arXiv subjects

Steven Bloemen

Publications and source records attributed to Steven Bloemen.

36 records · Page 2Linked to original sources

arXiv2018

Fast in-database cross-matching of high-cadence, high-density source lists with an up-to-date sky model

Coming high-cadence wide-field optical telescopes will image hundreds of thousands of sources per minute. Besides inspecting the near real-time data streams for transient and variability events, the accumulated data archive is a wealthy laboratory for making complementary scientific discoveries. The goal of this work is to optimise column-oriented database techniques to enable the construction of a full-source and light-curve database for large-scale surveys, that is accessible by the astronomical community. We adopted LOFAR's Transients Pipeline as the baseline and modified it to enable the processing of optical images that have much higher source densities. The pipeline adds new source lists to the archive database, while cross-matching them with the known cataloged sources in order to build a full light-curve archive. We investigated several techniques of indexing and partitioning the largest tables, allowing for faster positional source look-ups in the cross matching algorithms. We monitored all query run times in long-term pipeline runs where we processed a subset of IPHAS data that have image source density peaks over $170,000$ per field of view ($500,000$ deg$^{-2}$). Our analysis demonstrates that horizontal table partitions of declination widths of one-degree control the query run times. Usage of an index strategy where the partitions are densily sorted according to source declination yields another improvement. Most queries run in sublinear time and a few (<20%) run in linear time, because of dependencies on input source-list and result-set size. We observed that for this logical database partitioning schema the limiting cadence the pipeline achieved with processing IPHAS data is 25 seconds.

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arXiv2017

Convolutional Neural Networks for Transient Candidate Vetting in Large-Scale Surveys

Current synoptic sky surveys monitor large areas of the sky to find variable and transient astronomical sources. As the number of detections per night at a single telescope easily exceeds several thousand, current detection pipelines make intensive use of machine learning algorithms to classify the detected objects and to filter out the most interesting candidates. A number of upcoming surveys will produce up to three orders of magnitude more data, which renders high-precision classification systems essential to reduce the manual and, hence, expensive vetting by human experts. We present an approach based on convolutional neural networks to discriminate between true astrophysical sources and artefacts in reference-subtracted optical images. We show that relatively simple networks are already competitive with state-of-the-art systems and that their quality can further be improved via slightly deeper networks and additional preprocessing steps -- eventually yielding models outperforming state-of-the-art systems. In particular, our best model correctly classifies about 97.3% of all 'real' and 99.7% of all 'bogus' instances on a test set containing 1,942 'bogus' and 227 'real' instances in total. Furthermore, the networks considered in this work can also successfully classify these objects at hand without relying on difference images, which might pave the way for future detection pipelines not containing image subtraction steps at all.

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arXiv2016

Physics Of Eclipsing Binaries. II. Towards the Increased Model Fidelity

The precision of photometric and spectroscopic observations has been systematically improved in the last decade, mostly thanks to space-borne photometric missions and ground-based spectrographs dedicated to finding exoplanets. The field of eclipsing binary stars strongly benefited from this development. Eclipsing binaries serve as critical tools for determining fundamental stellar properties (masses, radii, temperatures and luminosities), yet the models are not capable of reproducing observed data well either because of the missing physics or because of insufficient precision. This led to a predicament where radiative and dynamical effects, insofar buried in noise, started showing up routinely in the data, but were not accounted for in the models. PHOEBE (PHysics Of Eclipsing BinariEs; http://phoebe-project.org) is an open source modeling code for computing theoretical light and radial velocity curves that addresses both problems by incorporating missing physics and by increasing the computational fidelity. In particular, we discuss triangulation as a superior surface discretization algorithm, meshing of rotating single stars, light time travel effect, advanced phase computation, volume conservation in eccentric orbits, and improved computation of local intensity across the stellar surfaces that includes photon-weighted mode, enhanced limb darkening treatment, better reflection treatment and Doppler boosting. Here we present the concepts on which PHOEBE is built on and proofs of concept that demonstrate the increased model fidelity.

