1,582 research outputs found
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
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
Stokes imaging, Doppler mapping and Roche tomography of the AM Her system V834 Cen
We report on new simultaneous phase resolved spectroscopic and polarimetric
observations of the polar (AM Herculis star) V834 Cen during a high state of
accretion. Strong emission lines and high levels of variable circular and
linear polarization are observed over the orbital period. The polarization data
is modelled using the Stokes imaging technique of Potter et al. The
spectroscopic emission lines are investigated using the Doppler tomography
technique of Marsh and Horne and the Roche tomography technique of Dhillon and
Watson. Up to now all three techniques have been used separately to investigate
the geometry and accretion dynamics in Cataclysmic Variables. For the first
time, we apply all three techniques to simultaneous data for a single system.
This allows us to compare and test each of the techniques against each other
and hence derive a better understanding of the geometry, dynamics and system
parameters of V834 Cen.Comment: 10 pages, 8 figures; Accepted for publication in MNRA
XMM-Newton observations of three short period polars: V347 Pav, GG Leo and EU UMa
We present phase-resolved XMM_Newton data of three short period polars: V347
Pav, GG Leo and EU UMa. All three systems show one dominant accretion region
which is seen for approximately half of the orbital cycle. GG Leo shows a
strong dip feature in its X-ray and UV light curves which is due to absorption
of X-rays from the accretion site by the accretion stream. The emission in the
case of EU UMa is dominated by soft X-rays: its soft/hard X-ray ratio is
amongst the highest seen in these objects. In contrast, GG Leo and V347 Pav
shows a ratio consistent with that predicted by the standard shock model. We
infer the mass of the white dwarf and explore the affect of restricting the
energy range on the derived parameters.Comment: accepted MNRA
ORFEUS II Far-UV Spectroscopy of AM Herculis
Six high-resolution (\lambda/\Delta\lambda ~ 3000) far-UV (\lambda\lambda =
910-1210 \AA) spectra of the magnetic cataclysmic variable AM Herculis were
acquired in 1996 November during the flight of the ORFEUS-SPAS II mission. AM
Her was in a high optical state at the time of the observations, and the
spectra reveal emission lines of O VI \lambda\lambda 1032, 1038, C III \lambda
977, \lambda 1176, and He II \lambda 1085 superposed on a nearly flat
continuum. Continuum flux variations can be described as per Gansicke et al. by
a ~ 20 kK white dwarf with a ~ 37 kK hot spot covering a fraction f~0.15 of the
surface of the white dwarf, but we caution that the expected Lyman absorption
lines are not detected. The O VI emission lines have narrow and broad component
structure similar to that of the optical emission lines, with radial velocities
consistent with an origin in the irradiated face of the secondary and the
accretion funnel, respectively. The density of the narrow- and broad-line
regions is n_{nlr} ~ 3\times 10^{10} cm^{-3} and n_{blr} ~ 1\times 10^{12}
cm^{-3}, respectively, yet the narrow-line region is optically thick in the O
VI line and the broad-line region is optically thin; apparently, the velocity
shear in the broad-line region allows the O VI photons to escape, rendering the
gas effectively optically thin. Unexplained are the orbital phase variations of
the emission-line fluxes.Comment: 15 pages, 6 Postscript figures; LaTeX format, uses aaspp4.sty;
table2.tex included separately because it must be printed sideways - see
instructions in the file; accepted on April 17, 1998 for publication in The
Astrophysical Journa
A Detection of an Anti-correlated Hard X-ray Lag in AM Herculis
Context {Earlier cross-correlation studies for AM Her were performed in
various energy range from optical to X-ray and suggested that it mostly shows a
high level of correlation but on occasion it shows a low level of correlation
or uncorrelation.} Aims {To investigate the degree of correlation between soft
(2-4 keV) and hard (9-20 keV) X-rays, we perform the cross-correlation study of
the X-ray data sets of AM Her obtained with {\it RXTE}.}
Methods {We cross-correlate the background-subtracted soft and hard X-ray
light curves using the XRONOS program crosscor and fit a model to the obtained
cross-correlation functions.}
Results {We detect a hard X-ray lag of s in a specific section of
energy-dependent light curve, where the soft X-ray (2-4 keV) intensity
decreases but the hard X-ray (9-20 keV) intensity increases. From a spectral
analysis, we find that the X-ray emission temperature increases during the
anti-correlated intensity variation. In two other observations, the
cross-correlation functions show a low level of correlation, which is
consistent with the earlier results performed in a different energy range.}
Conclusions {We report a detection of an anti-correlated hard X-ray lag of
190 s from the proto-type polar AM Her. The hard X-ray lag is detected
for the first time in the given energy range, and it is the longest lag among
those reported in magnetic cataclysmic variables. We discuss the implications
of our findings regarding the origin of the hard X-ray lag and the
anti-correlated intensity variation.}Comment: Accepted in A&A, 4 page
Stellar Motion around Spiral Arms: Gaia Mock Data
We compare the stellar motion around a spiral arm created in two different
scenarios, transient/co-rotating spiral arms and density-wave-like spiral arms.
