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1

Search for Dormant Black Holes in the OGLE Data

Kapusta M., Mróz P.

Acta Astronomica

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2023

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Vol. 73, No. 3

197--226

EN

Light curves of ellipsoidal variables collected by the Optical Gravitational Lensing Experiment (OGLE) were analyzed, in order to search for dormant black hole candidates. After the preselection based on the amplitude of ellipsoidal modulation, each object was investigated by means of the spectral energy distribution fit, which allowed us to select objects that are in close agreement with the spectrum of a single stellar object. After this final step of the preselection process, we were left with only fourteen objects that were then investigated in detail. For each candidate, we estimated basic physical parameters such as temperature, mass, luminosity, and, in some cases, radial velocity semi-amplitude. One of the objects turned out to be a spotted star while the rest are considered black-hole binary candidates. In the end, we present an alternative explanation for the ellipsoidal modulation in the form of contact binaries, which are not only vast in number, contrary to black-hole binaries, but are also in much better agreement with the radial velocity estimates for some of the systems analyzed. Even if the presented arguments suggest a noncompact character of the companion objects, each of them should be observed spectroscopically in order to verify the compact object hypothesis.

2

Candidates for Transiting Planets in OGLE-IV Galactic Bulge Fields

Mróz M., Pietrukowicz P., Poleski R., Udalski A., Szymański M. K., Gromadzki M., Ulaczyk K., Kozłowski S., Skowron J., Skowron D. M., Soszyński I., Mróz P., Ratajczak M., Rybicki K. A., Iwanek P., Wrona M.

Acta Astronomica

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2023

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Vol. 73, No. 2

127--158

EN

We present results of a search for transiting exoplanets in 10-yr long photometry with thousands of epochs taken in the direction of the Galactic bulge. This photometry was collected in the fourth phase of the Optical Gravitational Lensing Experiment (OGLE-IV). Our search covered ≈222 000 stars brighter than I=15.5 mag. Selected transits were verified using a probabilistic method. The search resulted in 99 high-probability candidates for transiting exoplanets. The estimated distances to these targets are between 0.4 kpc and 5.5 kpc, which is a significantly wider range than for previous transit searches. The planets found are Jupiter-size, with the exception of one (named OGLE-TR-1003b) located in the hot Neptune desert. If the candidate is confirmed, it can be important for studies of highly irradiated intermediate-size planets. The existing long-term, high-cadence photometry of our candidates increases the chances of detecting transit timing variations at long timescales. Selected candidates will be observed by the future NASA flagship mission, the Nancy Grace Roman Space Telescope, in its search for Galactic bulge microlensing events, which will further enhance the photometric coverage of these stars.

3

Studying the Physical Parameters of the Stellar Binary System Hip 42455 (HD 73900)

Abu-Dhaim A., Taani A., Tanineah D., Tamimi N., Mardini M., Al-Wardat M.

Acta Astronomica

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2022

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Vol. 72, No. 3

171--181

EN

We present the results of the combined spectrophotometric and dynamic analysis for the close visual binary star Hip 42455 (HD 73900, I 314). A model binary that consists of two main sequence stars with a clear magnitude difference, but completely different estimations of their spectral types, and different parallax measurements given by the four catalogs of the two main precise astrometric missions, Hipparcos and Gaia. The analysis follows Al-Wardat's method for analyzing binary and multiple stellar systems. It investigates the system's physical parameters using the parallax measurements given by the catalogs of Hip 1997, Hip 2007, Gaia DR2, and EDR3. The results were as follows: TeffA=6880±70 K, TeffB=5630±70 K, RA=1.554 R☉, RB=0.929 R☉, MA=1.44±0.15M ☉, MB=0.98±0.13M ☉, while the spectral type for the primary and secondary components are G7V and F2V, respectively. As a result, the new parallax for this system was given as πDyn=25.82±0.85, based on the best fit between the dynamical mass sum and the individual masses. This value lies between the old (1997) and new (2007) Hipparcos measurements, and is a bit far from those given by Gaia DR2 and Gaia EDR3.

4

Roche Modeling and Evolutionary History of Six Low Mass Contact Binary Systems

Alton K. B., Stępień K.