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arXiv2016

PTF1 J082340.04+081936.5: A hot subdwarf B star with a low mass white dwarf companion in an 87 minute orbit

We present the discovery of the hot subdwarf B star (sdB) binary PTF1 J082340.04+081936.5. The system has an orbital period P$_{\rm orb}=87.49668(1)$ min (0.060761584(10) days), making it the second-most compact sdB binary known. The lightcurve shows ellipsoidal variations. Under the assumption that the sdB primary is synchronized with the orbit, we find a mass $M_{\rm sdB}=0.45^{+0.09}_{-0.07}$ M$_\odot$, a companion white dwarf mass $M_{\rm WD}=0.46^{+0.12}_{-0.09}$ M$_\odot$ and a mass ratio $q = \frac{M_{\rm WD}}{M_{\rm sdB}}=1.03^{+0.10}_{-0.08}$. The future evolution was calculated using the MESA stellar evolution code. Adopting a canonical sdB mass of $M_{\rm sdB}=0.47$ M$_\odot$, we find that the sdB still burns helium at the time it will fill its Roche lobe if the orbital period was less than 106 min at the exit from the last common envelope phase. For longer common envelope exit periods the sdB will have stopped burning helium and turned into a C/O white dwarf at the time of contact. Comparing the spectroscopically derived log(g) and $T_{\rm eff}$ with our MESA models, we find that an sdB model with a hydrogen envelope mass of $5\times10^{-4} M_\odot$ matches the measurements at a post-common envelope age of 94 Myr, corresponding to a post-common envelope orbital period of 109 min which is close to the limit to start accretion while the sdB is still burning helium.

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arXiv2016

PLC-controlled cryostats for the BlackGEM and MeerLICHT detectors

BlackGEM (BG) is an array of telescopes, currently under development at the Radboud University Nijmegen and at NOVA. It targets the detection of the optical counterparts of gravitational waves. The first 3 BG telescopes are planned to be installed in 2018 at the La Silla observatory. A single prototype telescope, named MeerLICHT, will already be commissioned early 2017 in Sutherland to provide an optical complement for the MeerKAT radio array. The BG array consists of, initially, a set of 3 robotic 65-cm wide-field telescopes. Each telescope is equipped with a single STA1600 CCD detector with 10.5k x 10.5k 9-mum pixels that covers a 2.7 square degrees field of view. The cryostats for housing these detectors are developed and built at the KU Leuven University. The operational model of BG requires long periods of reliable ands-off operation. Therefore, we designed the cryostats for long vacuum hold time and we make use of a closed-cycle cooling system, based on Polycold PCC Joule-Thomson coolers. A single programmable logic controller (PLC) controls the cryogenic systems of several BG telescopes simultaneously, resulting in a highly reliable, cost-efficient and maintenance-friendly system. PLC-based cryostat control offers some distinct advantages, especially for a robotic facility. Apart of temperature monitoring and control, the PLC also monitors the vacuum quality, the power supply and the status of the PCC coolers (compressor power consumption and temperature, pressure in the gas lines, etc.). Furthermore, it provides an alarming system and safe and reproducible procedures for automatic cool down and warm up. The communication between PLC and higher-level software takes place via the OPC-UA protocol, offering a simple to implement, yet very powerful interface. Finally, a touch-panel display on the PLC provides the operator with a user-friendly and robust technical interface.

