arXiv subjects

K. Zebrun

Publications and source records attributed to K. Zebrun.

At least 19 recordsLinked to original sources

arXiv2009

The OGLE View of Microlensing towards the Magellanic Clouds. I. A Trickle of Events in the OGLE-II LMC data

We present the results from the OGLE-II survey (1996-2000) towards the Large Magellanic Cloud (LMC), which has the aim of detecting the microlensing phenomena caused by dark matter compact objects in the Galactic Halo (Machos). We use high resolution HST images of the OGLE fields and derive the correction for the number of monitored stars in each field. This also yield blending distributions which we use in 'catalogue level' Monte Carlo simulations of the microlensing events in order to calculate the detection efficiency of the events. We detect two candidates for microlensing events in the All Stars Sample, which translates into an optical depth of 0.43+-0.33x 10e-7. If both events were due to Macho the fraction of mass of compact dark matter objects in the Galactic halo would be 8+-6 per cent. This optical depth, however, along with the characteristics of the events, seems to be consistent with the self-lensing scenario, i.e., self-lensing alone is sufficient to explain the observed microlensing signal. Our results indicate a non-detection of Machos lensing towards the LMC with an upper limit on their abundance in the Galactic halo of 19 per cent for M=0.4 Msun and 10 per cent for masses between 0.01 and 0.2 Msun.

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arXiv2007

Binary Lenses in OGLE-III EWS Database. Season 2004

We present 19 binary lens candidates from OGLE-III Early Warning System database for the season of 2004. We have also found five events interpreted as single mass lensing of double sources. The candidates have been selected by visual light curves inspection. Examining the models of binary lenses of this and our previous studies (10 caustic crossing events of OGLE-II seasons 1997--1999 and 15 binary lens events of OGLE-III seasons 2002--2003) we find one case of extreme mass ratio binary (q ~ 0.005, a known planetary lens OGLE 2003-BLG-235/MOA 2003-BLG-53) and almost all other models with mass ratios in the range 0.1<q<1.0, which may indicate the division between planetary systems and binary stars.

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arXiv2007

The Optical Gravitational Lensing Experiment. OGLE-III Long Term Monitoring of the Gravitational Lens QSO 2237+0305

We present results of the long term monitoring of the gravitational lens QSO 2237+0305 conducted during the OGLE survey. Light curves of all four components of the lens obtained during the second phase of the OGLE project (OGLE-II; 1997-2000) are supplemented with the data collected in the OGLE-III phase in the observing seasons 2001-2006. Calibration procedures to tie the new OGLE-III data with already calibrated OGLE-II light curves are described. The resulting homogeneous OGLE data set is the most extensive photometric coverage of the gravitational lens QSO 2237+0305, spanning now one decade - the seasons from 1997 to 2006 - and revealing unique microlensing activity of this spectacular object. All photometric data of the gravitational lens QSO 2237+0305 collected by OGLE are available to the astronomical community from the OGLE Internet archive.

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arXiv2006

OGLE 2004-BLG-254: a K3 III Galactic Bulge Giant spatially resolved by a single microlens

We present an analysis of OGLE 2004-BLG-254, a high-magnification and relatively short duration microlensing event in which the source star, a Bulge K3-giant, has been spatially resolved by a point-like lens. We have obtained dense photometric coverage of the event light curve with OGLE and PLANET telescopes, as well as a high signal-to-noise ratio spectrum taken while the source was still magnified by 20, using the UVES/VLT spectrograph. Our dense coverage of this event allows us to measure limb darkening of the source star in the I and R bands. We also compare previous measurements of linear limb-darkening coefficients involving GK-giant stars with predictions from ATLAS atmosphere models. We discuss the case of K-giants and find a disagreement between limb-darkening measurements and model predictions, which may be caused by the inadequacy of the linear limb-darkening law.

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arXiv2006

Planetary Detection Efficiency of the Magnification 3000 Microlensing Event OGLE-2004-BLG-343

OGLE-2004-BLG-343 was a microlensing event with peak magnification A_{max}=3000+/-1100, by far the highest-magnification event ever analyzed and hence potentially extremely sensitive to planets orbiting the lens star. Due to human error, intensive monitoring did not begin until 43 minutes after peak, at which point the magnification had fallen to A~1200, still by far the highest ever observed. As the light curve does not show significant deviations due to a planet, we place upper limits on the presence of such planets by extending the method of Yoo et al. (2004b), which combines light-curve analysis with priors from a Galactic model of the source and lens populations, to take account of finite-source effects. This is the first event so analyzed for which finite-source effects are important, and hence we develop two new techniques for evaluating these effects. Somewhat surprisingly, we find that OGLE-2004-BLG-343 is no more sensitive to planets than two previously analyzed events with A_{max}~100, despite the fact that it was observed at ~12 times higher magnification. However, we show that had the event been observed over its peak, it would have been sensitive to almost all Neptune-mass planets over a factor of 5 of projected separation and even would have had some sensitivity to Earth-mass planets. This shows that some microlensing events being detected in current experiments are sensitive to very low-mass planets. We also give suggestions on how extremely high-magnification events can be more promptly monitored in the future.

