arXiv subjects

Pavlos Protopapas

Publications and source records attributed to Pavlos Protopapas.

126 records · Page 7Linked to original sources

arXiv2011

Period Estimation in Astronomical Time Series Using Slotted Correntropy

In this letter, we propose a method for period estimation in light curves from periodic variable stars using correntropy. Light curves are astronomical time series of stellar brightness over time, and are characterized as being noisy and unevenly sampled. We propose to use slotted time lags in order to estimate correntropy directly from irregularly sampled time series. A new information theoretic metric is proposed for discriminating among the peaks of the correntropy spectral density. The slotted correntropy method outperformed slotted correlation, string length, VarTools (Lomb-Scargle periodogram and Analysis of Variance), and SigSpec applications on a set of light curves drawn from the MACHO survey.

cs.IT↗

arXiv2011

QSO Selection Algorithm Using Time Variability and Machine Learning: Selection of 1,620 QSO Candidates from MACHO LMC Database

We present a new QSO selection algorithm using a Support Vector Machine (SVM), a supervised classification method, on a set of extracted times series features including period, amplitude, color, and autocorrelation value. We train a model that separates QSOs from variable stars, non-variable stars and microlensing events using 58 known QSOs, 1,629 variable stars and 4,288 non-variables using the MAssive Compact Halo Object (MACHO) database as a training set. To estimate the efficiency and the accuracy of the model, we perform a cross-validation test using the training set. The test shows that the model correctly identifies ~80% of known QSOs with a 25% false positive rate. The majority of the false positives are Be stars. We applied the trained model to the MACHO Large Magellanic Cloud (LMC) dataset, which consists of 40 million lightcurves, and found 1,620 QSO candidates. During the selection none of the 33,242 known MACHO variables were misclassified as QSO candidates. In order to estimate the true false positive rate, we crossmatched the candidates with astronomical catalogs including the Spitzer Surveying the Agents of a Galaxy's Evolution (SAGE) LMC catalog and a few X-ray catalogs. The results further suggest that the majority of the candidates, more than 70%, are QSOs.

astro-ph.IM↗

arXiv2010

Trans-Neptunian Objects with Hubble Space Telescope ACS/WFC

We introduce a novel search technique that can identify trans-neptunian objects in three to five exposures of a pointing within a single Hubble Space Telescope orbit. The process is fast enough to allow the discovery of candidates soon after the data are available. This allows sufficient time to schedule follow up observations with HST within a month. We report the discovery of 14 slow-moving objects found within 5\circ of the ecliptic in archival data taken with the Wide Field Channel of the Advanced Camera for Surveys. The luminosity function of these objects is consistent with previous ground-based and space-based results. We show evidence that the size distribution of both high and low inclination populations is similar for objects smaller than 100 km, as expected from collisional evolution models, while their size distribution differ for brighter objects. We suggest the two populations formed in different parts of the protoplanetary disk and after being dynamically mixed have collisionally evolved together. Among the objects discovered there is an equal mass binary with an angular separation ~ 0."53.

astro-ph.EP↗

arXiv2009

Finding Anomalous Periodic Time Series: An Application to Catalogs of Periodic Variable Stars

Catalogs of periodic variable stars contain large numbers of periodic light-curves (photometric time series data from the astrophysics domain). Separating anomalous objects from well-known classes is an important step towards the discovery of new classes of astronomical objects. Most anomaly detection methods for time series data assume either a single continuous time series or a set of time series whose periods are aligned. Light-curve data precludes the use of these methods as the periods of any given pair of light-curves may be out of sync. One may use an existing anomaly detection method if, prior to similarity calculation, one performs the costly act of aligning two light-curves, an operation that scales poorly to massive data sets. This paper presents PCAD, an unsupervised anomaly detection method for large sets of unsynchronized periodic time-series data, that outputs a ranked list of both global and local anomalies. It calculates its anomaly score for each light-curve in relation to a set of centroids produced by a modified k-means clustering algorithm. Our method is able to scale to large data sets through the use of sampling. We validate our method on both light-curve data and other time series data sets. We demonstrate its effectiveness at finding known anomalies, and discuss the effect of sample size and number of centroids on our results. We compare our method to naive solutions and existing time series anomaly detection methods for unphased data, and show that PCAD's reported anomalies are comparable to or better than all other methods. Finally, astrophysicists on our team have verified that PCAD finds true anomalies that might be indicative of novel astrophysical phenomena.

