arXiv subjects

I. Soszynski

Publications and source records attributed to I. Soszynski.

At least 181 records · Page 10Linked to original sources

arXiv2006

The Optical Gravitational Lensing Experiment. Variable Baseline Microlensing Events in the Galactic Bulge

We present the first systematic search for microlensing events with variability in their baselines using data from the third phase of the Optical Gravitational Lensing Experiment (OGLE-III). A total of 137 candidates (88 new) was discovered toward the Galactic bulge. Among these, 21 have periodic oscillations in their baselines, 111 are irregular variables and 5 are potential long period detached eclipsing binaries. This is about 10% of the total number of constant baseline events. They are hence quite common and can be regarded as a new type of exotic events, which allow the determination of extra parameters of the events. We show that microlensing of variable stars may allow us to break the degeneracy between the blending parameter and magnification. We note that in some cases variability hidden in the baseline due to strong blending may be revealed in highly magnified events and resemble other exotic microlensing behavior, including planetary deviation. A new system (VEWS) for detecting ongoing variable baseline microlensing events is presented.

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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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arXiv2006

The Araucaria Project: Distance to the Local Group Galaxy NGC 3109 from Near-Infrared Photometry of Cepheids

We present near-infrared J- and K-band photometry of 77 Cepheid variables in the Local Group galaxy NGC 3109. Combining our data with the previously published optical V- and I-band photometry of Cepheids in this galaxy we derive an accurate distance and interstellar reddening to NGC 3109. Adopting a distance modulus of 18.5 mag for the Large Magellanic Cloud, we obtain a true distance modulus to NGC 3109 of (m-M)_0 = 25.571 +- 0.024 mag (random error), corresponding to a distance of 1.30 +- 0.02 Mpc. The systematic uncertainty on this value (apart from the adopted LMC distance) is of the order of +-3%, the main contributors to this value being the uncertainty on the photometric zero points, and the effect of blending with unresolved companion stars. The total reddening determined from our multiwavelength solution is E(B-V)=0.087 +- 0.012 mag. About half of the reddening is produced internal to NGC 3109. Our distance result is consistent with previous determinations of the distance to NGC 3109, but has significantly reduced error bars.

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arXiv2006

The Araucaria Project. An Accurate Distance to the Local Group Galaxy NGC 6822 from Near-Infrared Photometry of Cepheid Variables

We have measured near-infrared magnitudes in the J and K bands for 56 Cepheid variables in the Local Group galaxy NGC 6822 with well-determined periods and optical light curves in the V and I bands. Using the template light curve approach of Soszynski, Gieren and Pietrzynski, accurate mean magnitudes were obtained from these data which allowed us to determine with unprecedented accuracy the distance to NGC 6822 from a multi-wavelength period-luminosity solution in the VIJK bands. From our data, we obtain a distance to NGC 6822 of (m-M)_{0} = 23.312 +- 0.021 (random error) mag, with an additional systematic uncertainty of about 3 %. This distance value is tied to an assumed LMC distance modulus of 18.50. From our multiwavelength approach, we find for the total (average) reddening to the NGC 6822 Cepheids E(B-V) = 0.356 +- 0.013 mag, which is in excellent agreement with a previous determination of McGonegal et al. from near-infrared photometry and implies significant internal reddening of the Cepheids in NGC 6822. Our present, definitive distance determination of NGC 6822 from Cepheids agrees within 2 % with the previous distance we had derived from optical photometry alone, but has significantly reduced error bars. Our Cepheid distance to NGC 6822 is in excellent agreement with the recent independent determination of Cioni and Habing from the I-band magnitude of the tip of the red giant branch. It also agrees well, within the errors, with the early determination of McGonegal et al. (1983) from random-phase H-band photometry of nine Cepheids.

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arXiv2006

The Araucaria Project. A Wide-Field Photometric Survey for Cepheid Variables in NGC 3109

We have obtained mosaic images of NGC 3109 in the V and I bands on 74 nights, spanning approximately one year. From these data, we have conducted an extensive search for Cepheid variables over the entire field of the galaxy, resulting in the discovery of 113 variables with periods ranging from 3.4 to 31.4 days. In this sample, 76 Cepheids, including many long-period variables, were not known before. For the previously known 45 Cepheids in this galaxy, our data proved that reported periods were wrong for 14 objects; for nearly all other previously known Cepheid variables we were able to significantly improve on the periods. We construct period-luminosity relations from our data and obtain reddening-corrected distance moduli of 25.72 $\pm$ 0.05 mag in V, and 25.66 $\pm$ 0.04 mag in I. The distance modulus derived form the reddening-independent V-I Wesenheit index turns out to be significantly shorter (25.54 $\pm$ 0.05 mag), which indicates that in addition to the foreground extinction of E(B-V) = 0.05 mag, there is an intrinsic to NGC 3109 redening of about 0.05 mag. Our distance obtained based on the reddening-free Wesenheit magnitudes is consistent with earlier distance determinations of NGC 3109 from Cepheids, and the tip of the red giant branch. We will improve on our distance and extinction determination combining our optical data with the follow-up near-infrared observations of a subsample of NGC 3109 Cepheids.