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arXiv2005

The Optical Gravitational Lensing Experiment. Miras and Semiregular Variables in the Large Magellanic Cloud

We use the OGLE-II and OGLE-III data in conjunction with the 2MASS near-infrared (NIR) photometry to identify and study Miras and Semiregular Variables (SRVs) in the Large Magellanic Cloud. We found in total 3221 variables of both types, populating two of the series of NIR period-luminosity (PL) sequences. The majority of these objects are double periodic pulsators, with periods belonging to both PL ridges. We indicate that in the period - Wesenheit index plane the oxygen-rich and carbon-rich AGB stars from the NIR PL sequences C, C' and D split into well separated ridges. Thus, we discover an effective method of distinguishing between O-rich and C-rich Miras, SRVs and stars with Long Secondary Periods using their V and I-band photometry. We present an empirical method of estimating the mean K_s magnitudes of the Long Period Variables using single-epoch K_s measurements and complete light curves in the I-band. We utilize these corrected magnitudes to show that the O-rich and C-rich Miras and SRVs follow somewhat different K_s-band PL relations.

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arXiv2005

Microlensing optical depth toward the Galactic Bulge using bright sources from OGLE-II

We present a measurement of the microlensing optical depth toward the Galactic Bulge based on 4 years of the OGLE-II survey using Red Clump Giant (RCG). Using 32 events we find tau=2.55_{-0.46}^{+0.57}* 10^{-6} at (l,b)=(1.16, -2.75). Taking into account the measured gradient along the Galactic latitude b, tau = [ (4.48+/- 2.37) + (0.78+/- 0.84)* b]* 10^{-6}, this value is consistent with previous measurements using RCG sources and recent theoretical predictions. We determine the microlensing parameters and select events using a model light curve with the flux blending. We find that ~38% of the OGLE-II events which appear to have RCG sources are actually due to much fainter stars blended with a bright companion. We show explicitly that model fits without blending result in similar tau estimates through partial cancellation of contributions from higher detection efficiency, underestimated time-scales and larger number of selected events. This approach, however, leads to biased time-scale distributions and event rates. Consequently, microlensing studies should carefully consider source confusion effects even for bright stars.

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arXiv2005

Mass Estimates for Some of the Binary Lenses in OGLE III Database

We model binary microlensing events OGLE 2003-BLG-170, 267, and 291. Source angular sizes are measured for the events 267 and 291. Model fits to the light curves give parallaxes for the events 267 and 291, and relative source sizes for 170 and 267. Selfconsistency arguments provide extra limits on the models of the event 291. As a result we obtain likelihood estimate of the lens mass for the event 170, mass measurement based on angular size and parallax for 267, and narrow limits on mass in the case of 291. Brown dwarfs are most likely candidates for some of the lens components. The influence of the binary lens rotation and the Earth parallax may be important but hard to distinguish when modeling relatively short lasting binary lens events.

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arXiv2005

The Optical Gravitational Lensing Experiment. Planetary and Low-Luminosity Object Transits in the Fields of Galactic Disk. Results of the 2003 OGLE Observing Campaigns

We present results of two observing campaigns conducted by the OGLE-III survey in the 2003 observing season aiming at the detection of new objects with planetary transiting companions. Six fields of 35'x35' each located in the Galactic disk were monitored with high frequency for several weeks in February - July 2003. Additional observations of three of these fields were also collected in the 2004 season. Altogether about 800 and 1500 epochs were collected for the fields of both campaigns, respectively. The search for low depth transits was conducted on about 230000 stars with photometry better than 15 mmag. It was focused on detection of planetary companions, thus clear non-planetary cases were not included in the final list of selected objects. Altogether we discovered 40 stars with shallow (<0.05 mag) flat-bottomed transits. In each case several individual transits were observed allowing determination of photometric elements. Additionally, the lower limits on radii of the primary and companion were calculated. From the photometric point of view the new OGLE sample contains many very good candidates for extrasolar transiting planets. However, only the future spectroscopic follow-up observations of the OGLE sample - determination of the amplitude of radial velocity and exclusion of blending possibilities - may allow to confirm their planetary status. In general, the transiting objects may be extrasolar planets, brown dwarfs, M-type dwarfs or fake transits caused by blending. All photometric data of objects with transiting companions discovered during the 2003 campaigns are available to the astronomical community from the OGLE Internet archive.

