243 research outputs found
Recent Strategies in Organic Reactions Catalyzed by Phase Transfer Catalysts and Analyzed by Gas Chromatography
The Geometry of PSR B0031-07
PSR B0031-07 is well known to exhibit three different modes of drifting
sub-pulses (mode A, B and C). It has recently been shown that in a
multifrequency observation, consisting of 2700 pulses, all driftmodes were
visible at low frequencies, while at 4.85 GHz only mode-A drift or non-drifting
emission was detected. This suggests that modes A and B are emitted in
sub-beams, rotating at a fixed distance from the magnetic axis, with the mode-B
sub-beams being closer to the magnetic axis than the mode-A sub-beams. Diffuse
emission between the sub-beams can account for the non-drifting emission. Using
the results of an analysis of simultaneous multifrequency observations of PSR
B0031-07, we set out to construct a geometrical model that includes emission
from both sub-beams and diffuse emission and describes the regions of the radio
emission of PSR B0031-07 at each emission frequency for driftmodes A and B.
Based on the vertical spacing between driftbands, we have determined the
driftmode of each sequence of drift. To restrict the model, we calculated
average polarisation and intensity characteristics for each driftmode and at
each frequency. The model reproduces the observed polarisation and intensity
characteristics, suggesting that diffuse emission plays an important role in
the emission properties of PSR B0031-07. The model further suggests that the
emission heights of this pulsar range from a few kilometers to a little over 10
kilometers above the pulsar surface. We also find that the relationships
between height and frequency of emission that follow from curvature radiation
and from plasma-frequency emission could not be used to reproduce the observed
frequency dependence of the width of the average intensity profiles.Comment: 15 pages, 9 figures, 8 tables, accepted for publication in A&
Women empowerment through green mussel (Perna viridis) farming : Focus on Kerala
Mussel farming is one of the best ideas for the
utilisation of natural resources as well as human
resources in a sustainable manner. It is an activity with
potential to empower women by giving them a chance to
earn additional income for their family, thereby making
them self-reliant. Thus, looking at the larger picture
mussel farming could bring about an improvement in
the socio-economic condition of families in the coastal
States of India. Additionally, it also ensures production
of healthy and protein rich food for the consumers
Book of Abstracts & Lead Articles The Second International Symposium Remote Sensing for Ecosystem Analysis and Fisheries
SAFARI (Societal Applications in Fisheries and Aquaculture using Remotely-Sensed
Imagery) is an initiative which provides a forum for coordination, at the international
level, of activities in global fisheries research and management. The forum is open to all
interested parties, including policy makers, research scientists, government managers, and
those involved in the fishing industries. SAFARI organizes international workshops and
symposia as a platform to discuss the latest research in Earth observation and fisheries
management, information sessions aimed at the fisheries industry, government officials
and resource managers, representation at policy meetings, and producing publications
relevant to the activities. SAFARI gains worldwide attention through collaboration
with other international networks, such as ChloroGIN (Chlorophyll Global Integrated
Network), IOCCG (International Ocean-Colour Coordinating Group), POGO (Partnership
for Observation of the Global Oceans) and the oceans and society: Blue Planet Initiative
of the intergovernmental organization, the Group on Earth Observations (GEO)
Local Pulsars; A note on the Birth-Velocity Distribution
We explore a simple model for the representation of the observed
distributions of the motions, and the characteristic ages of the local
population of pulsars. The principal difference from earlier models is the
introduction of a unique value, S, for the kick velocity with which pulsars are
born. We consider separately the proper motion components in galactic longitude
and latitude, and find that the distributions of the velocity components
parallel and perpendicular to the galactic plane are represented satisfactorily
by S=200 km/sec, and leave no room for a significant fraction of much higher
velocities. The successful proposition of a unique value for the kick velocity
may provide an interesting tool in attempts to understand the physical process
leading to the expulsion of the neutron star.Comment: To be published in JAA, 14 pages, 7 figure
Rocking the Lighthouse: Circumpulsar Asteroids and Radio Intermittency
We propose that neutral, circumpulsar debris entering the light cylinder can
account for many time-dependent pulsar phenomena that are otherwise difficult
to explain. Neutral material avoids propeller ejection and injects sufficient
charges -- after heating, evaporation, and ionization -- to alter current flows
and pair-production and thus trigger, detune, or extinguish coherent emission.
