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On Estimating Non-Uniform Density Distributions Using N Nearest Neighbors

Woźniak P. R., Kruszewski A.

Acta Astronomica

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2012

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Vol. 62, No. 4

409--417

EN

We consider density estimators based on the nearest neighbors method applied to discrete point distributions in spaces of arbitrary dimensionality. If the density is constant, the volume of a hypersphere centered at a random location is proportional to the expected number of points falling within the hypersphere radius. The distance to the N-th nearest neighbor alone is then a sufficient statistic for the density. In the non-uniform case the proportionality is distorted. We model this distortion by normalizing hypersphere volumes to the largest one and expressing the resulting distribution in terms of the Legendre polynomials. Using Monte Carlo simulations we show that this approach can be used to effectively address the trade-off between smoothing bias and estimator variance for sparsely sampled distributions.

2

The Optical Gravitational Lensing Experiment. Eclipsing Binary Stars in the Small Magellanic Cloud

Wyrzykowski Ł., Udalski A., Kubiak M., Szymański M. K., Żebruń K., Soszyński I., Woźniak P. R., Pietrzyński G., Szewczyk O.

Acta Astronomica

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2004

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Vol. 54, No. 1

1--17

EN

We present new version of the OGLE-II catalog of eclipsing binary stars detected in the Small Magellanic Cloud, based on Difference Image Analysis catalog of variable stars in the Magellanic Clouds containing data collected from 1997 to 2000. We found 1351 eclipsing binary stars in the central 2.4 square degree area of the SMC. 455 stars are newly discovered objects, not found in the previous release of the catalog. The eclipsing objects were selected with the automatic search algorithm based on the artificial neural network. The full catalog is accessible from the OGLE Internet archive.

3

The Optical Gravitational Lensing Experiment. Eclipsing Binary Stars in the Large Magellanic Cloud

Wyrzykowski Ł., Udalski A., Kubiak M., Szymański M., Żebruń K., Soszyński I., Woźniak P. R., Pietrzyński G., Szewczyk O.

Acta Astronomica

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2003

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Vol. 53, No. 1

1--25

EN

We present the catalog of 2580 eclipsing binary stars detected in 4.6 square degree area of the central parts of the Large Magellanic Cloud. The photometric data were collected during the second phase of the OGLE microlensing search from 1997 to 2000. The eclipsing objects were selected with the automatic search algorithm based on an artificial neural network. Basic statistics of eclipsing stars are presented. Also, the list of 36 candidates of detached eclipsing binaries for spectroscopic study and for precise LMC distance determination is provided. The full catalog is accessible from the OGLE Internet archive.

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The Optical Gravitational Lensing Experiment. Catalog of High Proper Motion Stars Towards the Magellanic Clouds

Soszyński I., Żebruń K., Udalski A., Woźniak P. R., Szymański M., Kubiak M., Pietrzyński G., Szewczyk O., Wyrzykowski Ł.

Acta Astronomica

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2002

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Vol. 52, No. 2

129--142

EN

We present a Catalog of high proper motion (HPM) stars detected in the foreground of central parts of the Magellanic Clouds. The Catalog contains 2161 objects in the 4.5 square degree area towards the LMC, and 892 HPM stars in the 2.4 square degree area towards the SMC. The Catalog is based on observations collected during four years of the OGLE-II microlensing survey. The Difference Image Analysis (DIA) of the images provided candidate HPM stars with proper motion as small as 4 mas/yr. These appeared as pseudo-variables, and were all measured astrometrically on all CCD images, providing typically about 400 data points per star. The reference frame was defined by the majority of background stars, most of them members of the Magellanic Clouds. The reflex motion due to solar velocity with respect to the local standards of rest is clearly seen. The largest proper motion in our sample is 363 mas/yr. Parallaxes were measured with errors smaller than 20% for several stars.

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Difference Image Analysis of the OGLE-II Bulge Data. III. Catalog of 200 000 Candidate Variable Stars

Woźniak P. R., Udalski A., Szymański M., Kubiak M., Pietrzyński G., Soszyński I., Żebruń K.

Acta Astronomica

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2002

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Vol. 52, No. 2

129--142

EN

We present the first edition of a catalog of variable stars from OGLE-II Galactic bulge data covering 3 years: 1997-1999. Typically 200-300 I band data points are available in 49 fields between -11 and 11 degrees in galactic longitude, totaling roughly 11 square degrees in sky coverage. Photometry was obtained using the Difference Image Analysis (DIA) software and tied to the OGLE data base with the DoPhot package. The present version of the catalog comprises 221 801 light curves. In this preliminary work the level of contamination by spurious detections is still about 10%. Parts of the catalog have only crude calibration, insufficient for distance determinations. The next, fully calibrated, edition will include the data collected in year 2000. The data is accessible via ftp. Due to the data volume, we also distribute DAT tapes upon request.

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The Optical Gravitational Lensing Experiment. Difference Image Analysis of LMC and SMC Data. The Method

Żebruń K., Soszyński I., Woźniak P. R.