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arXiv2004

The Optical Gravitational Lensing Experiment. Ellipsoidal Variability of Red Giants in the Large Magellanic Cloud

We used the OGLE-II and OGLE-III photometry of red giants in the Large Magellanic Cloud to select and study objects revealing ellipsoidal variability. We detected 1546 candidates for long period ellipsoidal variables and 121 eclipsing binary systems with clear ellipsoidal modulation. The ellipsoidal red giants follow a period-luminosity (PL) relationship (sequence E), and the scatter of the relation is correlated with the amplitude of variability: the larger the amplitude, the smaller the scatter. We note that some of the ellipsoidal candidates exhibit simultaneously OGLE Small Amplitude Red Giants pulsations. Thus, in some cases the Long Secondary Period (LSP) phenomenon can be explained by the ellipsoidal modulation. We also select about 1600 red giants with distinct LSP, which are not ellipsoidal variables. We discover that besides the sequence D in the PL diagram known before, the LSP giants form additional less numerous sequence for longer periods. We notice that the PL sequence of the ellipsoidal candidates is a direct continuation of the LSP sequence toward fainter stars, what might suggest that the LSP phenomenon is related to binarity but there are strong arguments against such a possibility. About 10% of the presented light curves reveal clear deformation by the eccentricity of the system orbits. The largest estimated eccentricity in our sample is about 0.4. All presented data, including individual BVI observations and finding charts are available from the OGLE Internet archive.

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arXiv2004

Variability-selected QSO candidates in OGLE-II Galactic Bulge fields

We present 97 QSO candidates in 48 Galactic Bulge fields of the Optical Gravitational Lensing Experiment II (OGLE-II) covering about 11 square degrees, which are selected via their variability. We extend light curves of variable objects which were detected in 3-year baseline in the OGLE-II variable star catalog to 4th year. We search for objects which are faint (16<I_0<18.5) and slowly variable during 4 years in this catalog by using the variogram/structure function. Finding the QSOs in such stellar-crowded and high extinction fields is challenging, but should be useful for the astrometric reference frame. Spectroscopic follow-up observations are required to confirm these QSO candidates. Follow-up observations are being prepared for four of these fields (BUL_SC1, 2, 32 and 45). Follow-up observations for other fields are strongly encouraged. The complete list and light curves of all 97 candidates are available in electronic format at http://www.astro.princeton.edu/~sumi/QSO-OGLEII/.

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arXiv2004

Search for Low-Mass Exoplanets by Gravitational Microlensing at High Magnification

Observations of the gravitational microlensing event MOA 2003-BLG-32/OGLE 2003-BLG-219 are presented for which the peak magnification was over 500, the highest yet reported. Continuous observations around the peak enabled a sensitive search for planets orbiting the lens star. No planets were detected. Planets 1.3 times heavier than Earth were excluded from more than 50 % of the projected annular region from approximately 2.3 to 3.6 astronomical units surrounding the lens star, Uranus-mass planets from 0.9 to 8.7 astronomical units, and planets 1.3 times heavier than Saturn from 0.2 to 60 astronomical units. These are the largest regions of sensitivity yet achieved in searches for extrasolar planets orbiting any star.

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arXiv2004

Binary Lenses in OGLE-III EWS Database. Seasons 2002--2003

We present 15 binary lens candidates from OGLE-III Early Warning System database for seasons 2002-2003. We also found 15 events interpreted as single mass lensing of double sources. The candidates were selected by visual light curves inspection. Examining the models of binary lenses of this and our previous study (10 caustic crossing events of OGLE-II seasons 1997-1999) we find one case of extreme mass ratio binary (q ~ 0.005) and the rest in the range 0.1<q<1.0, which may indicate the division between planetary systems and binary stars. There is no strong discrepancy between the expected and the observed distributions of mass ratios and separations for binary stars.

