134 research outputs found
Astronomical Data Mining with Neural Networks
No full textWe give a brief overview of artificial neural networks (ANNs), focusing on Kohonen networks (KNs). The two kinds of KNs will be described in detail: the unsupervised self-organizing map (SOM) and the supervised learning vector quantization (LVQ). We then apply these algorithms to two astronomical classification problems: the classification of broad absorption line quasars (BALQSOs) and of gamma-ray bursts (GRBs). In the context of BALQSOs, we find a BALQSO fraction of 10.4%, and compile a catalogue from the Sloan Digital Sky Survey (SDSS) using the supervised LVQ. This is currently the most complete BALQSO catalogue. We then apply the unsupervised SOM to GRB light curves obtained from the Burst and Transient Source Experiment (BATSE). Using only shape-dependent variables, we find that two classes are recovered: single-pulsed bursts (SPBs) and multi-pulsed bursts (MPBs). We show that these two network classes also have different observational properties that are independent of light curve shape (T90 and fluence), suggesting an intrinsic difference between the two. We conclude with some attempts to correlate our GRB result to previous studies and suggest improvements for future work
Machine learning from hard x-ray surveys: applications to magnetic cataclysmic variable studies
No full textWithin this thesis are discussed two main topics of contemporary astrophysics. The first is that of machine learning algorithms for astronomy whilst the second is that of magnetic cataclysmic variables (mCVs). To begin, an overview is given of ISINA: INTEGRAL Source Identification Network Algorithm. This machine learning algorithm, using random forests, is applied to the IBIS/ISGRI data set in order to ease the production of unbiased future soft gamma-ray source catalogues. The feature extraction process on an initial candidate list is described together with feature merging. Three trainng and testing sets are created in order to deal with the diverse time-scales encountered when dealing with the gamma-ray sky: one dealing with faint persistent source recognition, one dealing with strong persistent sources and a final one dealing with transients. For the latter, a new transient detection technique is introduced and described: the transient matrix. Finally the performance of the network is assessed and discussed using the testing set and some illustrative source examples. ISINA is also compared to the more conventional approach of visual inspection. Next mCVs are discussed, and in particular the properties arising from a hard X-ray selected sample which has proven remarkably efficient in detecting intermediate polars and asynchronous polars, two of the rarest type of cataclysmic variables (CVs). This thesis focuses particularly on the link between hard X-ray properties and spin/orbital periods. To this end, a new sample of these objects is constructed by cross-corelating candidate sources detected in INTEGRAL/IBIS observations against catalogues of known CVs. Also included in the analysis are hard X-ray Observations from Swift/BAT and SUZAKU/HXD in order to make the study more complete. It is found that most hard X-ray detected mCVs have Pspin/Porb<0.1 above the period gap. In this respect, attention is given to the very low number of detected systems in any ban between Pspin/Porb = 0.3 and Pspin/Porb = 1 and the apparent peak of the Pspin/Porb distribution at about 0.1. The observational features of the Pspin - Porb plane are discussed in the context of mCV evolution scenarios. Also presented is evidence for correlations between hard X-ray spectral hardness and Pspin, Porb and Pspin/Porb. An attempt to explain the observed correlations is made in th context of mCV evolution and accretion footpring geometrirs on the whit dwarf surface
Influence of Displacement Rate on Residual Shear Strength of Clays
No full textThis paper reports on the results of direct shear tests carried out under controlled displacement rate in the range of 10-4 -
102 mm/min, under different normal stresses, with different shear devices. The tests were carried out on a kaolin, a bentonite,
their mixtures with sand at various percentages, and the clayey soil of the Costa della Gaveta earthflow. The tests were
performed on specimens reconstituted with distilled water as well as with NaCl solutions at various concentrations. Positive rate
effects were exhibited by mixtures with c.f. higher than 50% and, consistently, by the natural clayey soil the c.f. of which is about
50%. The residual shear strength increases significantly for shear displacement rate higher than about 1 mm/min. The rate effect
increases with the pore solution concentration. The residual shear strength independence of displacement rate has been confirmed
in the range 10-6 - 10-1 mm/min by the results of shear tests performed under controlled shear stress, with varying chemical
conditions of the pore fluid
ISINA : INTEGRAL source identification network algorithm
No full textWe give an overview of ISINA: INTEGRAL Source Identification Network Algorithm. This machine learning algorithm, using random forests, is applied to the IBIS/ISGRI data set in order to ease the production of unbiased future soft gamma-ray source catalogues. First, we introduce the data set and the problems encountered when dealing with images obtained using the coded mask technique. The initial step of source candidate searching is introduced and an initial candidate list is created. A description of the feature extraction on the initial candidate list is then performed together with feature merging for these candidates. Three training and testing sets are created in order to deal with the diverse time-scales encountered when dealing with the gamma-ray sky. Three independent random forests are built: one dealing with faint persistent source recognition, one dealing with strong persistent sources and a final one dealing with transients. For the latter, a new transient detection technique is introduced and described: the transient matrix. Finally the performance of the network is assessed and discussed using the testing set and some illustrative source examples
The intrinsic fraction of broad-absorption line quasars
No full textWe carefully reconsider the problem of classifying broad-absorption line quasars (BALQSOs) and derive a new, unbiased estimate of the intrinsic BALQSO fraction from the Sloan Digital Sky Survey (SDSS) DR3 quasi-stellar object (QSO) catalogue. We first show that the distribution of objects selected by the so-called 'absorption index' (AI) is clearly bimodal in log AI, with only one mode corresponding to definite BALQSOs. The surprisingly high BALQSO fractions that have recently been inferred from AI-based samples are therefore likely to be overestimated. We then present two new approaches to the classification problem that are designed to be more robust than the AI, but also more complete than the traditional 'balnicity index' (BI). Both approaches yield observed BALQSO fractions around 13.5 per cent, while a conservative third approach suggests an upper limit of 18.3 per cent. Finally, we discuss the selection biases that affect our observed BALQSO fraction. After correcting for these biases, we arrive at our final estimate of the intrinsic BALQSO fraction. This is fBALQSO= 0.17 ± 0.01 (stat) ± 0.03 (sys) with an upper limit of fBALQSO? 0.23 . We conclude by pointing out that the bimodality of the log AI distribution may be evidence that the BAL-forming region has clearly delineated physical boundaries
Discovery of the orbital period in the supergiant fast X-ray transient IGR J17544-2619
No full textThe supergiant fast X-ray transient (SFXT) system IGR J17544?2619 has displayed many large outbursts in the past and is considered an archetypal example of SFXTs. A search of the INTEGRAL/ISGRI data archive from MJD 52698?54354 has revealed 11 outbursts and timing analysis of the light curve identifies a period of 4.926 ± 0.001 d which we interpret as the orbital period of the system. We find that large outbursts occasionally occur outside of periastron and place an upper limit for the radius of the supergiant of <23 R?