Relevant phenomena, with time scales from seconds to months, include nulls,
rotating radio transients (RRATs), rapid changes in pulse profile (``mode
changes''), variable subpulse drift rates, quasi-periodic bursts from B1931+24,
and torque variations. Over the 10 Myr lifetime of a canonical pulsar with
trillion-gauss surface magnetic field, less than a millionth of an Earth mass
of material is needed to modulate the Goldreich-Julian current by 100%.
Circumpulsar material originates from metal-rich, supernova fallback gas that
aggregates into asteroids. Debris disks can inject sufficient material on time
scales of interest, yet be too tenuous to form large planets detectable in
pulse timing data. Asteroid migration results from collisions and the
radiation-driven Yarkovsky and Poynting-Robertson effects. For B1931+24, an
asteroid in a ~day elliptical orbit pollutes the magnetosphere
stochastically through collisions with other debris. Injection is less likely
for hot, young and highly magnetized pulsars or millisecond pulsars that
pre-ionize any debris material well outside their small magnetospheres.
Injection effects will therefore be most prominent in long-period, cooler
pulsars, consistent with the distribution of relevant objects in perid and
period derivative. A pulsar's spin history and its radiation-beam orientation
may influence whether it displays nulling, RRATs and other effects.Comment: 20 pages, 6 figures, submitted to Ap
An Idealized Pulsar Magnetosphere: the Relativistic Force-Free Approximation
The non-dissipative relativistic force-free condition should be a good
approximation to describe the electromagnetic field in much of the pulsar
magnetosphere, but we may plausibly expect it to break down in singular
domains. Self-consistent magnetospheric solutions are found with field lines
closing both at and within the light-cylinder. In general, the detailed
properties of the solutions may be affected critically by the physics
determining the appropriate choice of equatorial boundary condition beyond the
light-cylinder.Comment: 31 pages, 3 figure
Inferring Initial Spin Periods for Neutron Stars in Composite Remnant
We propose a method to infer the initial spin period of pulsars residing in
composite supernova remnants. Such a remnant consists of both a plerionic and a
shell type component, corresponding respectively to the pulsar wind nebula
driven by the spindown luminosity of the central pulsar, and the blastwave
bounding the supernova remnant. Theoretical investigations including
hydrodynamical simulations have shown that at late times (~ 1,000 - 10,000
years), a simple scaling law connects the radius of the supernova shell to the
radius of the plerion. The energy content of the plerion and the total
mechanical energy of the supernova remnant enter into this scaling law. One can
use this scaling law to estimate the initial spin period of pulsars residing in
composite remnants. We discuss potential pitfalls of this method, including the
effect of a small remnant age and of strong radiative losses in the plerion.Comment: 4 pages, 2 figures, accepted by ApJ
Birth and Evolution of Isolated Radio Pulsars
We investigate the birth and evolution of Galactic isolated radio pulsars. We
begin by estimating their birth space velocity distribution from proper motion
measurements of Brisken et al. (2002, 2003). We find no evidence for
multimodality of the distribution and favor one in which the absolute
one-dimensional velocity components are exponentially distributed and with a
three-dimensional mean velocity of 380^{+40}_{-60} km s^-1. We then proceed
with a Monte Carlo-based population synthesis, modelling the birth properties
of the pulsars, their time evolution, and their detection in the Parkes and
Swinburne Multibeam surveys. We present a population model that appears
generally consistent with the observations. Our results suggest that pulsars
are born in the spiral arms, with a Galactocentric radial distribution that is
well described by the functional form proposed by Yusifov & Kucuk (2004), in
which the pulsar surface density peaks at radius ~3 kpc. The birth spin period
distribution extends to several hundred milliseconds, with no evidence of
multimodality. Models which assume the radio luminosities of pulsars to be
independent of the spin periods and period derivatives are inadequate, as they
lead to the detection of too many old simulated pulsars in our simulations.
Dithered radio luminosities proportional to the square root of the spin-down
luminosity accommodate the observations well and provide a natural mechanism
for the pulsars to dim uniformly as they approach the death line, avoiding an
observed pile-up on the latter. There is no evidence for significant torque
decay (due to magnetic field decay or otherwise) over the lifetime of the
pulsars as radio sources (~100 Myr). Finally, we estimate the pulsar birthrate
and total number of pulsars in the Galaxy.Comment: 27 pages, including 15 figures, accepted by Ap
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