We generate Gaia mock data from snapshots of the simulations following these
two scenarios using our stellar population code, SNAPDRAGONS, which takes into
account dust extinction and the expected Gaia errors. We compare the observed
rotation velocity around a spiral arm similar in position to the Perseus arm,
and find that there is a clear difference in the velocity features around the
spiral arm between the co-rotating spiral arm and the density-wave-like spiral
arm. Our result demonstrates that the volume and accuracy of the Gaia data are
sufficient to clearly distinguish these two scenarios of the spiral arms.Comment: 5 pages, 1 figure, to appear in the proceedings of "The Milky Way
Unravelled by Gaia: GREAT Science from the Gaia Data Releases", Barcelona,
1-5 December 2014, eds. N. Walton, F. Figueras, C. Soubira
Twisted accretion curtains in the intermediate polar FO Aquarii
We report on a ~37-ks XMM-Newton observation of the intermediate polar FO
Aquarii, presenting X-ray and UV data from the EPIC and OM cameras. We find
that the system has changed from its previously reported state of disc-overflow
accretion to one of purely disc-fed accretion. We detect the previously
reported `notch' feature in the X-ray spin pulse, and explain it as a partial
occultation of the upper accretion pole. Maximum flux of the quasi-sinusoidal
UV pulse coincides with the notch, in keeping with this idea. However, an
absorption dip owing to the outer accretion curtains occurs 0.27 later than the
expected phase, which implies that the accretion curtains are twisted, trailing
the magnetic poles. This result is the opposite of that reported in PQ Gem,
where accreting field lines were found to lead the pole. We discuss how such
twists relate to the accretion torques and thus the observed period changes of
the white dwarfs, but find no simple connection.Comment: 7 pages, 5 figures. Accepted by MNRA
Gas and Stellar Motions and Observational Signatures of Co-Rotating Spiral Arms
We have observed a snapshot of our N-body/Smoothed Particle Hydrodynamics
simulation of a Milky Way-sized barred spiral galaxy in a similar way to how we
can observe the Milky Way. The simulated galaxy shows a co-rotating spiral arm,
i.e. the spiral arm rotates with the same speed as the circular speed. We
observed the rotation and radial velocities of the gas and stars as a function
of the distance from our assumed location of the observer at the three lines of
sight on the disc plane, (l, b) = (90, 0), (120, 0) and (150,0) deg. We find
that the stars tend to rotate slower (faster) behind (at the front of) the
spiral arm and move outward (inward), because of the radial migration. However,
because of their epicycle motion, we see a variation of rotation and radial
velocities around the spiral arm. On the other hand, the cold gas component
shows a clearer trend of rotating slower (faster) and moving outward (inward)
behind (at the front of) the spiral arm, because of the radial migration. We
have compared the results with the velocity of the maser sources from Reid et
al. (2014), and find that the observational data show a similar trend in the
rotation velocity around the expected position of the spiral arm at l = 120
deg. We also compared the distribution of the radial velocity from the local
standard of the rest, V_LSR, with the APOGEE data at l = 90 deg as an example.Comment: 10 pages, 7 figures, accepted for publication in MNRA
The value of preventing malaria in Tembien, Ethiopia
The authors measure the monetary value households place on preventing malaria in Tembien, Tigray Region, Ethiopia. They estimate a household demand function for a hypothetical malaria vaccine and compute the value of preventing malaria as the household's maximum willingness to pay to provide vaccines for all family members. They contrast willingness to pay with the traditional costs of illness (medical costs and time lost because of malaria). Their results indicate that the value of preventing malaria with vaccines is about US6 a bed net), only a third of the population of a 200-person village would sleep under bed nets.Early Child and Children's Health,Public Health Promotion,Disease Control&Prevention,Health Monitoring&Evaluation,Economic Theory&Research,Health Monitoring&Evaluation,Economic Theory&Research,Climate Change,Environmental Economics&Policies,Early Child and Children's Health
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