Acta Astronomica

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2001

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Vol. 51, No. 4

303--316

EN

We describe the Difference Image Analysis (DIA) algorithms and software used to analyze four years (1997-2000) of OGLE-II photometric monitoring of the Magellanic Clouds, the calibration, the photometric error analysis and the search for variable stars. A preliminary analysis of photometric errors is based on the field LMC_SC2. A full catalog of more than 68 000 variable stars is presented in a separate publication.

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The Optical Gravitational Lensing Experiment. Difference Image Analysis of LMC and SMC Data. The Catalog

Żebruń K., Soszyński I., Woźniak P. R., Udalski A., Kubiak M., Szymański M., Pietrzyński G., Szewczyk O., Wyrzykowski Ł.

Acta Astronomica

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2001

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Vol. 51, No. 4

317--329

EN

We present the first edition of a catalog of variable stars found in the Magellanic Clouds using OGLE-II data obtained during four years: 1997-2000. The catalog covers about 7 square degrees of the sky - 21 fields in the Large Magellanic Cloud and 11 fields in the Small Magellanic Cloud. All variables were found with the Difference Image Analysis (DIA) software. The catalog is divided into two sections. The DC section contains FITS reference images (obtained by co-adding 20 best frames for each field) and profile photometry (DoPhot) of all variable stars on those images. The AC section contains flux variations and magnitudes of detected variable stars obtained with DIA as well as with DoPhot. The errors of magnitude measurements are 0.005 mag for the brightest stars (I<16 mag) then grow to 0.08 mag at 19 mag stars and to 0.3 mag at 20.5 mag. Typically, there are about 400 I-band data points and about 30 V and B-band data points for more than 68 000 variables. The stars with high proper motions were excluded from this catalog and will be presented in a separate paper. A detailed analysis and classification of variable stars will be presented elsewhere. The catalog is available in electronic form via FTP and through WWW interface from the OGLE Internet archive. The FTP catalog contains approximately 2 GB of data.

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Difference Image Analysis of the OGLE-II Bulge Data. II. Microlensing Events

Woźniak P. R., Udalski A., Szymański M., Kubiak M., Pietrzyński G., Soszyński I., Żebruń K.

Acta Astronomica

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2001

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Vol. 51, No. 3

175--219

EN

We present a sample of microlensing events discovered in the Difference Image Analysis (DIA) of the OGLE-II images collected during three observing seasons, 1997-1999. 4424 light curves pass our criteria on the presence of a brightening episode on top of a constant baseline. Among those, 512 candidate microlensing events were selected visually. We designed an automated procedure, which unambiguously selects up to 237 best events. Including eight candidate events recovered by other means, a total of 520 light curves are presented in this work. In addition to microlensing events, the larger sample contains certain types of transients, but is also strongly contaminated by artifacts. All 4424 light curves in the weakly filtered group are available electronically, with the intent of showing the gray zone between microlensing events and variable stars, as well as artifacts, to some extent inevitable in massive data reductions. We welcome suggestions for improving the selection process before the full analysis of complete four seasons of the OGLE-II bulge data. Selection criteria for binary events can also be investigated with our extended sample.

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Difference Image Analysis of the OGLE-II Bulge Data. I. The Method

Woźniak P. R.

Acta Astronomica

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2000

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Vol. 50, No. 4

421--450

EN

We present an implementation of the difference image photometry based on the Alard and Lupton optimal PSF matching algorithm. The most important feature distinguishing this method from the ones using Fourier divisions is that equations are solved in real space and the knowledge of each PSF is not required for determination of the convolution kernel. We evaluate the method and software on 380 GB of OGLE-II bulge microlensing data obtained in 1997-1999 observing seasons. The error distribution is Gaussian to better than 99% with the amplitude only 17% above the photon noise limit for faint stars. Over the entire range of the observed magnitudes the resulting scatter is improved by a factor of 2-3 compared to DoPhot photometry, currently a standard tool for massive stellar photometry in microlensing searches. For testing purposes the photometry of ≈4600 candidate variable stars and sample difference image data are provided for BUL_SC1 field. In the candidate selection process, very few assumptions have been made about the specific types of flux variations, which makes this data set well suited for general variability studies, including the development of the classification schemes.

10

Astrometric Shifts in the OGLE-1 Microlensing Events

Goldberg D. M., Woźniak P. R.

Acta Astronomica

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1998

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Vol. 48, No. 1

19--34

EN

We measure the astrometric shifts of the light centroids for the microlensing events in the OGLE-1 database. Of the 15 consistently detected events, 7 had significant shifts which we were able to measure with a fair degree of confidence. Those events with large shifts are also expected to be unresolved with a background "blend", and thus, we suggest that we have identified events in the OGLE-1 catalog which are strongly blended. Though we concentrate on the OGLE-1 database, and use the DOPHOT package in order to perform our analysis, we suggest that this shift is a generic effect, and should be observable in any crowded field.