3,652 research outputs found
Simultaneous optical polarimetry and X-ray data of the near synchronous polar RX J2115-5840
We present simultaneous optical polarimetry and X-ray data of the near
synchronous polar RX J2115-5840. We model the polarisation data using the
Stokes imaging technique of Potter et al. We find that the data are best
modelled using a relatively high binary inclination and a small angle between
the magnetic and spin axes. We find that for all spin-orbit beat phases, a
significant proportion of the accretion flow is directed onto the lower
hemisphere of the white dwarf, producing negative circular polarisation. Only
for a small fraction of the beat cycle is a proportion of the flow directed
onto the upper hemisphere. However, the accretion flow never occurs near the
upper magnetic pole, whatever the orientation of the magnetic poles. This
indicates the presence of a non-dipole field with the field strength at the
upper pole significantly higher. We find that the brightest parts of the hard
X-ray emitting region and the cyclotron region are closely coincident.Comment: 9 pages, accepted for publication in MNRAS 2 March 200
The lower boundary of the accretion column in magnetic cataclysmic variables
Using a parameterised function for the mass loss at the base of the
post-shock region, we have constructed a formulation for magnetically confined
accretion flows which avoids singularities, such as the infinity in density, at
the base associated with all previous formulations. With the further inclusion
of a term allowing for the heat input into the base from the accreting white
dwarf we are able also to obtain the hydrodynamic variables to match the
conditions in the stellar atmosphere. (We do not, however, carry out a mutually
consistent analysis for the match). Changes to the emitted X-ray spectra are
negligible unless the thickness of mass leakage region at the base approaches
or exceeds one percent of the height of the post-shock region. In this case the
predicted spectra from higher-mass white dwarfs will be harder, and fits to
X-ray data will predict lower white-dwarf masses than previous formulations.Comment: 13 pages, 6 figures, accepted for publication in MNRA
Logical Reduction of Metarules
International audienceMany forms of inductive logic programming (ILP) use metarules, second-order Horn clauses, to define the structure of learnable programs and thus the hypothesis space. Deciding which metarules to use for a given learning task is a major open problem and is a trade-off between efficiency and expressivity: the hypothesis space grows given more metarules, so we wish to use fewer metarules, but if we use too few metarules then we lose expressivity. In this paper, we study whether fragments of metarules can be logically reduced to minimal finite subsets. We consider two traditional forms of logical reduction: subsumption and entailment. We also consider a new reduction technique called derivation reduction, which is based on SLD-resolution. We compute reduced sets of metarules for fragments relevant to ILP and theoretically show whether these reduced sets are reductions for more general infinite fragments. We experimentally compare learning with reduced sets of metarules on three domains: Michalski trains, string transformations, and game rules. In general, derivation reduced sets of metarules outperform subsumption and entailment reduced sets, both in terms of predictive accuracies and learning times
First XMM-Newton observations of strongly magnetic cataclysmic variables I: spectral studies of DP Leo and WW Hor
We present an analysis of the X-ray spectra of two strongly magnetic
cataclysmic variables, DP Leo and WW Hor, made using XMM-Newton. Both systems
were in intermediate levels of accretion. Hard optically thin X-ray emission
from the shocked accreting gas was detected from both systems, while a soft
blackbody X-ray component from the heated surface was detected only in DP Leo.
We suggest that the lack of a soft X-ray component in WW Hor is due to the fact
that the accretion area is larger than in previous observations with a
resulting lower temperature for the re-processed hard X-rays. Using a
multi-temperature model of the post-shock flow, we estimate that the white
dwarf in both systems has a mass greater than 1 Msun. The implications of this
result are discussed. We demonstrate that the `soft X-ray excess' observed in
many magnetic cataclysmic variables can be partially attributed to using an
inappropriate model for the hard X-ray emission.Comment: Accepted by MNRAS as a letter, 5 pages, 2 figure
First X-ray observations of the polar CE Gru
We report the detection of the polar CE Gru in X-rays for the first time. We
find evidence for a dip seen in the hard X-ray light curve which we attribute
to the accretion stream obscuring the accretion region in the lower hemisphere
of the white dwarf. The X-ray spectrum can be fitted using only a shock model:
there is no distinct soft X-ray component. We suggest that this is because the
reprocessed component is cool enough so that it is shifted into the UV. We
determine a mass for the white dwarf of ~1.0Msun.Comment: Accepted for publication in MNRAS, 6 page
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Open Learning Network: the evidence of OER impact
Much of the initial work on Open Educational Resources (OER) has inevitably concentrated on how to produce the resources themselves and to establish the idea in the community. It is now eight years since the term OER was first used and more than ten years since the concept of open content was described and a greater focus is now emerging on the way in which OER can influence policy and change the way in which educational systems help people learn. The Open University UK and Carnegie Mellon University are working in partnership on the OLnet (Open Learning Network), funded by The William and Flora Hewlett Foundation with the aims to search out the evidence for use and reuse of OER and to establish a network for information sharing about research in the field. This means both gathering evidence and developing approaches for how to research and understand ways to learn in a more open world, particularly linked to OER, but also looking at other influences
Learning programs by learning from failures
We describe an inductive logic programming (ILP) approach called learning
from failures. In this approach, an ILP system (the learner) decomposes the
learning problem into three separate stages: generate, test, and constrain. In
the generate stage, the learner generates a hypothesis (a logic program) that
satisfies a set of hypothesis constraints (constraints on the syntactic form of
hypotheses). In the test stage, the learner tests the hypothesis against
training examples. A hypothesis fails when it does not entail all the positive
examples or entails a negative example. If a hypothesis fails, then, in the
constrain stage, the learner learns constraints from the failed hypothesis to
prune the hypothesis space, i.e. to constrain subsequent hypothesis generation.
For instance, if a hypothesis is too general (entails a negative example), the
constraints prune generalisations of the hypothesis. If a hypothesis is too
specific (does not entail all the positive examples), the constraints prune
specialisations of the hypothesis. This loop repeats until either (i) the
learner finds a hypothesis that entails all the positive and none of the
negative examples, or (ii) there are no more hypotheses to test. We introduce
Popper, an ILP system that implements this approach by combining answer set
programming and Prolog. Popper supports infinite problem domains, reasoning
about lists and numbers, learning textually minimal programs, and learning
recursive programs. Our experimental results on three domains (toy game
problems, robot strategies, and list transformations) show that (i) constraints
drastically improve learning performance, and (ii) Popper can outperform
existing ILP systems, both in terms of predictive accuracies and learning
times.Comment: Accepted for the machine learning journa
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