Acta Astronomica

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2021

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Vol. 71, No. 2

123--161

EN

We describe six eclipsing binary systems that were first detected by the ROTSE-I telescope and ultimately shown to be low mass contact binaries (LMCB). New photometric data (B, V and Ic) acquired from V473 Cam, PV Com, EI CVn, V2790 Ori, V2802 Ori and PS Vir were used to calculate an orbital period for each system along with updated ephemerides. All systems were subject to secular changes in orbital period with evidence for a third gravitationally bound body in V473 Cam, EI CVn, V2790 Ori, and PS Vir. Since a total eclipse is observed in all six systems, Roche modeling based on the Wilson-Devinney (WD) code yielded in each case a well-constrained photometric value for the mass ratio. Potential progenitors of these LMCBs were evaluated using an evolutionary model of cool close binaries. The best fits were obtained when the initial detached binaries have orbital periods ranging from 2.70 d to 3.71 d, total masses between 1.36 M and 1.53 M with mass ratios ranging from 2.34 to 4.04. The model progenitors require between 7.75-10.9 Gyr to attain the presently observed orbital period and physical attributes. Collectively, the models show that about half of orbital periods will increase in the future leading to a common envelope resulting from the evolutionary expansion of the accretor whereas the other half will decrease resulting in the overflow of the outer critical Roche surface and an ultimate merger.

5

Massive Search for Spot- and Facula-Crossing Events in 1598 Exoplanetary Transit Light Curves

Baluev R. V., Sokov E. N., Sokova I. A., Shaidulin V. Sh., Veselova A. V., Aitov V. N., Mitiani G. Sh., Valeev A. F., Gadelshin D. R., Gutaev A. G., Beskin G. M., Valyavin G. G., Antonyuk K., Barkaoui K., Gillon M., Jehin E., Delrez L., Guðmundsson S., Dale H. A., Fernández-Lajús E., Di Sisto R. P., Bretton M., Wunsche A., Hentunen V.-P., Shadick S., Jongen Y., Kang W., Kim T., Pakštienė E., Qvam J. K. T., Knight C. R., Guerra P., Marchini A., Salvaggio F., Papini R., Evans P., Salisbury M., Garlitz J., Esseiva N., Ogmen Y., Bosch-Cabot P., Selezneva A., Hinse T. C.

Acta Astronomica

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2021

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Vol. 71, No. 1

25--53

EN

We developed a dedicated statistical test for a massive detection of spot- and facula-crossing anomalies in multiple exoplanetary transit light curves, based on the frequentist p-value thresholding. This test was used to augment our algorithmic pipeline for transit light curves analysis. It was applied to 1598 amateur and professional transit observations of 26 targets being monitored in the EXPANSION project. We detected 109 statistically significant candidate events revealing a roughly 2:1 asymmetry in favor of spots-crossings over faculae-crossings. Although some candidate anomalies likely appear non-physical and originate from systematic errors, such asymmetry between negative and positive events should indicate a physical difference between the frequency of star spots and faculae. Detected spot-crossing events also reveal positive correlation between their amplitude and width, possibly due to spot size correlation. However, the frequency of all detectable crossing events appears just about a few per cent, so they cannot explain excessive transit timing noise observed for several targets.

6

Ground-Based Observations of the ZZ Ceti Star HS~1625+1231

Kalup Cs., Bognár Zs., Sódor Á.

Acta Astronomica

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2021

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Vol. 71, No. 4

281--295

EN

We present the results of our detailed light curve analysis of the ZZ Ceti star HS 1625+1231. We collected photometric time series data at Konkoly Observatory on 14 nights, and performed Fourier analysis of these data sets. We detected 11 significant frequencies, where six of them are found to be independent pulsation modes in the 514-881 s period range. By utilizing these frequencies, we performed preliminary asteroseismic investigations to give constraints on the main physical parameters, and to derive seismic distances for the star. Finally, we compared the astrometric distance provided by the Gaia EDR3 data with those seismic distances. Our selected model, considering both the spectroscopic measurements and the distance value provided by Gaia, has Teff=11 000 K and M*=0.60 M☉.

7

Analysis of the Most Precise Light Curves of HAT-P-36 Detrended from Spot Signals

Yalçınkaya S., Baştürk Ö., El Helweh F., Esmer E. M., Yörükoğlu O., Yılmaz M., Şenavcı H. V., Kılıçoğlu T., Selam S. O.