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arXiv2006

Microlens OGLE-2005-BLG-169 Implies Cool Neptune-Like Planets are Common

We detect a Neptune mass-ratio (q~8e-5) planetary companion to the lens star in the extremely high-magnification (A~800) microlensing event OGLE-2005-BLG-169. If the parent is a main-sequence star, it has mass M~0.5 M_sun implying a planet mass of ~13 M_earth and projected separation of ~2.7 AU. When intensely monitored over their peak, high-magnification events similar to OGLE-2005-BLG-169 have nearly complete sensitivity to Neptune mass-ratio planets with projected separations of 0.6 to 1.6 Einstein radii, corresponding to 1.6--4.3 AU in the present case. Only two other such events were monitored well enough to detect Neptunes, and so this detection by itself suggests that Neptune mass-ratio planets are common. Moreover, another Neptune was recently discovered at a similar distance from its parent star in a low-magnification event, which are more common but are individually much less sensitive to planets. Combining the two detections yields 90% upper and lower frequency limits f=0.37^{+0.30}_{-0.21} over just 0.4 decades of planet-star separation. In particular, f>16% at 90% confidence. The parent star hosts no Jupiter-mass companions with projected separations within a factor 5 of that of the detected planet. The lens-source relative proper motion is μ~7--10 mas/yr, implying that if the lens is sufficiently bright, I<23.8, it will be detectable by HST by 3 years after peak. This would permit a more precise estimate of the lens mass and distance, and so the mass and projected separation of the planet. Analogs of OGLE-2005-BLG-169Lb orbiting nearby stars would be difficult to detect by other methods of planet detection, including radial velocities, transits, or astrometry.

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arXiv2006

Discovery of a Cool Planet of 5.5 Earth Masses Through Gravitational Microlensing

In the favoured core-accretion model of formation of planetary systems, solid planetesimals accumulate to build up planetary cores, which then accrete nebular gas if they are sufficiently massive. Around M-dwarf stars (the most common stars in our Galaxy), this model favours the formation of Earth-mass to Neptune-mass planets with orbital radii of 1 to 10 astronomical units (AU), which is consistent with the small number of gas giant planets known to orbit M-dwarf host stars. More than 170 extrasolar planets have been discovered with a wide range of masses and orbital periods, but planets of Neptune's mass or less have not hitherto been detected at separations of more than 0.15 AU from normal stars. Here we report the discovery of a 5.5 (+5.5/-2.7) M_earth planetary companion at a separation of 2.6 (+1.5/-0.6) AU from a 0.22 (+0.21/-0.11) M_solar M-dwarf star. (We propose to name it OGLE-2005-BLG-390Lb, indicating a planetary mass companion to the lens star of the microlensing event.) The mass is lower than that of GJ876d, although the error bars overlap. Our detection suggests that such cool, sub-Neptune-mass planets may be more common than gas giant planets, as predicted by the core accretion theory.

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arXiv2006

The Araucaria Project. Bright Variable Stars in NGC 6822 from a Wide-Field Imaging Survey

We have performed a search for variable stars in the dwarf irregular galaxy NGC 6822 using wide-field multi-epoch VI photometry down to a limiting magnitude $V$ $\sim$ 22. Apart from the Cepheid variables in this galaxy already reported in an earlier paper by Pietrzynski et al. (2004), we have found 1019 "non-periodic" variable stars, 50 periodically variable stars with periods ranging from 0.12 to 66 days and 146 probably periodic variables. Twelve of these stars are eclipsing binaries and fifteen are likely new, low-amplitude Cepheids. Interestingly, seven of these Cepheid candidates have periods longer than 100 days, have very low amplitudes (less than 0.2 mag in $I$), and are very red. They could be young, massive Cepheids still embedded in dusty envelopes. The other objects span a huge range in colours and represent a mixture of different types of luminous variables. Many of the variables classified as non-periodic in the present study may turn out to be {\it periodic} variables once a much longer time baseline will be available to study them. We provide the catalogue of photometric parameters and show the atlas of light curves for the new variable stars. Our present catalogue is complementary to the one of Baldacci et al. (2005) which has focussed on very short-period and fainter variables in a subfield in NGC 6822.