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arXiv2016

Tiling strategies for optical follow-up of gravitational wave triggers by wide field of view telescopes

Binary neutron stars are among the most promising candidates for joint gravitational-wave and electromagnetic astronomy. The goal of this work is to investigate the strategy of using gravitational wave sky-localizations for binary neutron star systems, to search for electromagnetic counterparts using wide field of view optical telescopes. We examine various strategies of scanning the gravitational wave sky-localizations on the mock 2015-16 gravitational-wave events. We propose an optimal tiling-strategy that would ensure the most economical coverage of the gravitational wave sky-localization, while keeping in mind the realistic constrains of transient optical astronomy. Our analysis reveals that the proposed tiling strategy improves the sky-localization coverage over naive contour-covering method. The improvement is more significant for observations conducted using larger field of view telescopes, or for observations conducted over smaller confidence interval of gravitational wave sky-localization probability distribution. Next, we investigate the performance of the tiling strategy for telescope arrays and compare their performance against monolithic giant field of view telescopes. We observed that distributing the field of view of the telescopes into arrays of multiple telescopes significantly improves the coverage efficiency by as much as 50% over a single large FOV telescope in 2016 localizations while scanning around 100 sq. degrees. Finally, we studied the ability of optical counterpart detection by various types of telescopes. In Our analysis for a range of wide field-of-view telescopes we found improvement in detection upon sacrificing coverage of localization in order to achieve greater observation depth for very large field-of-view - small aperture telescopes, especially if the intrinsic brightness of the optical counterparts are weak.

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arXiv2016

Kepler Eclipsing Binary Stars. VIII. Identification of False Positive Eclipsing Binaries and Re-extraction of New Light Curves

The Kepler Mission has provided unprecedented, nearly continuous photometric data of $\sim$200,000 objects in the $\sim$105 deg$^{2}$ field of view from the beginning of science operations in May of 2009 until the loss of the second reaction wheel in May of 2013. The Kepler Eclipsing Binary Catalog contains information including but not limited to ephemerides, stellar parameters and analytical approximation fits for every known eclipsing binary system in the Kepler Field of View. Using Target Pixel level data collected from Kepler in conjunction with the Kepler Eclipsing Binary Catalog, we identify false positives among eclipsing binaries, i.e. targets that are not eclipsing binaries themselves, but are instead contaminated by eclipsing binary sources nearby on the sky and show eclipsing binary signatures in their light curves. We present methods for identifying these false positives and for extracting new light curves for the true source of the observed binary signal. For each source, we extract three separate light curves for each quarter of available data by optimizing the signal-to-noise ratio, the relative percent eclipse depth and the flux eclipse depth. We present 289 new eclipsing binaries in the Kepler Field of View that were not targets for observation, and these have been added to the Catalog. An online version of this Catalog with downloadable content and visualization tools is maintained at http://keplerEBs.villanova.edu.

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arXiv2015

Kepler Eclipsing Binary Stars. VII. The Catalog of Eclipsing Binaries Found in the Entire Kepler Data-Set

The primary Kepler Mission provided nearly continuous monitoring of ~200,000 objects with unprecedented photometric precision. We present the final catalog of eclipsing binary systems within the 105 square degree Kepler field of view. This release incorporates the full extent of the data from the primary mission (Q0-Q17 Data Release). As a result, new systems have been added, additional false positives have been removed, ephemerides and principal parameters have been recomputed, classifications have been revised to rely on analytical models, and eclipse timing variations have been computed for each system. We identify several classes of systems including those that exhibit tertiary eclipse events, systems that show clear evidence of additional bodies, heartbeat systems, systems with changing eclipse depths, and systems exhibiting only one eclipse event over the duration of the mission. We have updated the period and galactic latitude distribution diagrams and included a catalog completeness evaluation. The total number of identified eclipsing and ellipsoidal binary systems in the Kepler field of view has increased to 2878, 1.3% of all observed Kepler targets. An online version of this catalog with downloadable content and visualization tools is maintained at http://keplerEBs.villanova.edu.