Acta Astronomica

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2021

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Vol. 71, No. 3

223--242

EN

We study the most precise light curves of the planet-host HAT-P-36 that we obtained from the ground primarily with a brand-new 80 cm telescope (T80) very recently installed at Ankara University Kreiken Observatory (AUKR), Turkey and also from the space with Transiting Exoplanet Survey Satellite (TESS). The main objective of the study is to analyze the Transit Timing Variations (TTV) observed in the hot-Jupiter type planet HAT-P-36 b, a strong candidate for orbital decay. Our analysis is based on our own observations as well as those acquired by professional and amateur observers since its discovery. HAT-P-36 displays out-of-transit variability as well as light curve anomalies during the transits of its planet due to stellar spots. We collected and detrended from these anomalies all complete transit light curves we had access to. We modeled transits with EXOFAST and measured the mid-transit times forming a homogeneous data set for a TTV analysis. We found an increase in the orbital period of HAT-P-36 b at a rate of 0.014 s per year from the best fitting quadratic function. This increase is only found in the TTV constructed from the mid-transit times measured from detrended light curves. We refined the values of the system parameters by modeling with EXOFASTv2 the Spectral Energy Distribution of the host star, its archival radial velocity observations from multiple instruments, and the most precise transit light curves from the space and ground covering a wide range of wavelengths. We also analyzed the out-of-transit variability from TESS observations to search for potential rotational modulations through a frequency analysis. We report a statistically significant periodicity in the TESS light curve at 4.22±0.02 d, which might have been caused by instrumental systematics but should be tracked in the future observations of the target.

8

A Detection Threshold in the Amplitude Spectra Calculated from TESS Time-Series Data

Baran A. S., Koen C.

Acta Astronomica

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2021

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Vol. 71, No. 2

113--121

EN

We present results of time-series data simulation. We aimed at estimating the threshold used for detecting signals in amplitude spectra, calculated from simulating TESS photometry of up to one year duration. We selected the threshold at a false alarm probability FAP=0.1% and derived S/N ratios between 4.6 and 5.7 depending on the data cadence and coverage. We also provide a formula to estimate the threshold for any FAP adopted and a given number of data points. Our result confirms that, to avoid spurious detection, space-based photometry may require substantially higher S/N than that typically being employed for ground-based data.

9

One New Variable Candidate and Six Non-variable Stars at the ZZ Ceti Instability Strip

Bognár Zs., Kalup Cs., Sódor Á.

Acta Astronomica

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2019

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Vol. 69, No. 1

55--62

EN

We present our results on the continuation of our survey searching for new ZZ Ceti stars, inspired by the recently launched TESS space mission. The seven targets were bright DA-type white dwarfs located close to the empirical ZZ Ceti instability strip. We successfully identified one new pulsator candidate, namely PM J22299+3024, derived detection limits for possible pulsations of four objects for the first time, and determined new detection limits for two targets.

10

CCD Washington Photometry of Ten Open Clusters or Candidates Projected Close to the Galactic Plane

Clariá J. J., Parisi M. C., Palma T., Ahumada A. V., Oviedo C. G.

Acta Astronomica

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2019

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Vol. 69, No. 1

1--23

EN

We present high-quality CCD photometry in the Washington system C and T1 passbands down to T1≈19.5 mag in the fields of ten Galactic open clusters or candidates projected close to the Galactic plane, namely: ESO 313-SC03, BH 54, Ruprecht 87, ESO 129-SC32, BH 217, Collinder 347, Basel 5, Ruprecht 144, Archinal 1 and Berkeley 82. Four of these objects are located toward the Galactic center within a solid angle of 21°. No photoelectric or CCD photometry in the optical domain has been so far reported for five of these objects. Cluster radii are estimated from radial density profiles in the cluster fields. Using the cluster Washington (C-T1,T1) color-magnitude diagrams, statistically cleaned from field star contamination, we estimate reddening, heliocentric distance and age of the clusters by fitting Padova theoretical isochrones computed for the Washington system. In all cases, the best fittings were obtained with nearly solar metal content isochrones. Both radial density profiles and color-magnitude diagrams show that we are dealing with real open clusters, except for Ruprecht 87 and Archinal 1 that are found to be probably not physical systems. Differential reddening appears to be present across the fields of ESO 313-SC03, ESO 129-SC32, BH 217, Collinder 347 and Basel 5. The studied open clusters are located at d☉=1.0-5.0 kpc from the Sun and at Galactocentric distances RGC=6.0-10.6 kpc, with mean reddening E(B-V) in the range of 0.10-1.30 mag and ages between 5 Myr (Collinder 347) and ≈1000 Myr (Basel 5). The estimated linear cluster radii are in the range of 0.4-3.2 pc. In general terms, the results obtained show fairly good agreement with previous photometric results. In some clusters, however, considerable differences are found between the present results and previous ones determined using near-infrared photometric data. The current study provides new open cluster parameters and some revisions to the open cluster catalogs.