cs.LG↗

arXiv2009

De-Trending Time Series for Astronomical Variability Surveys

We present a de-trending algorithm for the removal of trends in time series. Trends in time series could be caused by various systematic and random noise sources such as cloud passages, changes of airmass, telescope vibration or CCD noise. Those trends undermine the intrinsic signals of stars and should be removed. We determine the trends from subsets of stars that are highly correlated among themselves. These subsets are selected based on a hierarchical tree clustering algorithm. A bottom-up merging algorithm based on the departure from normal distribution in the correlation is developed to identify subsets, which we call clusters. After identification of clusters, we determine a trend per cluster by weighted sum of normalized light-curves. We then use quadratic programming to de-trend all individual light-curves based on these determined trends. Experimental results with synthetic light-curves containing artificial trends and events are presented. Results from other de-trending methods are also compared. The developed algorithm can be applied to time series for trend removal in both narrow and wide field astronomy.

astro-ph↗

arXiv2009

A Bayesian approach to the analysis of time symmetry in light curves: Reconsidering Scorpius X-1 occultations

We present a new approach to the analysis of time symmetry in light curves, such as those in the x-ray at the center of the Scorpius X-1 occultation debate. Our method uses a new parameterization for such events (the bilogistic event profile) and provides a clear, physically relevant characterization of each event's key features. We also demonstrate a Markov Chain Monte Carlo algorithm to carry out this analysis, including a novel independence chain configuration for the estimation of each event's location in the light curve. These tools are applied to the Scorpius X-1 light curves presented in Chang et al. (2007), providing additional evidence based on the time series that the events detected thus far are most likely not occultations by TNOs.

astro-ph.IM↗

arXiv2009

A Search for Occultations of Bright Stars by Small Kuiper Belt Objects using Megacam on the MMT

We conducted a search for occultations of bright stars by Kuiper Belt Objects (KBOs) to estimate the density of sub-km KBOs in the sky. We report here the first results of this occultation survey of the outer solar system conducted in June 2007 and June/July 2008 at the MMT Observatory using Megacam, the large MMT optical imager. We used Megacam in a novel shutterless continuous--readout mode to achieve high precision photometry at 200 Hz. We present an analysis of 220 star hours at signal-to-noise ratio of 25 or greater. The survey efficiency is greater than 10% for occultations by KBOs of diameter d>=0.7 km, and we report no detections in our dataset. We set a new 95% confidence level upper limit for the surface density Σ_N(d) of KBOs larger than 1 km: Σ_N(d>=1 km) <= 2.0e8 deg^-2, and for KBOs larger than 0.7 km Σ_N(d>= 0.7 km) <= 4.8e8 deg^-2.

astro-ph.EP↗

arXiv2009

Reverberation in the UV-Optical Continuum Brightness Fluctuations of MACHO Quasar 13.5962.237

We examine the nature of brightness fluctuations in the UV-Optical spectral region of an ordinary quasar with 881 optical brightness measurements made during the epoch 1993 - 1999. We find evidence for systematic trends having the character of a pattern of reverberations following an initial disturbance. The initial pulses have brightness increases of order 20% and pulse widths of 50 days, and the reverberations have typical amplitudes of 12% with longer mean pulse widths of order 80 days and pulse separations of order 90 days. The repeat pattern occurs over the same time scales whether the initial disturbance is a brightening or fading. The lags of the pulse trains are comparable to the lags seen previously in reverberation of the broad blue-shifted emission lines following brightness disturbances in Seyfert galaxies, when allowance is made for the mass of the central object. In addition to the burst pulse trains, we find evidence for a semi-periodicity with a time scale of 2 years. These strong patterns of brightness fluctuations suggest a method of discovering quasars from photometric monitoring alone, with data of the quality expected from large brightness monitoring programs like Pann-Stars and LSST.