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arXiv2006

The Araucaria Project. The Distance to the Local Group Galaxy IC 1613 from Near-Infrared Photometry of Cepheid Variables

We have measured accurate near-infrared magnitudes in the J and K bands of 39 Cepheid variables in IC 1613 with well-determined periods and optical VI light curves. Using the template light curve approach of Soszy{ń}ski, Gieren and Pietrzy{ń}ski, accurate mean magnitudes were obtained from these data which allowed to determine the distance to IC 1613 relative to the LMC from a multiwavelength period-luminosity solution in the optical VI and near-IR JK bands, with an unprecedented accuracy. Our result for the IC 1613 distance is $(m-M)_{0} = 24.291 \pm 0.014$ (random error) mag, with an additional systematic uncertainty smaller than 2%. From our multiwavelength approach, we find for the total (average) reddening to the IC 1613 Cepheids $E(B-V) = 0.090 \pm 0.007$ mag,which is significantly higher than the foreground reddening of about 0.03 mag,showing the presence of appreciable dust extinction inside the galaxy. Our data suggest that the extinction law in IC 1613 is very similar to the galactic one.Our distance result agrees, within the uncertainties, with two earlier infrared Cepheid studies in this galaxy of Macri et al. (from HST data on 4 Cepheids), and McAlary et al. (from ground-based H-band photometry of 10 Cepheids), but our result has reduced the total uncertainty on the distance to IC 1613 (relative to the LMC) to less than 3%. With distances to nearby galaxies from Cepheid infrared photometry at this level of accuracy, which are currently being obtained in our Araucaria Project, it seems possible to significantly reduce the systematic uncertainty of the Hubble constant as derived from the HST Key Project approach, by improving the calibration of the metallicity effect on PL relation zero points, and by improving the distance determination to the LMC.

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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

Mean JHK Magnitudes of Fundamental-Mode Cepheids from Single-Epoch Observations

We present an empirical method for converting single-point near-infrared J, H, and K measurements of fundamental-mode Cepheids to mean magnitudes, using complete light curves in V or I bands. The algorithm is based on the template light curves in the near-infrared bandpasses. The mean uncertainty of the method is estimated to about 0.03 mag, which is smaller than the uncertainties obtained in other approaches to the problem in the literature.

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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

A Jovian-mass Planet in Microlensing Event OGLE-2005-BLG-071

We report the discovery of a several-Jupiter mass planetary companion to the primary lens star in microlensing event OGLE-2005-BLG-071. Precise (<1%) photometry at the peak of the event yields an extremely high signal-to-noise ratio detection of a deviation from the light curve expected from an isolated lens. The planetary character of this deviation is easily and unambiguously discernible from the gross features of the light curve. Detailed modeling yields a tightly-constrained planet-star mass ratio of q=m_p/M=0.0071+/-0.0003. This is the second robust detection of a planet with microlensing, demonstrating that the technique itself is viable and that planets are not rare in the systems probed by microlensing, which typically lie several kpc toward the Galactic center.

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arXiv2005

The Araucaria Project. Near-Infrared Photometry of Cepheid Variables in the Sculptor Galaxy NGC 300

We have obtained deep near-infrared images in J and K filters of three fields in the Sculptor galaxy NGC 300 with the ESO VLT and ISAAC camera. For 16 Cepheid variables in these fields, we have determined J and K magnitudes at two different epochs, and have derived their mean magnitudes in these bands. The slopes of the resulting period-luminosity relations are in very good agreement with the slopes of these relations measured in the LMC by Persson et al. Fitting the LMC slopes to our data, we have derived distance moduli in J and K. Using these values together with the values derived in the optical V and I bands in our previous work, we have determined an improved total reddening for NGC 300 of E(B-V)=0.096 +/- 0.006 mag, which yields extremely consistent values for the absorption-corrected distance modulus of the galaxy from VIJK bands. Our distance result for NGC 300 from this combined optical/near infrared Cepheid study is (m-M)_0 = 26.37 +/- 0.04 (random) +/- 0.03 (systematic) mag and is tied to an adopted true LMC distance modulus of 18.50 mag. Both random and systematic uncertainties are dominated by photometric errors, while errors due to reddening, metallicity effects and crowding are less important. Our distance determination is consistent with the earlier result from near-infrared (H-band) photometry of two Cepheids in NGC 300 by Madore et al., but far more accurate. Our distance value also agrees with the HST Key Project result of Freedman et al., and with the recent distance estimate for NGC 300 from Butler et al. from the TRGB I-band magnitude when our improved reddening is used to calculate the absorption corrections. Our distance results from the different optical and near-infrared bands indicate that the reddening law in NGC 300 must be very similar to the Galactic one.

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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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