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arXiv2015

BEER analysis of Kepler and CoRoT light curves: IV. Discovery of four new low-mass white dwarf companions in the Kepler data

We report the discovery of four short-period eclipsing systems in the Kepler light curves, consisting of an A-star primary and a low-mass white dwarf (WD) secondary (dA+WD) - KIC 4169521, KOI-3818, KIC 2851474, and KIC 9285587. The systems show BEaming, Ellipsoidal and Reflection (BEER) phase modulations together with primary and secondary eclipses. These add to the 6 Kepler and 18 WASP short-period eclipsing dA+WD binaries that were previously known. The light curves, together with follow-up spectroscopic observations, allow us to derive the masses, radii, and effective temperatures of the two components of the four systems. The orbital periods, of 1.17-3.82 days, and WD masses, of 0.19-0.22 Msun, are similar to those of the previously known systems. The WD radii of KOI-3818, KIC 2851474, and KIC 9285587 are 0.026, 0.035, and 0.026 Rsun, respectively, the smallest WD radii derived so far for short-period eclipsing dA+WD binaries. These three binaries extend the previously known population to older systems with cooler and smaller WD secondaries. KOI-3818 displays evidence for a fast-rotating primary and a minute but significant eccentricity of ~0.0015. These features are probably the outcome of the mass-transfer process.

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arXiv2015

Looking at the bright side - The story of AA Dor as revealed by its cool companion

Irradiation effects in close binaries are crucial for a reliable determination of system parameters and understanding the close binary evolution. We study irradiated light originating from the low mass component of an eclipsing system comprising a hot subdwarf primary and a low mass companion, to precisely interpret their high precision photometric and spectroscopic data, and accurately determine their system and surface parameters. We re-analyse the archival VLT/UVES spectra of AA Dor system where irradiation features have already been detected. After removing the predominant contribution of the hot subdwarf primary, the residual spectra reveal more than 100 emission lines from the heated side of the secondary with maximum intensity close to the phases around secondary eclipse. We analyse 22 narrow emission lines of the irradiated secondary, mainly of OII, with a few CII lines. Their phase profiles constrain the emission region of the heated side to a radius $\geq$ 95% of the radius of the secondary. The shape of their velocity profiles reveals two distinct asymmetry features one at the quadrature and the other at the secondary eclipse. We identify more than 70 weaker emission lines originating from HeI, NII, SiIII, CaII and MgII. We correct the radial velocity semi-amplitude of the center-of-light to the centre-of-mass of the secondary and calculate accurate masses of both components. The resulting masses $M_{1}$=0.46 $\pm$ 0.01$M_{\odot}$ and $M_{2}$=0.079 $\pm$ 0.002$M_{\odot}$ are in perfect accordance with those of a canonical hot subdwarf primary and a low mass star just at the substellar limit for the companion. We compute a first generation atmosphere model of the irradiated low mass secondary, which matches the observed spectrum well. We find an indication of an extended atmosphere of the irradiated secondary star.

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arXiv2014

The APOKASC Catalog: An Asteroseismic and Spectroscopic Joint Survey of Targets in the Kepler Fields

We present the first APOKASC catalog of spectroscopic and asteroseismic properties of 1916 red giants observed in the Kepler fields. The spectroscopic parameters provided from the Apache Point Observatory Galactic Evolution Experiment project are complemented with asteroseismic surface gravities, masses, radii, and mean densities determined by members of the Kepler Asteroseismology Science Consortium. We assess both random and systematic sources of error and include a discussion of sample selection for giants in the Kepler fields. Total uncertainties in the main catalog properties are of order 80 K in Teff , 0.06 dex in [M/H], 0.014 dex in log g, and 12% and 5% in mass and radius, respectively; these reflect a combination of systematic and random errors. Asteroseismic surface gravities are substantially more precise and accurate than spectroscopic ones, and we find good agreement between their mean values and the calibrated spectroscopic surface gravities. There are, however, systematic underlying trends with Teff and log g. Our effective temperature scale is between 0-200 K cooler than that expected from the Infrared Flux Method, depending on the adopted extinction map, which provides evidence for a lower value on average than that inferred for the Kepler Input Catalog (KIC). We find a reasonable correspondence between the photometric KIC and spectroscopic APOKASC metallicity scales, with increased dispersion in KIC metallicities as the absolute metal abundance decreases, and offsets in Teff and log g consistent with those derived in the literature. We present mean fitting relations between APOKASC and KIC observables and discuss future prospects, strengths, and limitations of the catalog data.