astro-ph.GA↗

arXiv2009

Event Discovery in Time Series

The discovery of events in time series can have important implications, such as identifying microlensing events in astronomical surveys, or changes in a patient's electrocardiogram. Current methods for identifying events require a sliding window of a fixed size, which is not ideal for all applications and could overlook important events. In this work, we develop probability models for calculating the significance of an arbitrary-sized sliding window and use these probabilities to find areas of significance. Because a brute force search of all sliding windows and all window sizes would be computationally intractable, we introduce a method for quickly approximating the results. We apply our method to over 100,000 astronomical time series from the MACHO survey, in which 56 different sections of the sky are considered, each with one or more known events. Our method was able to recover 100% of these events in the top 1% of the results, essentially pruning 99% of the data. Interestingly, our method was able to identify events that do not pass traditional event discovery procedures.

astro-ph.IM↗

arXiv2007

Eclipsing binary stars in the Large and Small Magellanic Clouds from the MACHO project: The Sample

We present a new sample of 4634 eclipsing binary stars in the Large Magellanic Cloud (LMC), expanding on a previous sample of 611 objects and a new sample of 1509 eclipsing binary stars in the Small Magellanic Cloud (SMC), that were identified in the light curve database of the MACHO project. We perform a cross correlation with the OGLE-II LMC sample, finding 1236 matches. A cross correlation with the OGLE-II SMC sample finds 698 matches. We then compare the LMC subsamples corresponding to center and the periphery of the LMC and find only minor differences between the two populations. These samples are sufficiently large and complete that statistical studies of the binary star populations are possible.

astro-ph↗

arXiv2005

Fast identification of transits from light-curves

We present an algorithm that allows fast and efficient detection of transits, including planetary transits, from light-curves. The method is based on building an ensemble of fiducial models and compressing the data using the MOPED algorithm. We describe the method and demonstrate its efficiency by finding planet-like transits in simulated Pan-STARRS light-curves. We show that that our method is independent of the size of the search space of transit parameters. In large sets of light-curves, we achieve speed up factors of order of $10^{8}$ times over the full $\chi2$ search. We discuss how the algorithm can be used in forthcoming large surveys like Pan-STARRS and LSST and how it may be optimized for future space missions like Kepler and COROT where most of the processing must be done on board.

astro-ph↗

arXiv1997

Nucleon-Antinucleon Interaction from the Skyrme Model: II. Beyond the Product Ansatz

We calculate the full static interaction of a Skyrmion and an anti-Skyrmion as a function of separation and relative grooming. From this, using projection methods and Born-Oppenheimer mixing, we obtain the nucleon antinucleon interaction. We find agreement with the major features of the empirical interaction including the strong central attraction and the sharp onset of annihilation at about 1 fm.

nucl-th↗

arXiv1997

Application of the Kerman-Klein method to the solution of a spherical shell model for a deformed rare-earth nucleus

Core-particle coupling models are made viable by assuming that core properties such as matrix elements of multipole and pairing operators and excitation spectra are known independently. From the completeness relation, it is seen, however, that these quantities are themselves algebraic functions of the calculated core-particle amplitudes. For the deformed rare-earth nucleus 158Gd, we find that these sum rules are well-satisfied for the ground state band, implying that we have found a self-consistent solution of the non-linear Kerman-Klein equations.

