4,763 research outputs found
Pulsar Spin--Velocity Alignment: Further Results and Discussion
The reported alignment between the projected spin-axes and proper motion
directions of pulsars is revisited in the light of new data from Jodrell Bank
and Effelsberg. The present investigation uses 54 pulsars, the largest to date
sample of pulsars with proper-motion and absolute polarisation, to study this
effect. Our study has found strong evidence for pulsar spin-velocity alignment,
excluding that those two vectors are completely uncorrelated, with >99%
confidence. Although we cannot exclude the possibility of orthogonal
spin-velocity configurations, comparison of the data with simulations shows
that the scenario of aligned vectors is more likely than that of the orthogonal
case. Moreover, we have determined the spread of velocities that a spin-aligned
and spin-orthogonal distribution of kicks must have to produce the observed
distribution of spin-velocity angle offsets. If the observed distribution of
spin-velocity offset angles is the result of spin-aligned kicks, then we find
that the distribution of kick-velocity directions must be broad with
{\sigma}_v~30\degree if the orthogonal-kick scenario is assumed, then the
velocity distribution is much narrower with {\sigma}_v<10\degree. Finally, in
contrast to previous studies, we have performed robustness tests on our data,
in order to determine whether our conclusions are the result of a statistical
and/or systematic bias. The conclusion of a correlation between the spin and
velocity vectors is independent of a bias introduced by subsets in the total
sample. Moreover, we estimate that the observed alignment is robust to within
10% systematic uncertainties on the determination of the spin-axis direction
from polarisation data.Comment: 20 pages, 7 figures, 1 Table, accepted in MNRA
Allopatric differentiation in the Marcusenius macrolepidotus species complex in southern and eastern Africa: the resurrection of M. pongolensis and M. angolensis, and the description of two new species (Mormyridae, Teleostei)
We critically compared local populations of the bulldog fish, Marcusenius macrolepidotus (Peters 1852), from different watersheds, from the furthest south (28° South, South Africa) to the Equator in Kenya. We ascertained allopatric differentiation from topotypical M. macrolepidotus from the Lower Zambezi River (Mozambique) in morphology, electric organ discharges, and molecular genetics for: (1) samples from the Okavango and Upper Zambezi Systems (Botswana and Namibia), (2) samples from South Africa's rivers draining into the Indian Ocean, and (3) samples from the East African Tana River (Kenya). Significant genetic distances in the mitochondrial cytochrome b gene and differing ISSR-PCR profiles corroborate differentiation between the four taxa. We resurrect M. pongolensis (Fowler, 1934) for South Africa (sample 2), and M. angolensis (Boulenger, 1905) for the Quanza River/Angola. We recognize M. altisambesi sp. n. for the Upper Zambezi/Okavango specimens (sample 1), and M. devosi sp. n. for those from Kenya (sample 3)
A survey of the operating functions of university and college bureaus of business research
Thesis (M.B.A.)--Boston Universit
Opening Access to Fresh Air\u27s Archives
Producing a daily public radio show while preserving past content and metadata for public access is both time-consuming and difficult, particularly for a small staff untrained in archival techniques. Over the past decade, however, Fresh Air with Terry Gross has made a concerted effort to preserve its collection, by digitizing and standardizing the metadata in its archives in order to ensure future public access to its material. In this paper, we detail why Fresh Air’s archives and other audio-dominant collections deserve such urgent attention, and present a case study for how a small public radio institution successfully managed an archival project and rethought its asset management strategy
Using deep learning to construct stochastic local search SAT solvers with performance bounds
The Boolean Satisfiability problem (SAT) is the most prototypical NP-complete
problem and of great practical relevance. One important class of solvers for
this problem are stochastic local search (SLS) algorithms that iteratively and
randomly update a candidate assignment. Recent breakthrough results in
theoretical computer science have established sufficient conditions under which
SLS solvers are guaranteed to efficiently solve a SAT instance, provided they
have access to suitable "oracles" that provide samples from an
instance-specific distribution, exploiting an instance's local structure.
Motivated by these results and the well established ability of neural networks
to learn common structure in large datasets, in this work, we train oracles
using Graph Neural Networks and evaluate them on two SLS solvers on random SAT
instances of varying difficulty. We find that access to GNN-based oracles
significantly boosts the performance of both solvers, allowing them, on
average, to solve 17% more difficult instances (as measured by the ratio
between clauses and variables), and to do so in 35% fewer steps, with
improvements in the median number of steps of up to a factor of 8. As such,
this work bridges formal results from theoretical computer science and
practically motivated research on deep learning for constraint satisfaction
problems and establishes the promise of purpose-trained SAT solvers with
performance guarantees.Comment: 15 pages, 9 figures, code available at
https://github.com/porscheofficial/sls_sat_solving_with_deep_learnin
Structural and ferromagnetic properties of an orthorhombic phase of MnBi stabilized with Rh additions
The article addresses the possibility of alloy elements in MnBi which may
modify the thermodynamic stability of the NiAs-type structure without
significantly degrading the magnetic properties. The addition of small amounts
of Rh and Mn provides an improvement in the thermal stability with some
degradation of the magnetic properties. The small amounts of Rh and Mn
additions in MnBi stabilize an orthorhombic phase whose structural and magnetic
properties are closely related to the ones of the previously reported
high-temperature phase of MnBi (HT~MnBi). To date, the properties of the
HT~MnBi, which is stable between and ~K, have not been studied in
detail because of its transformation to the stable low-temperature MnBi
(LT~MnBi), making measurements near and below its Curie temperature difficult.
The Rh-stabilized MnBi with chemical formula MnRhBi
[] adopts a new superstructure of the NiAs/NiIn structure
family. It is ferromagnetic below a Curie temperature of ~K. The critical
exponents of the ferromagnetic transition are not of the mean-field type but
are closer to those associated with the Ising model in three dimensions. The
magnetic anisotropy is uniaxial; the anisotropy energy is rather large, and it
does not increase when raising the temperature, contrary to what happens in
LT~MnBi. The saturation magnetization is approximately ~/f.u. at low
temperatures. While this exact composition may not be application ready, it
does show that alloying is a viable route to modifying the stability of this
class of rare-earth-free magnet alloys.Comment: 9 pages, 10 figure
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