We present an evaluation of the performance of an automated classification of the Hipparcos periodic variable stars into 26 types. The sub-sample with the most reliable variability types available in ...the literature is used to train supervised algorithms to characterize the type dependencies on a number of attributes. The most useful attributes evaluated with the random forest methodology include, in decreasing order of importance, the period, the amplitude, the V−I colour index, the absolute magnitude, the residual around the folded light-curve model, the magnitude distribution skewness and the amplitude of the second harmonic of the Fourier series model relative to that of the fundamental frequency. Random forests and a multi-stage scheme involving Bayesian network and Gaussian mixture methods lead to statistically equivalent results. In standard 10-fold cross-validation (CV) experiments, the rate of correct classification is between 90 and 100 per cent, depending on the variability type. The main mis-classification cases, up to a rate of about 10 per cent, arise due to confusion between SPB and ACV blue variables and between eclipsing binaries, ellipsoidal variables and other variability types. Our training set and the predicted types for the other Hipparcos periodic stars are available online.
We present a study on galaxy detection and shape classification using topometric clustering algorithms. We first use the DBSCAN algorithm to extract, from CCD frames, groups of adjacent pixels with ...significant fluxes and we then apply the DENCLUE algorithm to separate the contributions of overlapping sources. The DENCLUE separation is based on the localization of pattern of local maxima, through an iterative algorithm, which associates each pixel to the closest local maximum. Our main classification goal is to take apart elliptical from spiral galaxies. We introduce new sets of features derived from the computation of geometrical invariant moments of the pixel group shape and from the statistics of the spatial distribution of the DENCLUE local maxima patterns. Ellipticals are characterized by a single group of local maxima, related to the galaxy core, while spiral galaxies have additional groups related to segments of spiral arms. We use two different supervised ensemble classification algorithms: Random Forest and Gradient Boosting. Using a sample of NOT approximately equal to 24,000 galaxies taken from the Galaxy Zoo 2 main sample with spectroscopic redshifts, and we test our classification against the Galaxy Zoo 2 catalogue. We find that features extracted from our pipeline give, on average, an accuracy of NOT approximately equal to 93 per cent, when testing on a test set with a size of 20 per cent of our full data set, with features deriving from the angular distribution of density attractor ranking at the top of the discrimination power.
We present the results of a new search for galaxy-scale strong lensing systems in CFHTLS Wide. Our lens-finding technique involves a preselection of potential lens galaxies, applying simple cuts in ...size and magnitude. We then perform a Principal Component Analysis of the galaxy images, ensuring a clean removal of the light profile. Lensed features are searched for in the residual images using the clustering topometric algorithm DBSCAN. We find 1098 lens candidates that we inspect visually, leading to a cleaned sample of 109 new lens candidates. Using realistic image simulations we estimate the completeness of our sample and show that it is independent of source surface brightness, Einstein ring size (image separation) or lens redshift. We compare the properties of our sample to previous lens searches in CFHTLS. Including the present search, the total number of lenses found in CFHTLS amounts to 678, which corresponds to ∼4 lenses per square degree down to i(AB) = 24.8. This is equivalent to ∼60.000 lenses in total in a survey as wide as Euclid, but at the CFHTLS resolution and depth.
Context.Globular clusters are the simplest stellar systems in which structural parameters are found to correlate with their masses and luminosities. Aims.To investigate whether the brightest globular ...clusters in the giant elliptical galaxies are similar to the less luminous globular clusters like those found in Local Group galaxies, we study the velocity dispersion and structural parameter correlations of a sample of bright globular clusters in the nearest giant elliptical galaxy NGC 5128 (Centaurus A). Methods.The UVES echelle spectrograph on the ESO Very Large Telescope (VLT) was used to obtain high-resolution spectra of 23 bright globular clusters in NGC 5128, and 10 clusters were observed with EMMI in echelle mode with the ESO New Technology Telescope. The two datasets have 5 clusters in common, while one cluster observed with UVES had too low a signal-to-noise ratio. Hence the total number of clusters analysed in this work is 27, more than doubling the previously known sample. Their spectra were cross-correlated with template spectra to measure the central velocity dispersion for each target. The structural parameters were either taken from the existing literature, or in cases where this was not available, we derived them from our VLT FORS1 images taken under excellent seeing conditions, using the ISHAPE software. The velocity dispersion and structural parameter measurements were used to obtain masses and mass-to-luminosity ratios ($M/L_V$) for 22 clusters. Results.The masses of the clusters in our sample range from Mvir = 105-107 $M_{\odot}$, and the average $M/L_V$ is 3 ± 1. The three globular clusters harbouring X-ray point sources are the second, third, and sixth most massive in our sample. The most massive cluster, HCH99-18, is also the brightest and the largest. It has a mass (Mvir = 1.4$\times$107 $M_{\odot}$) that is an order of magnitude higher than the most massive clusters in the Local Group and a high $M/L_V$ ratio (4.7 ± 1.2). We briefly discuss possible formation scenarios for this object. Conclusions.The correlations of structural parameters, velocity dispersion, masses, and $M/L_V$ for the bright globular clusters in NGC 5128 extend the properties established for the most massive Local Group clusters towards those characteristic of dwarf elliptical nuclei and ultra-compact dwarf galaxies (UCDs). The detection of the mass-radius and the mass-$M/L_V$ relations for the globular clusters with masses higher than ~ 2 $\times$ 106 $M_{\odot}$ provides the missing link between “normal” old globular clusters, young massive clusters, and evolved objects like UCDs.