11

Hybrid Pulsations and Tidal Splitting detected in the Kepler Eclipsing and Spotted Binary System KIC 6048106

Samadi Ghadim A., Lampens P., Jassur D. M.

Acta Astronomica

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2018

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Vol. 68, No. 4

425--447

EN

We present a new asteroseismic analysis of KIC 6048106, a Kepler Algol-type eclipsing binary star in a circularized orbit with Porb=1.559361±0.000036 d. Based on a physical model for the binary and its corresponding set of fundamental parameters (Teff=7033±187 K, M1=1.55±0.11 M☉, R1=1.58±0.12 R☉ and Teff=4522±103 K, M2=0.33±0.07 M☉, R2=1.78±0.16 R☉, respectively for the primary and the secondary component), we obtained the residual light curve after removal of the full binary model, including a 290-d activity cycle for the secondary component. In this work, we used the method of Fourier analysis of the residual light curve in combination with least squares optimization for the frequency analysis. We detected seven dominant, independent gravity (g) modes as well as 34 low-amplitude acoustic (p) modes. The g modes in the range of 1.96–2.85 d-1 have a mean spacing of ΔΠmean=1517.92±131.54 s. Though of much lower amplitude, additional significant frequencies were detected in the intervals 7.49–15.2 d-1 and 19–22.5 d-1 (i.e., in the p mode region), with corresponding dominant modes νmax1=11.745±0.001 d-1 and νmax2=20.960±0.002 d-1 From its position in the HR diagram, we conclude that the primary component is the source of the detected hybrid pulsations. Consequently, the pulsation constants, Q, of the high frequencies cover the range of 0.028–0.064 d. Furthermore, ν43 (19.037±0.002 d-1) might correspond to the fundamental radial mode (Q=0.033±0.007 d). The other frequencies in the range of 19–22.5 d-1 could be radial or non-radial overtone modes. Moreover, the low-amplitude p modes show an equidistant splitting by forb, which we interpret as tidal splitting following theoretical predictions.

12

Cool Spot and Flare Activities of a RS CVn Binary KIC 7885570

Kunt M., Dal H. A.

Acta Astronomica

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2017

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Vol. 67, No. 4

345--362

EN

We present here the results of our studies on the physical nature and chromospheric activity of a RS CVn binary KIC 7885570 based on the Kepler Mission data. Assuming the primary component temperature, 6530 K, the temperature of the secondary component was found to be 5732±4 K. The mass ratio of the components (q) was found to be 0.43±0.01, while the inclination (i) of the system - 80.6°±0.1°. Additionally, the data were separated into 35 subsets to model the sinusoidal variation due to the rotational modulation, using the SpotModel program, as the light curve analysis indicated the chromospherically active secondary component. It was found that there are generally two spotted areas, whose radii, longitudes and latitudes are rapidly changing, located around the latitudes of +50° and +90° on the active component. Moreover, 113 flares were detected and their parameters were computed from the available data. The One Phase Exponential Association function model was derived from the parameters of these flares. Using the regression calculations, the Plateau value was found to be 1.9815±0.1177, while the half-life value was computed as 3977.2 s. In addition, the flare frequency (N1) - the flare number per hour, was estimated to be 0.00362 h-1, while flare frequency (N2 - the flare-equivalent duration emitted per hour, was computed as 0.00001. Finally, the times of eclipses were computed for 278 minima of the light curves, whose analysis indicated that the chromosphere activity nature of the system causes some effects on these minima times. Comparing the chromospheric activity patterns with the analogues of the secondary component, it is seen that the magnetic activity level is remarkably low. However, it is still at the expected level according to the B-V color index of 0.643 mag for the secondary component.

