1,081 research outputs found
A Simple Model for Pulse Profiles from Precessing Pulsars, with Special Application to Relativistic Binary PSR B1913+16
We study the observable pulse profiles that can be generated from precessing
pulsars. A novel coordinate system is defined to aid visualization of the
observing geometry. Using this system we explore the different families of
profiles that can be generated by simple, circularly symmetric beam shapes. An
attempt is then made to fit our model to the observations of relativistic
binary PSR B1913+16. It is found that while qualitatively similar pulse
profiles can be produced, this minimal model is insufficient for an accurate
match to the observational data. Consequently, we confirm that the emission
beam of PSR B1913+16 must deviate from circular symmetry, as first reported by
Weisberg and Taylor. However, the approximate fits obtained suggest that it may
be sufficient to consider only minimal deviations from a circular beam in order
to explain the data. We also comment on the applicability of our analysis
technique to other precessing pulsars, both binary and isolated.Comment: 35 pages and 8 figures. Published versio
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Assessing rotation-invariant feature classification for automated wildebeest population counts
Accurate and on-demand animal population counts are the holy grail for wildlife conservation organizations throughout the world because they enable fast and responsive adaptive management policies. While the collection of image data from camera traps, satellites, and manned or unmanned aircraft has advanced significantly, the detection and identification of animals within images remains a major bottleneck since counting is primarily conducted by dedicated enumerators or citizen scientists. Recent developments in the field of computer vision suggest a potential resolution to this issue through the use of rotation-invariant object descriptors combined with machine learning algorithms. Here we implement an algorithm to detect and count wildebeest from aerial images collected in the Serengeti National Park in 2009 as part of the biennial wildebeest count. We find that the per image error rates are greater than, but comparable to, two separate human counts. For the total count, the algorithm is more accurate than both manual counts, suggesting that human counters have a tendency to systematically over or under count images. While the accuracy of the algorithm is not yet at an acceptable level for fully automatic counts, our results show this method is a promising avenue for further research and we highlight specific areas where future research should focus in order to develop fast and accurate enumeration of aerial count data. If combined with a bespoke image collection protocol, this approach may yield a fully automated wildebeest count in the near future
De Sitter space and perpetuum mobile
We give general arguments that any interacting non--conformal {\it classical}
field theory in de Sitter space leads to the possibility of constructing a
perpetuum mobile. The arguments are based on the observation that massive free
falling particles can radiate other massive particles on the classical level as
seen by the free falling observer. The intensity of the radiation process is
non-zero even for particles with any finite mass, i.e. with a wavelength which
is within the causal domain. Hence, we conclude that either de Sitter space can
not exist eternally or that one can build a perpetuum mobile.Comment: 11 pages revtex, no figures. Added discussion to strengthen
conclusio
The Simultaneous Evolution of Author and Paper Networks
There has been a long history of research into the structure and evolution of
mankind's scientific endeavor. However, recent progress in applying the tools
of science to understand science itself has been unprecedented because only
recently has there been access to high-volume and high-quality data sets of
scientific output (e.g., publications, patents, grants), as well as computers
and algorithms capable of handling this enormous stream of data. This paper
reviews major work on models that aim to capture and recreate the structure and
dynamics of scientific evolution. We then introduce a general process model
that simultaneously grows co-author and paper-citation networks. The
statistical and dynamic properties of the networks generated by this model are
validated against a 20-year data set of articles published in the Proceedings
of the National Academy of Science. Systematic deviations from a power law
distribution of citations to papers are well fit by a model that incorporates a
partitioning of authors and papers into topics, a bias for authors to cite
recent papers, and a tendency for authors to cite papers cited by papers that
they have read. In this TARL model (for Topics, Aging, and Recursive Linking),
the number of topics is linearly related to the clustering coefficient of the
simulated paper citation network.Comment: 14 pages, 6 figures, 2 table
Orientation of native versus translocated juvenile lesser spotted eagles <i>(Clanga pomarina)</i> on the first autumn migration
Orientation of native versus translocated juvenile lesser spotted eagles <i>(Clanga pomarina)</i> on the first autumn migration
