109 research outputs found
New Neighbours: Modelling the Growing Population of Gamma-ray Millisecond Pulsars
The Fermi Large Area Telescope, in collaboration with several groups from the
radio community, have had marvellous success at uncovering new gamma-ray
millisecond pulsars (MSPs). In fact, MSPs now make up a sizable fraction of the
total number of known gamma-ray pulsars. The MSP population is characterized by
a variety of pulse profile shapes, peak separations, and radio-to-gamma phase
lags, with some members exhibiting nearly phase-aligned radio and gamma-ray
light curves (LCs). The MSPs' short spin periods underline the importance of
including special relativistic effects in LC calculations, even for emission
originating from near the stellar surface. We present results on modelling and
classification of MSP LCs using standard pulsar model geometries.Comment: 4 pages, 2 figures, proceedings of the ICREA Workshop on The
High-Energy Emission from Pulsars and their Systems (HEEPS), Sant Cugat,
Spai
Generalized nonreciprocity in an optomechanical circuit via synthetic magnetism and reservoir engineering
Synthetic magnetism has been used to control charge neutral excitations for
applications ranging from classical beam steering to quantum simulation. In
optomechanics, radiation-pressure-induced parametric coupling between optical
(photon) and mechanical (phonon) excitations may be used to break time-reversal
symmetry, providing the prerequisite for synthetic magnetism. Here we design
and fabricate a silicon optomechanical circuit with both optical and mechanical
connectivity between two optomechanical cavities. Driving the two cavities with
phase-correlated laser light results in a synthetic magnetic flux, which in
combination with dissipative coupling to the mechanical bath, leads to
nonreciprocal transport of photons with 35dB of isolation. Additionally,
optical pumping with blue-detuned light manifests as a particle non-conserving
interaction between photons and phonons, resulting in directional optical
amplification of 12dB in the isolator through direction. These results indicate
the feasibility of utilizing optomechanical circuits to create a more general
class of nonreciprocal optical devices, and further, to enable novel
topological phases for both light and sound on a microchip.Comment: 18 pages, 8 figures, 4 appendice
Multiwavelength Observations of Pulsar Wind Nebulae
The extended nebulae formed as pulsar winds expand into their surroundings
provide information about the composition of the winds, the injection history
from the host pulsar, and the material into which the nebulae are expanding.
Observations from across the electromagnetic spectrum provide constraints on
the evolution of the nebulae, the density and composition of the surrounding
ejecta, the geometry of the central engines, and the long-term fate of the
energetic particles produced in these systems. Such observations reveal the
presence of jets and wind termination shocks, time-varying compact emission
structures, shocked supernova ejecta, and newly formed dust. Here I provide a
broad overview of the structure of pulsar wind nebulae, with specific examples
from observations extending from the radio band to very-high-energy gamma-rays
that demonstrate our ability to constrain the history and ultimate fate of the
energy released in the spin-down of young pulsars.Comment: 20 pages, 11 figures. Invited review to appear in Proc. of the
inaugural ICREA Workshop on "The High-Energy Emission from Pulsars and their
Systems" (2010), eds. N. Rea and D. Torres, (Springer Astrophysics and Space
Science series
Beyond the myth of the supernova-remnant origin of cosmic rays
The origin of Galactic cosmic-ray ions has remained an enigma for almost a
century. Although it has generally been thought that they are accelerated in
the shock waves associated with powerful supernova explosions-for which there
have been recent claims of evidence-the mystery is far from resolved. In fact,
we may be on the wrong track altogether in looking for isolated regions of
cosmic-ray acceleration.Comment: Nature Progress Revie
Fermi Large Area Telescope observations of PSR J1836+5925
The discovery of the gamma-ray pulsar PSR J1836+5925, powering the formerly
unidentified EGRET source 3EG J1835+5918, was one of the early accomplishments
of the Fermi Large Area Telescope (LAT). Sitting 25 degrees off the Galactic
plane, PSR J1836+5925 is a 173 ms pulsar with a characteristic age of 1.8
million years, a spindown luminosity of 1.1 erg s, and a
large off-peak emission component, making it quite unusual among the known
gamma-ray pulsar population. We present an analysis of one year of LAT data,
including an updated timing solution, detailed spectral results and a long-term
light curve showing no indication of variability. No evidence for a surrounding
pulsar wind nebula is seen and the spectral characteristics of the off-peak
emission indicate it is likely magnetospheric. Analysis of recent XMM
observations of the X-ray counterpart yields a detailed characterization of its