13

Analysis of Photometric Uncertainties in the OGLE-IV Galactic Bulge Microlensing Survey Data

Skowron J., Udalski A., Kozłowski S., Szymański M. K., Mróz P., Wyrzykowski Ł., Poleski R., Pietrukowicz P., Ulaczyk K., Pawlak M., Soszyński I.

Acta Astronomica

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2016

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Vol. 66, No. 1

1--13

EN

We present a statistical assessment of both, observed and reported, photometric uncertainties in the OGLE-IV Galactic bulge microlensing survey data. This dataset is widely used for the detection of variable stars, transient objects, discovery of microlensing events, and characterization of the exo-planetary systems. Large collections of RR Lyr stars and Cepheids discovered by the OGLE project toward the Galactic bulge provide light curves based on this dataset. We describe the method of analysis, and provide the procedure, which can be used to update preliminary photometric uncertainties, provided with the light curves, to the ones reflecting the actual observed scatter at a given magnitude and for a given CCD detector of the OGLE-IV camera. This is of key importance for data modeling, in particular, for the correct estimation of the goodness of fit.

14

On the Atmospheric Extinction Reduction Procedure in Multiband Wide-Field Photometric Surveys

Zakharov A., Mironov A., Biryukov A., Kroussanova N., Prokhorov M., Beskin G., Karpov S., Bondar S., Ivanov E., Perkov A., Sasyuk V.

Acta Astronomica

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2015

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Vol. 65, No. 2

197--204

EN

We propose an improved method for the atmospheric extinction reduction within optical photometry. Our method is based on the simultaneous multicolor observations of photometric standards. Such data are now available within the modern wide-field sky surveys and contain a large amount of information about instant atmospheric conditions. So, it became possible to estimate the extinction parameters on the basis of a quite short observational dataset and, hence, to trace the rapid stars twinkling accurately. Having been developed for a new MiniMegaTORTORa observational system, the proposed method can be adopted for a wide range of modern observational programs.

15

OGLE-IV: Fourth Phase of the Optical Gravitational Lensing Experiment

Udalski A., Szymański M. K., Szymański G.

Acta Astronomica

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2015

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Vol. 65, No. 1

1--38

EN

We present both the technical overview and main science drivers of the fourth phase of the Optical Gravitational Lensing Experiment (hereafter OGLE-IV). OGLE-IV is currently one of the largest sky variability surveys worldwide, targeting the densest stellar regions of the sky. The survey covers over 3000 square degrees in the sky and monitors regularly over a billion sources. The main targets include the inner Galactic Bulge and the Magellanic System. Their photometry spans the range of 12

16

Empirical Conversions of Broad-Band Optical and Infrared Magnitudes to Monochromatic Continuum Luminosities for Active Galactic Nuclei

Kozłowski S.

Acta Astronomica

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2015

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Vol. 65, No. 3

251--265

EN

We use public data for 105 783 quasars from The Sloan Digital Sky Survey (SDSS) Data Release 7 (DR7) that include spectral monochromatic luminosities at 5100 Å, 3000 Å, and 1350 Å, and the corresponding observed broad-band ugriz, VRI (converted), JHK and WISE magnitudes, and derive broad-band-to-monochromatic luminosity ratios independent of a cosmological model. The ratios span the redshift range of z = 0.1 ÷ 4.9 and may serve as a proxy for measuring the bolometric luminosity, broad line region (BLR) radii and/or black hole masses, whenever flux-calibrated spectra are unavailable or the existing spectra have low signal-to-noise ratios. They are provided both in tabular and parametric form.

17

The Optical Gravitational Lensing Experiment. Gaia South Ecliptic Pole Field as Seen by OGLE-IV

Soszyński I., Udalski A., Poleski R., Kozłowski S., Wyrzykowski Ł., Pietrukowicz P., Szymański M. K., Kubiak M., Pietrzyński G., Ulaczyk K., Skowron J.