The rms-flux relation in accreting white dwarfs: another nova-like variable and the first dwarf nova
No full textWe report on the detection of the linear rms-flux relation in two
accreting white dwarf binary systems: V1504 Cyg and KIC 8751494. The
rms-flux relation relates the absolute root-mean-square (rms)
variability of the light curve to its mean flux. The light curves
analysed were obtained with the Kepler satellite at a 58.8 s cadence.
The rms-flux relation was previously detected in only one other
cataclysmic variable (CV), MV Lyr. This result reinforces the ubiquity
of the linear rms-flux relation as a characteristic property of
accretion-induced variability, since it has been observed in several
black hole binaries, neutron star binaries and active galactic nuclei.
Moreover, its detection in V1504 Cyg is the first time the rms-flux
relation has been detected in a dwarf nova-type CV during quiescence.
This result, together with previous studies, hence points towards a
common physical origin of accretion-induced variability, independent of
the size, mass or type of the central accreting compact object
On the Raman O VI and related lines in classical novae
No full textWe critically examine the recent claimed detection of Raman-scattered O VI at around 6830 Å in the iron curtain stage spectra of the classical CO nova V339 Del. The observed line variations are compatible in the profile and timing of emission line strength with an excited state transition of neutral carbon. Line formation in classical nova ejecta is physically very different from what it is in symbiotic binaries, in which the O VI emission line is formed within the wind of the companion red giant at low differential velocity. The ejecta velocity and density structure prevent the scattering from producing analogous features. High velocity gradient outflows, such as winds, are subject to the same constraints. There might, however, be a broadband spectropolarimetric signature of the Raman process and also Rayleigh scattering at some stage in the expansion. We show that the neutral carbon spectrum, hitherto underexploited for novae, is especially useful as a probe of the structure of the ejecta during the early, optically thick stages of the expansion.sponsorship: Based on observations made with the Mercator Telescope, operated on the island of La Palma by the Flemish Community, at the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofsica de Canarias. Further based on observations obtained with the HERMES spectrograph, which is supported by the Fund for Scientific Research of Flanders (FWO), Belgium, the Research Council of K.U.Leuven, Belgium, the Fonds National de la Recherche Scientifique (F.R.S.-FNRS), Belgium, the Royal Observatory of Belgium, the Observatoire de Geneve, Switzerland, and the Thuringer Landessternwarte Tautenburg, Germany. S.N.S. warmly thanks Elena Mason, Greg Schwarz, Sumner Starrfield, Francois Teyssier, and Patrick Woudt for invaluable discussions and exchanges. S. S. acknowledges funding from the FWO Pegasus Marie Curie fellowship program. We thank Alejandra Sans Fuentes, Roy Stensen, and Maria Suveges for carrying out some of the observations. We thank the referee, Bob Williams, for very helpful suggestions. (Fund for Scientific Research of Flanders (FWO), Belgium, Research Council of K.U.Leuven, Belgium, Fonds National de la Recherche Scientifique (F.R.S.-FNRS), Belgium, Royal Observatory of Belgium, Observatoire de Geneve, Switzerland, Thuringer Landessternwarte Tautenburg, Germany, FWO Pegasus Marie Curie fellowship program)status: Publishe
Classifying optical (out)bursts in cataclysmic variables: the distinct observational characteristics of dwarf novae, micronovae, stellar flares and magnetic gating
No full textCataclysmic variables can experience short optical brightenings, which are commonly attributed to phenomena such as dwarf novae outbursts, micronovae, donor flares, or magnetic gating bursts. Since these events exhibit similar observational characteristics, their identification has often been ambiguous. In particular, magnetic gating bursts and micronovae have been suggested as alternative interpretations of the same phenomena. Here we show that the timescales and energies separate the optical brightenings into separate clusters consistent with their different classifications. This suggests that micronovae and magnetic gating bursts are in fact separate phenomena. Based on our findings, we develop diagnostic diagrams that can distinguish between these bursts/flares based on their properties. We demonstrate the effectiveness of this approach on observations of a newly identified intermediate polar, CTCV J0333-4451, which we classify as a magnetic gating system. CTCV J0333-4451 is the third highest spin-to-orbital period ratio intermediate polar with magnetic gating, suggesting that these bursts are common among these rare systems.</p
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