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arXiv2004

Constraints on Planetary Companions in the Magnification A=256 Microlensing Event: OGLE-2003-BLG-423

We develop a new method of modeling microlensing events based on a Monte Carlo simulation that incorporates both a Galactic model and the constraints imposed by the observed characteristics of the event. The method provides an unbiased way to analyze the event especially when parameters are poorly constrained by the observed lightcurve. We apply this method to search for planetary companions of the lens in OGLE-2003-BLG-423, whose maximum magnification A_max=256+-43 (or A_max=400+-115 from the lightcurve data alone) is the highest among single-lens events ever recorded. The method permits us, for the first time, to place constraints directly in the planet-mass/projected-physical-separation plane rather than in the mass-ratio/Einstein-radius plane as was done previously. For example, Jupiter-mass companions of main-sequence stars at 2.5 AU are excluded with 80% efficiency.

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arXiv2004

The Optical Gravitational Lensing Experiment. Small Amplitude Variable Red Giants in the Magellanic Clouds

We present analysis of the large sample of variable red giants from the Large and Small Magellanic Clouds detected during the second phase of the Optical Gravitational Lensing Experiment (OGLE-II) and supplemented with OGLE-III photometry. Comparing pulsation properties of detected objects we find that they constitute two groups with clearly distinct features. In this paper we analyze in detail small amplitude variable red giants (about 15400 and 3000 objects in the LMC and SMC, respectively). The vast majority of these objects are multi-periodic. At least 30% of them exhibit two modes closely spaced in the power spectrum, what likely indicates non-radial oscillations. About 50% exhibit additional so called Long Secondary Period. To distinguish between AGB and RGB red giants we compare PL diagrams of multi-periodic red giants located above and below the tip of the Red Giant Branch (TRGB). The giants above the TRGB form four parallel ridges in the PL diagram. Among much more numerous sample of giants below the TRGB we find objects located on the low luminosity extensions of these ridges, but most of the stars are located on the ridges slightly shifted in logP. We interpret the former as the second ascent AGB red giants and the latter as the first ascent RGB objects. Thus, we empirically show that the pulsating red giants fainter than the TRGB are a mixture of RGB and AGB giants. Finally, we compare the Petersen diagrams of the LMC, SMC and Galactic bulge variable red giants and find that they are basically identical indicating that the variable red giants in all these different stellar environments share similar pulsation properties.

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arXiv2004

WR 20a is an Eclipsing Binary: Accurate Determination of Parameters for an Extremely Massive Wolf-Rayet System

We present a high-precision I-band light curve for the Wolf-Rayet binary WR 20a, obtained as a sub-project of the Optical Gravitational Lensing Experiment. Rauw et al. have recently presented spectroscopy for this system, strongly suggesting extremely large minimum masses of 70.7 +/- 4.0 Mo and 68.8 +/- 3.8 Mo for the component stars of the system, with the exact values depending strongly on the period of the system. We detect deep eclipses of about 0.4 mag in the light curve of WR 20a, confirming and refining the suspected period of P=3.686 days and deriving an inclination angle of i=74.5 +/- 2 deg. Using these photometric data and the radial velocity data of Rauw et al., we derive the masses for the two components of WR 20a to be 83.0 +/- 5.0 Mo and 82.0 +/- 5.0 Mo. Therefore, WR 20a is confirmed to consist of two extremely massive stars and to be the most massive binary known with an accurate mass determination.

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arXiv2004

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

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.

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arXiv2004

OGLE-2003-BLG-238: Microlensing Mass Estimate of an Isolated Star

Microlensing is the only known direct method to measure the masses of stars that lack visible companions. In terms of microlensing observables, the mass is given by M=(c^2/4G)\tilde r_E θ_E and so requires the measurement of both the angular Einstein radius, θ_E, and the projected Einstein radius, \tilde r_E. Simultaneous measurement of these two parameters is extremely rare. Here we analyze OGLE-2003-BLG-238, a spectacularly bright (I_min=10.3), high-magnification (A_max = 170) microlensing event. Pronounced finite source effects permit a measurement of θ_E = 650 uas. Although the timescale of the event is only t_E = 38 days, one can still obtain weak constraints on the microlens parallax: 4.4 AU < \tilde r_E < 18 AU at the 1 σlevel. Together these two parameter measurements yield a range for the lens mass of 0.36 M_sun < M < 1.48 M_sun. As was the case for MACHO-LMC-5, the only other single star (apart from the Sun) whose mass has been determined from its gravitational effects, this estimate is rather crude. It does, however, demonstrate the viability of the technique. We also discuss future prospects for single-lens mass measurements.

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