Acta Astronomica

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2012

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Vol. 62, No. 3

219--245

EN

We present a comprehensive analysis of the Gaia South Ecliptic Pole (GSEP) field, 5.3 square degrees area around the South Ecliptic Pole on the outskirts of the LMC, based on the data collected during the fourth phase of the Optical Gravitational Lensing Experiment, OGLE-IV. The GSEP field will be observed during the commissioning phase of the ESA Gaia space mission for testing and calibrating the Gaia instruments. We provide the photometric maps of the GSEP region containing the mean VI photometry of all detected stellar objects and their equatorial coordinates. We show the quality and completeness of the OGLE-IV photometry and color-magnitude diagrams of this region. We conducted an extensive search for variable stars in the GSEP field leading to the discovery of 6789 variable stars. In this sample we found 132 classical Cepheids, 686 RR Lyr type stars, 2819 long-period, and 1377 eclipsing variables. Several objects deserving special attention were also selected, including a new classical Cepheid in a binary eclipsing system. To provide empirical data for the Gaia Science Alert system we also conducted a search for optical transients. We discovered two firm type Ia supernovae and nine additional supernova candidates. To facilitate future Gaia supernovae detections we prepared a list of more than 1900 galaxies to redshift about 0.1 located in the GSEP field. Finally, we present the results of astrometric study of the GSEP field. With the 26 months time base of the presented here OGLE-IV data, proper motions of stars could be detected with the accuracy reaching 2 mas/yr. Astrometry allowed to distinguish galactic foreground variable stars detected in the GSEP field from LMC objects and to discover about 50 high proper motion stars (proper motion ≥ 100 mas/yr). Among them three new nearby white dwarfs were found. All data presented in this paper are available to the astronomical community from the OGLE Internet archive.

18

Photometric Maps Based on the OGLE-III Shallow Survey in the Large Magellanic Cloud

Ulaczyk K., Szymański M. K., Udalski A., Kubiak M., Pietrzyński G., Soszyński I., Wyrzykowski Ł., Poleski R., Gieren W., Walker A. R., Garcia-Varela A.

Acta Astronomica

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2012

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Vol. 62, No. 3

247--268

EN

We present photometric maps based on data from the shallow survey in the Large Magellanic Cloud performed as the supplementary project during the third phase of the Optical Gravitational Lensing Experiment. They cover about 40 square degrees in the LMC and contain mean calibrated VI photometry and astrometry of about 1.7 million stars. The magnitudes of the registered objects range from 9.1 to 18.5. We discuss the quality of data and present color-magnitude diagrams of selected fields. The maps together with the main LMC photometric maps are available to the astronomical community from the OGLE Internet archive.

19

The Optical Gravitational Lensing Experiment. OGLE-III Photometric Maps of the Galactic Bulge Fields

Szymański M. K., Udalski A., Soszyński I., Kubiak M., Pietrzyński G., Poleski R., Wyrzykowski Ł., Ulaczyk K.

Acta Astronomica

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2011

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Vol. 61, No. 2

83--102

EN

We present OGLE-III Photometric Maps of the Galactic bulge fields observed during the third phase of the OGLE project. This paper describes the last, concluding set of maps based on OGLE-III data. The maps contain precise, calibrated VI photometry of about 340 million stars from 267 fields in the Galactic bulge observed during entire OGLE-III phase (2002-2009), covering about 92 square degrees in the sky. Precise astrometry of these objects is also provided. We briefly discuss the photometry procedures and the quality of the data. We also present sample data and color-magnitude diagrams of the observed fields. All photometric data are available to the astronomical community from the OGLE Internet archive.

20

The Optical Gravitational Lensing Experiment. OGLE-III Photometric Maps of the Galactic Disk Fields

Szymański M. K., Udalski A., Soszyński I., Kubiak M., Pietrzyński G., Poleski R., Wyrzykowski Ł., Ulaczyk K.

Acta Astronomica

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2010

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Vol. 60, No. 4

295--304

EN

We present OGLE-III Photometric Maps of the Galactic disk fields observed during the OGLE-III campaigns for low luminosity transiting objects that led to the discovery of the first transitng exoplanets. The maps contain precise, calibrated VI photometry of about 9 million stars from 21 OGLE-III fields in the Galactic disk observed in the years 2002-2009 and covering more than 7 square degrees in the sky. Precise astrometry of these objects is also provided. We discuss quality of the data and present a few color-magnitude diagrams of the observed fields. All photometric data are available to the astronomical community from the OGLE Internet archive.