spectrum, which, like Geminga, is consistent with that of a neutron star
showing evidence for both magnetospheric and thermal emission.Comment: Accepted to Astrophysical Journa
Beetle (Coleoptera: Scirtidae) Facilitation of Larval Mosquito Growth in Tree Hole Habitats is Linked to Multitrophic Microbial Interactions
Container-breeding mosquitoes, such as Aedes triseriatus, ingest biofilms and filter water column microorganisms directly to obtain the bulk of their nutrition. Scirtid beetles often co-occur with A. triseriatus and may facilitate the production of mosquito adults under low-resource conditions. Using molecular genetic techniques and quantitative assays, we observed changes in the dynamics and composition of bacterial and fungal communities present on leaf detritus and in the water column when scirtid beetles co-occur with A. triseriatus. Data from terminal restriction fragment polymorphism analysis indicated scirtid presence alters the structure of fungal communities in the water column but not leaf-associated fungal communities. Similar changes in leaf and water bacterial communities occurred in response to mosquito presence. In addition, we observed increased processing of leaf detritus, higher leaf-associated enzyme activity, higher bacterial productivity, and higher leaf-associated fungal biomass when scirtid beetles were present. Such shifts suggest beetle feeding facilitates mosquito production indirectly through the microbial community rather than directly through an increase in available fine particulate organic matter
Specific Evolution of F1-Like ATPases in Mycoplasmas
F1F0 ATPases have been identified in most bacteria, including mycoplasmas which have very small genomes associated with a host-dependent lifestyle. In addition to the typical operon of eight genes encoding genuine F1F0 ATPase (Type 1), we identified related clusters of seven genes in many mycoplasma species. Four of the encoded proteins have predicted structures similar to the α, β, γ and ε subunits of F1 ATPases and could form an F1-like ATPase. The other three proteins display no similarity to any other known proteins. Two of these proteins are probably located in the membrane, as they have three and twelve predicted transmembrane helices. Phylogenomic studies identified two types of F1-like ATPase clusters, Type 2 and Type 3, characterized by a rapid evolution of sequences with the conservation of structural features. Clusters encoding Type 2 and Type 3 ATPases were assumed to originate from the Hominis group of mycoplasmas. We suggest that Type 3 ATPase clusters may spread to other phylogenetic groups by horizontal gene transfer between mycoplasmas in the same host, based on phylogeny and genomic context. Functional analyses in the ruminant pathogen Mycoplasma mycoides subsp. mycoides showed that the Type 3 cluster genes were organized into an operon. Proteomic analyses demonstrated that the seven encoded proteins were produced during growth in axenic media. Mutagenesis and complementation studies demonstrated an association of the Type 3 cluster with a major ATPase activity of membrane fractions. Thus, despite their tendency toward genome reduction, mycoplasmas have evolved and exchanged specific F1-like ATPases with no known equivalent in other bacteria. We propose a model, in which the F1-like structure is associated with a hypothetical X0 sector located in the membrane of mycoplasma cells
Effects of anthropogenic activities on the heavy metal levels in the clams and sediments in a tropical river
The present study aimed to assess the effects of anthropogenic activities on the heavy metal levels in the Langat River by transplantation of Corbicula javanica. In addition, potential ecological risk indexes (PERI) of heavy metals in the surface sediments of the river were also investigated. The correlation analysis revealed that eight metals (As, Co, Cr, Fe, Mn, Ni, Pb and Zn) in total soft tissue (TST) while five metals (As, Cd, Cr, Fe and Mn) in shell have positively and significantly correlation with respective metal concentration in sediment, indicating the clams is a good biomonitor of the metal levels. Based on clustering patterns, the discharge of dam impoundment, agricultural activities and urban domestic waste were identified as three major contributors of the metals in Pangsun, Semenyih and Dusun Tua, and Kajang, respectively. Various geochemical indexes for a single metal pollutant (geoaccumulation index (I geo), enrichment factors (EF), contamination factor (C f) and ecological risk (Er)) all agreed that Cd, Co, Cr, Cu, Fe, Mn, Ni and Zn are not likely to cause adverse effect to the river ecosystem, but As and Pb could pose a potential ecological risk to the river ecosystem. All indexes (degree of contamination (C d), combined pollution index (CPI) and PERI) showed that overall metal concentrations in the tropical river are still within safe limit. River metal pollution was investigated. Anthropogenic activities were contributors of the metal pollution. Geochemical indexes showed that metals are within the safe limit
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