26,178 research outputs found
Short term aperiodic variability of X-ray binaries: its origin and implications
In this review I briefly describe the latest advances in studies of aperiodic
variability of accreting X-ray binaries and outline the model which currently
describe the majority of observational appearances of variability of accreting
sources in the best way. Then I concentrate on the case of luminous accreting
neutron star binaries (in the soft/high spectral state), where study of
variability of X-ray emission of sources allowed us to resolve long standing
problem of disentangling the contribution of accretion disk and
boundary/spreading layer components to the time average spectrum of sources.
The obtained knowledge of the shape of the spectrum of the boundary layer
allowed us to make estimates of the mass and radii of accreting neutron stars.Comment: 11 pages, 5 figures. Proceedings article of the conference "Cool
Discs, Hot Flows: The Varying Faces of Accreting Compact Objects", Ed. M.
Axelsson, AIP Conference Proceedings 105
Automated physical classification in the SDSS DR10. A catalogue of candidate Quasars
We discuss whether modern machine learning methods can be used to
characterize the physical nature of the large number of objects sampled by the
modern multi-band digital surveys. In particular, we applied the MLPQNA (Multi
Layer Perceptron with Quasi Newton Algorithm) method to the optical data of the
Sloan Digital Sky Survey - Data Release 10, investigating whether photometric
data alone suffice to disentangle different classes of objects as they are
defined in the SDSS spectroscopic classification. We discuss three groups of
classification problems: (i) the simultaneous classification of galaxies,
quasars and stars; (ii) the separation of stars from quasars; (iii) the
separation of galaxies with normal spectral energy distribution from those with
peculiar spectra, such as starburst or starforming galaxies and AGN. While
confirming the difficulty of disentangling AGN from normal galaxies on a
photometric basis only, MLPQNA proved to be quite effective in the three-class
separation. In disentangling quasars from stars and galaxies, our method
achieved an overall efficiency of 91.31% and a QSO class purity of ~95%. The
resulting catalogue of candidate quasars/AGNs consists of ~3.6 million objects,
of which about half a million are also flagged as robust candidates, and will
be made available on CDS VizieR facility.Comment: Accepted for publication by MNRAS, 13 pages, 6 figure
Challenges in Disentangling Independent Factors of Variation
We study the problem of building models that disentangle independent factors
of variation. Such models could be used to encode features that can efficiently
be used for classification and to transfer attributes between different images
in image synthesis. As data we use a weakly labeled training set. Our weak
labels indicate what single factor has changed between two data samples,
although the relative value of the change is unknown. This labeling is of
particular interest as it may be readily available without annotation costs. To
make use of weak labels we introduce an autoencoder model and train it through
constraints on image pairs and triplets. We formally prove that without
additional knowledge there is no guarantee that two images with the same factor
of variation will be mapped to the same feature. We call this issue the
reference ambiguity. Moreover, we show the role of the feature dimensionality
and adversarial training. We demonstrate experimentally that the proposed model
can successfully transfer attributes on several datasets, but show also cases
when the reference ambiguity occurs.Comment: Submitted to ICLR 201
Disentangling Factors of Variation with Cycle-Consistent Variational Auto-Encoders
Generative models that learn disentangled representations for different
factors of variation in an image can be very useful for targeted data
augmentation. By sampling from the disentangled latent subspace of interest, we
can efficiently generate new data necessary for a particular task. Learning
disentangled representations is a challenging problem, especially when certain
factors of variation are difficult to label. In this paper, we introduce a
novel architecture that disentangles the latent space into two complementary
subspaces by using only weak supervision in form of pairwise similarity labels.
Inspired by the recent success of cycle-consistent adversarial architectures,
we use cycle-consistency in a variational auto-encoder framework. Our
non-adversarial approach is in contrast with the recent works that combine
adversarial training with auto-encoders to disentangle representations. We show
compelling results of disentangled latent subspaces on three datasets and
compare with recent works that leverage adversarial training
Disentangling instrumental broadening
A new procedure aiming at disentangling the instrumental profile broadening
and the relevant X-ray powder diffraction (XRPD) profile shape is presented.
The technique consists of three steps: denoising by means of wavelet
transforms, background suppression by morphological functions and deblurring by
a Lucy--Richardson damped deconvolution algorithm. Real XRPD intensity profiles
of ceria samples are used to test the performances. Results show the robustness
of the method and its capability of efficiently disentangling the instrumental
broadening affecting the measurement of the intrinsic physical line profile.
These features make the whole procedure an interesting and user-friendly tool
for the pre-processing of XRPD data.Comment: 9 pages, 1 table, 1 figure; typos correcte
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