2,066 research outputs found
Synchrotron Pair Production Equilibrium in Relativistic Magnetic Reconnection
Magnetic reconnection is ubiquitous in astrophysical systems, and in many
such systems, the plasma suffers from significant cooling due to synchrotron
radiation. We study relativistic magnetic reconnection in the presence of
strong synchrotron cooling, where the ambient magnetization is high
and the magnetic compactness of the system is of order unity. In
this regime, pair production from synchrotron photons is inevitable,
and this process can regulate the magnetization surrounding the
current sheet. We investigate this self-regulation analytically and find a
self-consistent steady state for a given magnetic compactness of the system and
initial magnetization. This result helps estimate the self-consistent upstream
magnetization in systems where plasma density is poorly constrained, and can be
useful for a variety of astrophysical systems. As illustrative examples, we
apply it to study the properties of reconnecting current sheets near the
supermassive black hole of M87, as well as the equatorial current sheet outside
the light cylinder of the Crab pulsar.Comment: 11 pages, 2 figures, submitted to ApJ. Comments welcome
Coherent control of indirect excitonic qubits in optically driven quantum dot molecules
We propose an optoelectronic scheme to define and manipulate an indirect
neutral exciton qubit within a quantum dot molecule. We demonstrate coherent
dynamics of indirect excitons resilient against decoherence effects, including
direct exciton spontaneous recombination. For molecules with large interdot
separation, the exciton dressed spectrum yields an often overlooked avoided
crossing between spatially indirect exciton states. Effective two level system
Hamiltonians are extracted by Feshbach projection over the multilevel exciton
configurations. An adiabatic manipulation of the qubit states is devised using
time dependent electric field sweeps. The exciton dynamics yields the necessary
conditions for qubit initialization and near unitary rotations in the
picosecond time scale, driven by the system internal dynamics. Despite the
strong influence of laser excitation, charge tunneling, and interdot
dipole-dipole interactions, the effective relaxation time of indirect excitons
is much longer than the direct exciton spontaneous recombination time,
rendering indirect excitons as potential elemental qubits in more complex
schemes.Comment: Submitted to PRB, 11 pages and 6 figure
Separation of Test-Free Propositional Dynamic Logics over Context-Free Languages
For a class L of languages let PDL[L] be an extension of Propositional
Dynamic Logic which allows programs to be in a language of L rather than just
to be regular. If L contains a non-regular language, PDL[L] can express
non-regular properties, in contrast to pure PDL.
For regular, visibly pushdown and deterministic context-free languages, the
separation of the respective PDLs can be proven by automata-theoretic
techniques. However, these techniques introduce non-determinism on the automata
side. As non-determinism is also the difference between DCFL and CFL, these
techniques seem to be inappropriate to separate PDL[DCFL] from PDL[CFL].
Nevertheless, this separation is shown but for programs without test operators.Comment: In Proceedings GandALF 2011, arXiv:1106.081
correction novel chemical probes for the investigation of nonribosomal peptide assembly
Correction for 'Novel chemical probes for the investigation of nonribosomal peptide assembly' by Y. T. Candace Ho et al., Chem. Commun., 2017, 53, 7088â7091
Birminghamâs Eastside story: making steps towards sustainability?
Sustainability has come to play a dominant discursive role in the UK planning system, particularly relating to urban regeneration. The purpose of this paper is to evaluate the role that sustainability plays in a major regeneration
programme, known as Eastside, currently underway in Birmingham, the UK. That this ÂŁ6 billion redevelopment is now widely talked about by such key players as Birmingham City Council and the Regional Development Agency, Advantage West Midlands, as having a central sustainability agenda points to the growing importance of the ideal of sustainability in planning and regeneration agendas. In this paper, we investigate in detail how and why sustainability has become part of the planning discourse for Eastside and critically evaluate what impact, if any, this is having on public policy decision-making
Out of the Orient: Post-Tethyan transoceanic and trans-Arabian routes fostered the spread of Baorini skippers in the Afrotropics
The origin of taxa presenting a disjunct distribution between Africa and Asia has puzzled biogeographers for more than a century. This biogeographic pattern has been hypothesized to be the result of transoceanic long-distance dispersal, Oligocene dispersal through forested corridors, Miocene dispersal through the Arabian Peninsula or passive dispersal on the rifting Indian plate. However, it has often been difficult to pinpoint the mechanisms at play. We investigate biotic exchange between the Afrotropics and the Oriental region during the Cenozoic, a period in which geological changes altered landmass connectivity. We use Baorini skippers (Lepidoptera, Hesperiidae) as a model, a widespread clade of butterflies in the Old World tropics with a disjunct distribution between the Afrotropics and the Oriental region. We use anchored phylogenomics to infer a robust evolutionary tree for Baorini skippers and estimate divergence times and ancestral ranges to test biogeographic hypotheses. Our phylogenomic tree recovers strongly supported relationships for Baorini skippers and clarifies the systematics of the tribe. Dating analyses suggest that these butterflies originated in the Oriental region, Greater Sunda Islands, and the Philippines in the early Miocene c. 23 Ma. Baorini skippers dispersed from the Oriental region towards Africa at least five times in the past 20 Ma. These butterflies colonized the Afrotropics primarily through trans-Arabian geodispersal after the closure of the Tethyan seaway in the mid-Miocene. Range expansion from the Oriental region towards the African continent probably occurred via the Gomphotherium land bridge through the Arabian Peninsula. Alternative scenarios invoking long-distance dispersal and vicariance are not supported. The Miocene climate change and biome shift from forested areas to grasslands possibly facilitated geodispersal in this clade of butterflies.Directorate for Biological Sciences. Grant Numbers: DEBâ1541500, DEBâ1541560.Peer reviewe
Looking on the bright side: biased attention and the human serotonin transporter gene
Humans differ in terms of biased attention for emotional stimuli and these biases can confer differential resilience and vulnerability to emotional disorders. Selective processing of positive emotional information, for example, is associated with enhanced sociability and well-being while a bias for negative material is associated with neuroticism and anxiety. A tendency to selectively avoid negative material might also be associated with mental health and well-being. The neurobiological mechanisms underlying these cognitive phenotypes are currently unknown. Here we show for the first time that allelic variation in the promotor region of the serotonin transporter gene (5-HTTLPR) is associated with differential biases for positive and negative affective pictures. Individuals homozygous for the long allele (LL) showed a marked bias to selectively process positive affective material alongside selective avoidance of negative affective material. This potentially protective pattern was absent among individuals carrying the short allele (S or SL). Thus, allelic variation on a common genetic polymorphism was associated with the tendency to selectively process positive or negative information. The current study is important in demonstrating a genotype-related alteration in a well-established processing bias, which is a known risk factor in determining both resilience and vulnerability to emotional disorders
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