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arXiv2014

Kepler Eclipsing Binary Stars. V. Identification of 31 Eclipsing Binaries in the K2 Engineering Data-set

Over 2500 eclipsing binaries were identified and characterized from the ultra-precise photometric data provided by the Kepler space telescope. Kepler is now beginning its second mission, K2, which is proving to again provide ultra-precise photometry for a large sample of eclipsing binary stars. In the 1951 light curves covering 12 days in the K2 engineering data-set, we have identified and determined the ephemerides for 31 eclipsing binaries that demonstrate the capabilities for eclipsing binary science in the upcoming campaigns in K2. Of those, 20 are new discoveries. We describe both manual and automated approaches to harvesting the complete set of eclipsing binaries in the K2 data, provide identifications and details for the full set of eclipsing binaries present in the engineering data-set, and discuss the prospects for application of eclipsing binary searches in the K2 mission.

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arXiv2013

Large Frame-Transfer Detectors for the MAIA Imager

MAIA, the Mercator Advanced Imager for Asteroseismology, is a new fast-cadence 3-channel photometric instrument. It is installed on the 1.2-m Mercator telescope at the Roque de Los Muchachos Observatory in La Palma, Spain. MAIA comprises 3 cameras that simultaneously observe the same 9.4 x 14.1 arcmin field in 3 different colour bands (u, g and r). Each camera is based on a very large frame-transfer detector (CCD42-C0) of 2kx6k pixels, specially designed for rapid time-series photometry. These CCDs were originally developed by e2v for ESA's cancelled Eddington space mission and are now on permanent loan to the Institute of Astronomy of the KU Leuven, Belgium. The acquisition system of MAIA uses a single ARC GEN-III controller, custom programmed to allow differing exposure times for each of the three CCDs. Predefined sequences synchronize each read-out with the shortest integration time. Detectors that are not read-out at the end of an exposure continue integrating and can be read-out in one of the subsequent cycles. This read method takes full advantage of the frame-transfer functionality of the CCD42-C0 detectors and allows optimisation of the exposure times for each wavelength band. This then gives a similar exposure depth in each of the 3 arms despite the fact that the UV channel typically sees much less flux. We present the CCD42-C0 detectors, their characterisation, including a thorough analysis of their non-linearity, and the MAIA data-acquisition system.

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arXiv2013

MAIA, a three-channel imager for asteroseismology: instrument design

MAIA, an acronym for Mercator Advanced Imager for Asteroseismology, is a three-channel instrument that targets fast-cadence three-colour photometry, installed at the 1.2-m Mercator telescope at the Roque de los Muchachos at La Palma (Canary Islands, Spain). This instrument observes a 9.4x14.1arcmin2 Field-of-View simultaneously in three wavelength bands on three large frame-transfer CCDs. These detectors were developed for ESA's cancelled Eddington space mission and were offered on permanent loan to the Institute of Astronomy (KU Leuven, Belgium). MAIA uses its own ugr photometric system that is a crude approximation of the SDSS system. The instrument is designed to perform multi-colour observations for asteroseismology, with specific emphasis on subdwarf and white dwarf single and binary stars. We describe the design of the instrument, discuss key components, and report on its performance and first results.

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arXiv2013

Kepler's Unparalleled Exploration of the Time Dimension

We show that the Kepler spacecraft in two-reaction wheel mode of operation is very well suited for the study of eclipsing binary star systems. Continued observations of the Kepler field will provide the most enduring and long-term valuable science. It will enable the discovery and characterization of eclipsing binaries with periods greater than 1 year - these are the most important, yet least understood binaries for habitable-zone planet background considerations. The continued mission will also enable the investigation of hierarchical multiple systems (discovered through eclipse timing variations), and provide drastically improved orbital parameters for circumbinary planetary systems.