nucl-th↗

arXiv1996

Possible solution of the Coriolis attenuation problem

The most consistently useful simple model for the study of odd deformed nuclei, the particle-rotor model (strong coupling limit of the core-particle coupling model) has nevertheless been beset by a long-standing problem: It is necessary in many cases to introduce an ad hoc parameter that reduces the size of the Coriolis interaction coupling the collective and single-particle motions. Of the numerous suggestions put forward for the origin of this supplementary interaction, none of those actually tested by calculations has been accepted as the solution of the problem. In this paper we seek a solution of the difficulty within the framework of a general formalism that starts from the spherical shell model and is capable of treating an arbitrary linear combination of multipole and pairing forces. With the restriction of the interaction to the familiar sum of a quadrupole multipole force and a monopole pairing force, we have previously studied a semi-microscopic version of the formalism whose framework is nevertheless more comprehensive than any previously applied to the problem. We obtained solutions for low-lying bands of several strongly deformed odd rare earth nuclei and found good agreement with experiment, except for an exaggerated staggering of levels for K=1/2 bands, which can be understood as a manifestation of the Coriolis attenuation problem. We argue that within the formalism utilized, the only way to improve the physics is to add interactions to the model Hamiltonian. We verify that by adding a magnetic dipole interaction of essentially fixed strength, we can fit the K=1/2 bands without destroying the agreement with other bands. In addition we show that our solution also fits 163Er, a classic test case of Coriolis attenuation that we had not previously studied.

nucl-th↗

arXiv1996

Derivation and assessment of strong coupling core-particle model from the Kerman-Klein-Dönau-Frauendorf theory

We review briefly the fundamental equations of a semi-microscopic core-particle coupling method that makes no reference to an intrinsic system of coordinates. We then demonstrate how an intrinsic system can be introduced in the strong coupling limit so as to yield a completely equivalent formulation. It is emphasized that the conventional core-particle coupling calculation introduces a further approximation that avoids what has hitherto been the most time-consuming feature of the full theory, and that this approximation can be introduced either in the intrinsic system, the usual case, or in the laboratory system, our preference. A new algorithm is described for the full theory that largely removes the difference in complexity between the two types of calculation. Comparison of the full and approximate theories for some representative cases provides a basis for the assessment of the accuracy of the traditional approach. We find that for well-deformed nuclei, e.g. 157Gd and 157Tb, the core-coupling method and the full theory give similar results.

nucl-th↗

arXiv1996

Application of a semi-microscopic core-particle coupling method to the backbending in odd deformed nuclei

In two previous papers, the Kerman-Klein-Donau-Frauendorf (KKDF) model was used to study rotational bands of odd deformed nuclei. Here we describe backbending for odd nuclei using the same model. The backbending in the neighboring even nuclei is described by a phenomenological two band model, and this core is then coupled to a large single-particle space, as in our previous work. The results obtained for energies and M1 transition rates are compared with experimental data for 165Lu and for energies alone to the experimental data for 179W. For the case of 165Lu comparison is also made with previous theoretical work.

nucl-th↗

arXiv1995

Further application of a semi-microscopic core-particle coupling method to the properties of Gd155,157, and Dy159

In a previous paper a semi-microscopic core-particle coupling method that includes the conventional strong coupling core-particle model as a limiting case, was applied to spectra and electromagnetic properties of several well-deformed odd nuclei. This work, coupled a large single-particle space to the ground state bands of the neighboring even cores. In this paper, we generalize the theory to include excited bands of the cores, such as beta and gamma bands, and thereby show that the resulting theory can account for the location and structure of all bands up to about 1.5 MeV.

nucl-th↗

arXiv1993

Calculation of the properties of the rotational bands of $^{155,157}$Gd

We reexamine the long-standing problem of the microscopic derivation of a particle-core coupling model. We base our research on the Klein-Kerman approach, as amended by Dönau and Frauendorf. We describe the formalism to calculate energy spectra and transition strengths in some detail. We apply our formalism to the rotational nuclei $^{155,157}$Gd, where recent experimental data requires an explanation. We find no clear evidence of a need for Coriolis attenuation.

nucl-th↗