Context.During the first year in operation, INTEGRAL, the European Space Agency's γ-ray observatory, has detected more than 28 new bright sources in the galactic plane which emit the bulk of their ...emission above 10 keV. Aims.To understand the nature of those sources we have obtained follow-up observations in the X-ray band with XMM-Newton. Methods.We derive accurate X-ray positions, propose infrared counterparts and study the source high energy long and short term variability and spectra. Results.70% of the sources are strongly absorbed ($N_{\rm H}\geq10^{23}$ atom cm-2). More than half of these absorbed sources show pulsations with periods ranging from 139 to 1300 s, i.e., they are slow X-ray pulsars. The candidate infrared counterparts are not as strongly absorbed demonstrating that part of the absorbing matter is local to the sources. Conclusions.Many of these new sources are supergiant high-mass X-ray binaries (HMXB) in which the stellar wind of the companion star is accreted onto the compact object. The large local absorption in these new sources can be understood if the compact objects are buried deep in their stellar winds. These new objects represent half of the population of supergiant HMXB.
We present a novel automated methodology to detect and classify periodic variable stars in a large data base of photometric time series. The methods are based on multivariate Bayesian statistics and ...use a multistage approach. We applied our method to the ground-based data of the Trans-Atlantic Exoplanet Survey (TrES) Lyr1 field, which is also observed by the Kepler satellite, covering ∼26 000 stars. We found many eclipsing binaries as well as classical non-radial pulsators, such as slowly pulsating B stars, γ Doradus, β Cephei and δ Scuti stars. Also a few classical radial pulsators were found.
Abstract
We propose a robust principal component analysis framework for the exploitation of multiband photometric measurements in large surveys. Period search results are improved using the ...time-series of the first principal component due to its optimized signal-to-noise ratio. The presence of correlated excess variations in the multivariate time-series enables the detection of weaker variability. Furthermore, the direction of the largest variance differs for certain types of variable stars. This can be used as an efficient attribute for classification. The application of the method to a subsample of Sloan Digital Sky Survey Stripe 82 data yielded 132 high-amplitude δ Scuti variables. We also found 129 new RR Lyrae variables, complementary to the catalogue of Sesar et al., extending the halo area mapped by Stripe 82 RR Lyrae stars towards the Galactic bulge. The sample also comprises 25 multiperiodic or Blazhko RR Lyrae stars.
The INTEGRAL Science Data Centre (ISDC) T. J.-L. Courvoisier; Walter, R.; Beckmann, V. ...
Astronomy and astrophysics (Berlin),
11/2003, Letnik:
411, Številka:
1
Journal Article
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The INTEGRAL Science Data Centre (ISDC) provides the INTEGRAL data and means to analyse them to the scientific community. The ISDC runs a gamma ray burst alert system that provides the position of ...gamma ray bursts on the sky within seconds to the community. It operates a quick-look analysis of the data within few hours that detects new and unexpected sources as well as it monitors the instruments. The ISDC processes the data through a standard analysis the results of which are provided to the observers together with their data.
Abstract
We present an automated classification of stars exhibiting periodic, non-periodic and irregular light variations. The Hipparcos catalogue of unsolved variables is employed to complement the ...training set of periodic variables of Dubath et al. with irregular and non-periodic representatives, leading to 3881 sources in total which describe 24 variability types. The attributes employed to characterize light-curve features are selected according to their relevance for classification. Classifier models are produced with random forests and a multistage methodology based on Bayesian networks, achieving overall misclassification rates under 12 per cent. Both classifiers are applied to predict variability types for 6051 Hipparcos variables associated with uncertain or missing types in the literature.
The decline of the high mass X-ray transient V 0332+53 during the Dec. 2004 to Feb. 2005 outburst is analysed from the data recorded by INTEGRAL. The flux is shown to decrease exponentially until ...2005 Feb. 10, with a decay time scale of ~30 days above 20 keV and ~20 days at lower energies, and to decrease linearly thereafter. The energy spectrum is well modelled throughout the decay by a power law with a folding energy of ~7.5 keV, and with two cyclotron absorption features. The folding energy does not vary significantly over the decay, but the spectrum becomes harder with time. Most importantly, we show that the parameters describing the fundamental cyclotron line around 27 keV do vary with time: its energy and depth increase (by about 17% for the energy in ~6 weeks), while its width decreases. These changes of the cyclotron line parameters are interpreted as resulting from a change in the extent of the cyclotron scattering region. Two quasi-periodic oscillations are also observed at various times during the observations, one at 0.05 Hz and another one near the pulsation frequency around 0.23 Hz.
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