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arXiv2012

Photometrically derived masses and radii of the planet and star in the TrES-2 system

We measure the mass and radius of the star and planet in the TrES-2 system using 2.7 years of observations by the Kepler spacecraft. The light curve shows evidence for ellipsoidal variations and Doppler beaming on a period consistent with the orbital period of the planet with amplitudes of 2.79+0.44-0.62 and 3.44+0.32-0.37 parts per million (ppm) respectively, and a difference between the day and night side planetary flux of 3.41+0.55-0.82 ppm. We present an asteroseismic analysis of solar-like oscillations on TrES-2A which we use to calculate the stellar mass of 0.94+/-0.05 MSun and radius of 0.95+/-0.02 RSun. Using these stellar parameters, a transit model fit and the phase curve variations, we determine the planetary radius of 1.162+0.020-0.024 RJup and derive a mass for TrES-2b from the photometry of 1.44+/-0.21 MJup. The ratio of the ellipsoidal variation to the Doppler beaming amplitudes agrees to better than 2σ with theoretical predications, while our measured planet mass and radius agree within 2-σ of previously published values based on spectroscopic radial velocity measurements. We measure a geometric albedo of 0.0136+0.0022-0.0033 and an occultation (secondary eclipse) depth of 6.5+1.7-1.8 ppm which we combined with the day/night planetary flux ratio to model the atmosphere of TReS-2b. We find an atmosphere model that contains a temperature inversion is strongly preferred. We hypothesize that the Kepler bandpass probes a significantly greater atmospheric depth on the night side relative to the day side.

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arXiv2012

The Transiting Circumbinary Planets Kepler-34 and Kepler-35

Most Sun-like stars in the Galaxy reside in gravitationally-bound pairs of stars called "binary stars". While long anticipated, the existence of a "circumbinary planet" orbiting such a pair of normal stars was not definitively established until the discovery of Kepler-16. Incontrovertible evidence was provided by the miniature eclipses ("transits") of the stars by the planet. However, questions remain about the prevalence of circumbinary planets and their range of orbital and physical properties. Here we present two additional transiting circumbinary planets, Kepler-34 and Kepler-35. Each is a low-density gas giant planet on an orbit closely aligned with that of its parent stars. Kepler-34 orbits two Sun-like stars every 289 days, while Kepler-35 orbits a pair of smaller stars (89% and 81% of the Sun's mass) every 131 days. Due to the orbital motion of the stars, the planets experience large multi-periodic variations in incident stellar radiation. The observed rate of circumbinary planets implies > ~1% of close binary stars have giant planets in nearly coplanar orbits, yielding a Galactic population of at least several million.

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arXiv2012

Kepler Eclipsing Binary Stars. III. Classification of Kepler Eclipsing Binary Light Curves with Locally Linear Embedding

We present an automated classification of 2165 \textit{Kepler} eclipsing binary (EB) light curves that accompanied the second \textit{Kepler} data release. The light curves are classified using Locally Linear Embedding, a general nonlinear dimensionality reduction tool, into morphology types (detached, semi-detached, overcontact, ellipsoidal). The method, related to a more widely used Principal Component Analysis, produces a lower-dimensional representation of the input data while preserving local geometry and, consequently, the similarity between neighboring data points. We use this property to reduce the dimensionality in a series of steps to a one-dimensional manifold and classify light curves with a single parameter that is a measure of "detachedness" of the system. This fully automated classification correlates well with the manual determination of morphology from the data release, and also efficiently highlights any misclassified objects. Once a lower-dimensional projection space is defined, the classification of additional light curves runs in a negligible time and the method can therefore be used as a fully automated classifier in pipeline structures. The classifier forms a tier of the \textit{Kepler} EB pipeline that pre-processes light curves for the artificial intelligence based parameter estimator.

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