads to an evolutionarily stable rate of investment in altruism that decreases smoothly with the level of mobility. A rather different adaptive pattern emerges in species with slowly accelerating costs: high altruism evolves at low mobility, whereas a quasi-selfish state is promoted in more mobile species. The high adaptive level of altruism can be predicted solely from habitat connectedness and physiological parameters that characterize the pattern of cost. We also show that environmental changes that cause increased mobility in those highly altruistic species can beget selection-driven self-extinction, which may contribute to the rarity of social species

Adaptive Dynamics and Technological Change

This paper is about the emergence of technological variety arising from market interaction and technological innovation. Existing products in the market compete with innovative ones resulting in a slow and continuous evolution of the underlying technological characteristics of successful products. When technological evolution reaches an equilibrium, it can either be an ESS (Evolutionary Stable Strategy), where marginally innovative products do not penetrate the market, or a branching point, where new products coexist along with established ones. Thus, technological branching can give rise to product variety. In the paper we first introduce Adaptive Dynamics (AD), a recently proposed theory of evolutionary processes, aiming at modelling various features of technological change. Then, a first application of AD in economics is presented and discussed in detail. The limitations of the AD approach, as well as some promising further applications in economics and social sciences, are briefly discussed at the concluding section

Evolutionary impact assessment of the North Sea plaice fishery

There is growing evidence that fishing causes evolution in life-history traits that affect the productivity of fish stocks. Here we explore the impact of fisheries-induced evolu-tion (FIE) on the productivity of North Sea plaice using an eco-genetic individual-based model by comparing management scenarios with and without an evolutionary re-sponse. Under status-quo management, plaice evolve towards smaller size at age, ear-lier maturation, and higher reproductive investment. Current reference points of maximum sustainable yield (MSY) and corresponding fishing-mortality rate ( ) that ignore FIE will decrease and cannot be considered sustainable. The nature and extent of the change through FIE depend on fishing effort and selectivity. The adverse evolutionary effects can be reduced – and even reversed – by implementing a dome-shaped exploitation pattern protecting the large fish. The evolutionarily sustainable maximum yield can be obtained by combining such a dome-shaped exploitation pattern with a reduction in fishing mortality and an increase in mesh size; it is similar to the maximum sustainable yield that would apply if life-history traits were static. Fisheries managers will need to trade off the short-term loss in yield associated with evolutionarily informed management with the long-term loss in yield FIE causes under evolutionarily uninformed management

Dimer, trimer and FFLO liquids in mass- and spin-imbalanced trapped binary mixtures in one dimension

We present a systematic investigation of attractive binary mixtures in presence of both spin- and mass-imbalance in one dimensional setups described by the Hubbard model. After discussing typical cold atomic experimental realizations and the relation between microscopic and effective parameters, we study several many-body features of trapped Fermi-Fermi and Bose-Bose mixtures such as density profiles, momentum distributions and correlation functions by means of numerical density-matrix-renormalization-group and Quantum Monte Carlo simulations. In particular, we focus on the stability of Fulde-Ferrell-Larkin-Ovchinnikov, dimer and trimer fluids in inhomogeneous situations, as typically realized in cold gas experiments due to the harmonic confinement. We finally consider possible experimental signatures of these phases both in the presence of a finite polarization and of a finite temperature.Comment: 19 pages, 25 figure

Two models of the influenza A M2 channel domain: verification by comparison

Background: The influenza M2 protein is a simple membrane protein, containing a single transmembrane helix. It is representative of a very large family of single-transmembrane helix proteins. The functional protein is a tetramer, with the four transmembrane helices forming a proton-permeable channel across the bilayer. Two independently derived models of the M2 channel domain are compared, in order to assess the success of applying molecular modelling approaches to simple membrane proteins.Results: The Cα RSMD between the two models is 1.7 å. Both models are composed of a left-handed bundle of helices, with the helices tilted roughly 15° relative to the (presumed) bilayer normal. The two models have similar pore radius profiles, with a pore cavity lined by the Ser31 and Gly34 residues and a pore constriction formed by the ring of His37 residues.Conclusions:Independent studies of M2 have converged on the same structural model for the channel domain. This model is in agreement with solid state NMR data. In particular, both model and NMR data indicate that the M2 helices are tilted relative to the bilayer normal and form a left-handed bundle. Such convergence suggests that, at least for simple membrane proteins, restraints-directed modelling might yield plausible models worthy of further computational and experimental investigation

Small world effects in evolution

For asexual organisms point mutations correspond to local displacements in the genotypic space, while other genotypic rearrangements represent long-range jumps. We investigate the spreading properties of an initially homogeneous population in a flat fitness landscape, and the equilibrium properties on a smooth fitness landscape. We show that a small-world effect is present: even a small fraction of quenched long-range jumps makes the results indistinguishable from those obtained by assuming all mutations equiprobable. Moreover, we find that the equilibrium distribution is a Boltzmann one, in which the fitness plays the role of an energy, and mutations that of a temperature.Comment: 13 pages and 5 figures. New revised versio

Partitioning of plant functional trait variation into phenotypic plasticity and neutral components reveals functional differences among neotropical tree species

Background: Tropical plant communities exhibit extraordinary species richness and functional diversity in highly heterogeneous environments. Albeit the fact that such environmental filtering shapes local species composition and associated plant functional traits, it remains elusive to what extend tropical vegetation might be able to acclimate to environmental changes via phenotypic plasticity, which could be a critical determinant affecting the resistance and resilience of tropical vegetation to projected climate change. Methods: Based on a dataset compiled from 345 individuals and comprising 34 tropical tree species we here investigated the role of phenotypic plasticity versus non-plastic variation among key plant functional traits, i.e. wood density, maximum height, leaf thickness, leaf area, specific leaf area, leaf dry mass, nitrogen and phosphorus content. We hypothesized that trait variation due to plasticity is driven by environmental variability independently of spatial effects due to geographic distance between forest stands, whereas non-plastic variation increases with geographic distance due to adaption of the plant community to the local environment. Based on these hypotheses we partitioned total observed trait variation into phenotypic plasticity and neutral components and quantified respective amount of variation related to environmental filtering and neutral community assembly. Results: We found that trait variation was strongly related to spatial factors, thus often masking phenotypic plasticity in response to environmental cues. However, respective environmental factors differed among plant functional traits, such that leaf traits varied in association with light regime and soil nutrient content, whereas wood traits were related to topography and soil water content. Our results further suggest that phenotypic plasticity increased with the range size of congeneric tree species, indicating less plasticity within range restricted endemics compared to their widespread congeners. Conclusions: Differences in phenotypic trait plasticity affect stress tolerance and range size of tropical tree species, therefore endemic species could be especially prone to projected climate change