The ontogeny of migration routines used by wild birds remains unresolved. Here we investigated the migratory orientation of juvenile lesser spotted eagles (LSE; Clanga pomarina) based on translocation and satellite tracking. Between 2004 and 2016, 85 second-hatched juveniles (Abels) were reared in captivity for release into the declining German population, including 50 birds that were translocated 940 km from Latvia. In 2009, we tracked 12 translocated juveniles, as well as eight native juveniles and nine native adults, to determine how inexperienced birds come to use strategic migration routes. Native juveniles departed around the same time as the adults and six of eight used the eastern flyway around the Mediterranean, which was used by all adults. In contrast, translocated juveniles departed on average 6 days before native LSEs, and five travelled southward and died in the central Mediterranean region. Consequently, fewer translocated juveniles (4/12) than native juveniles (7/8) reached Africa. We conclude that juvenile LSEs have a much better chance of learning the strategic southeastern flyway if they leave at an appropriate time to connect with experienced elders upon departure. It is not clear why translocated juveniles departed so early. Regardless, by the end of the year, most juveniles had perished, whether they were translocated (10/12) or not (6/8). The small number of surviving translocated juveniles thus still represents a significant increase in the annual productivity of the German LSE population in 2009
Studies in Structure Formation in theories with a repulsive long range gravitational force
This article reports on emergence of structures in a class of alternative
theories of gravity. These theories do not have any horizon, flatness, initial
cosmological singularity and (possibly) quantization problems. The model is
characterised by a dynamically induced gravitational constant with a ``wrong''
sign corresponding to repulsive gravitation on the large scale. A non - minimal
coupling of a scalar field in the model can give rise to non - topological
solitons in the theory. This results in domains (gravity - balls) inside which
an effective, canonical, attractive gravitational constant is induced. We
consider simulations of the formation and evolution of such solutions. Starting
with a single gravity - ball, we consider its fragmentation into smaller (lower
mass) balls - evolving by mutual repulsion. After several runs, we have been
able to identify two parameters: the strength of the long range gravitational
constant and the size of the gravity balls, which can be used to generate
appropriate two point correlations of the distribution of these balls.Comment: Normal Tex, 7 pages, 5 figures available from the author
Physics of Interpulse Emission in Radio Pulsars
The magnetized induced Compton scattering off the particles of the
ultrarelativistic electron-positron plasma of pulsar is considered. The main
attention is paid to the transverse regime of the scattering, which holds in a
moderately strong magnetic field. We specifically examine the problem on
induced transverse scattering of the radio beam into the background, which
takes place in the open field line tube of a pulsar. In this case, the
radiation is predominantly scattered backwards and the scattered component may
grow considerably. Based on this effect, we for the first time suggest a
physical explanation of the interpulse emission observed in the profiles of
some pulsars. Our model can naturally account for the peculiar spectral and
polarization properties of the interpulses. Furthermore, it implies a specific
connection of the interpulse to the main pulse, which may reveal itself in the
consistent intensity fluctuations of the components at different timescales.
Diverse observational manifestations of this connection, including the moding
behavior of PSR B1822-09, the peculiar temporal and frequency structure of the
giant interpulses in the Crab pulsar, and the intrinsic phase correspondence of
the subpulse patterns in the main pulse and the interpulse of PSR B1702-19, are
discussed in detail. It is also argued that the pulse-to-pulse fluctuations of
the scattering efficiency may lead to strong variability of the interpulse,
which is yet to be studied observationally. In particular, some pulsars may
exhibit transient interpulses, i.e. the scattered component may be detectable
only occasionally.Comment: 28 pages, 2 figures. Accepted for publication in Ap
Competition, predation, and migration: individual choice patterns of Serengeti migrants captured by hierarchical models
Large-herbivore migrations occur across gradients of food quality or food abundance that are generally determined by underlying geographic patterns in rainfall, elevation, or latitude, in turn causing variation in the degree of interspecific competition and the exposure to predators. However, the role of top-down effects of predation as opposed to the bottom-up effects of competition for resources in shaping migrations is not well understood. We studied 30 GPS radio-collared wildebeest and zebra migrating seasonally in the Serengeti-Mara ecosystem to ask how predation and food availability differentially affect the individual movement patterns of these co-migrating species. A hierarchical analysis of movement trajectories (directions and distances) in relation to grass biomass, high-quality food patches, and predation risk show that wildebeest tend to move in response to food quality, with little attention to predation risk. In contrast, individual zebra movements reflect a balance between the risk of predation and the access to high-quality food of sufficient biomass. Our analysis shows how two migratory species move in response to different attributes of the same landscape. Counterintuitively and in contrast to most other animal movement studies, we find that both species move farther each day when resources are locally abundant than when they are scarce. During the wet season when the quality of grazing is at its peak, both wildebeest and zebra move the greatest distances and do not settle in localized areas to graze for extended periods. We propose that this punctuated movement in highquality patches is explained by density dependency, whereby large groups of competing individuals (up to 1.65 million grazers) rapidly deplete the localized grazing opportunities. These findings capture the roles of predation and competition in shaping animal migrations, which are often claimed but rarely measured
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