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OGLE Collection of Star Clusters. New Objects in the Magellanic Bridge and the Outskirts of the Small Magellanic Cloud

Sitek M., Szymański M. K., Udalski A., Skowron D. M., Kostrzewa-Rutkowska Z., Skowron J., Karczmarek P., Cieślar M., Wyrzykowski Ł., Kozłowski S., Pietrukowicz P., Soszyński I., Mróz P., Pawlak M., Poleski R., Ulaczyk K.

Acta Astronomica

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2017

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Vol. 67, No. 4

363--378

EN

The Magellanic System (MS) encompasses the nearest neighbors of the Milky Way, the Large (LMC) and Small (SMC) Magellanic Clouds, and the Magellanic Bridge (MBR). This system contains a diverse sample of star clusters. Their parameters, such as the spatial distribution, chemical composition and age distribution yield important information about the formation scenario of the whole Magellanic System. Using deep photometric maps compiled in the fourth phase of the Optical Gravitational Lensing Experiment (OGLE-IV) we present the most complete catalog of star clusters in the Magellanic System ever constructed from homogeneous, long time-scale photometric data. In this second paper of the series, we show the collection of star clusters found in the area of about 360 square degrees in the MBR and in the outer regions of the SMC. Our sample contains 198 visually identified star cluster candidates, 75 of which were not listed in any of the previously published catalogs. The new discoveries are mainly young small open clusters or clusters similar to associations.

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Robust Filtering of Artifacts in Difference Imaging for Rapid Transients Detection

Klencki J., Wyrzykowski Ł., Kostrzewa-Rutkowska Z., Udalski A.

Acta Astronomica

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2016

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Vol. 66, No. 1

15--29

EN

Real-time analysis and classification of observational data collected within synoptic sky surveys is a huge challenge due to constant growth of data volumes. Machine learning techniques are often applied in order to perform this task automatically. The current bottleneck of transients detection in most surveys is the process of filtering numerous artifacts from candidate detection. We present a new method for automated artifact filtering based on hierarchical unsupervised classifier employing Self-Organizing Maps (SOMs). The system accepts 97% of real transients and removes 97.5% of artifacts when tested on the OGLE-IV Transient Detection System. The improvement of the artifacts filtering allows for single-frame-based detection of transients within OGLE-IV, which now alerts on transient discoveries in less than 15 minutes from the image acquisition.

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OGLE Collection of Star Clusters. New Objects in the Outskirts of the Large Magellanic Cloud

Sitek M., Szymański M. K., Skowron D. M., Udalski A., Kostrzewa-Rutkowska Z., Skowron J., Karczmarek P., Cieślar M., Wyrzykowski Ł., Kozłowski S., Pietrukowicz P., Soszyński I., Mróz P., Pawlak M., Poleski R., Ulaczyk K.

Acta Astronomica

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2016

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Vol. 66, No. 3

255--267

EN

The Magellanic System (MS), consisting of the Large Magellanic Cloud (LMC), the Small Magellanic Cloud (SMC) and the Magellanic Bridge (MBR), contains diverse sample of star clusters. Their spatial distribution, ages and chemical abundances may provide important information about the history of formation of the whole System. We use deep photometric maps derived from the images collected during the fourth phase of the Optical Gravitational Lensing Experiment (OGLE-IV) to construct the most complete catalog of star clusters in the Large Magellanic Cloud using the homogeneous photometric data. In this paper we present the collection of star clusters found in the area of about 225 square degrees in the outer regions of the LMC. Our sample contains 679 visually identified star cluster candidates, 226 of which were not listed in any of the previously published catalogs. The new clusters are mainly young small open clusters or clusters similar to associations.

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OGLE-IV Real-Time Transient Search

Wyrzykowski Ł., Kostrzewa-Rutkowska Z., Kozłowski S., Udalski A., Poleski R., Skowron J., Blagorodnova N., Kubiak M., Szymański M. K., Pietrzyński G., Soszyński I., Ulaczyk K., Pietrukowicz P., Mróz Michał

Acta Astronomica

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2014

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Vol. 64, No. 3

197--232

EN

We present the design and first results of a real-time search for transients within the 650 sq. deg. area around the Magellanic Clouds, conducted as part of the OGLE-IV project and aimed at detecting supernovae, novae and other events. The average sampling of about four days from September to May, yielded a detection of 238 transients in 2012/2013 and 2013/2014 seasons. The superb photometric and astrometric quality of the OGLE data allows for numerous applications of the discovered transients. We use this sample to prepare and train a Machine Learning-based automated classifier for early light curves, which distinguishes major classes of transients with more than 80% of correct answers. Spectroscopically classified 49 supernovae Type Ia are used to construct a Hubble Diagram with statistical scatter of about 0.3 mag and fill the least populated region of the redshifts range in the Union sample. We investigate the influence of host galaxy environments on supernovae statistics and find the mean host extinction of AI=0.19±0.10 mag and AV=0.39±0.21 mag based on a subsample of supernovae Type Ia. We show that the positional accuracy of the survey is of the order of 0.5 pixels (0.13'') and that the OGLE-IV Transient Detection System is capable of detecting transients within the nuclei of galaxies. We present a few interesting cases of nuclear transients of unknown type. All data on the OGLE transients are made publicly available to the astronomical community via the OGLE website.