arXiv subjects

P. Pietrukowicz

Publications and source records attributed to P. Pietrukowicz.

At least 91 records · Page 5Linked to original sources

arXiv2018

The OGLE Collection of Variable Stars. Type II Cepheids in the Magellanic System

We present a nearly complete collection of type II Cepheids in the Magellanic System. The sample consists of 338 objects: 285 and 53 variables in the Large and Small Magellanic Clouds, respectively. Based on the pulsation periods and light-curve morphology, we classified 118 of our type II Cepheids as BL Herculis, 120 as W Virginis, 34 as peculiar W Virginis, and 66 as RV Tauri stars. For all objects, we publish time-series VI photometry obtained during the OGLE-IV survey, from 2010 to the end of 2017. We present the most interesting individual objects in our collection: 16 type II Cepheids showing additional eclipsing or ellipsoidal variability, two RV Tau variables more than 2.5 mag fainter than other stars of this type in the LMC, an RVb star that drastically decreased the amplitude of the long-period modulation, type II Cepheids exhibiting significant amplitude and period changes, and an RV Tau star which experiences interchanges of deep and shallow minima. We show that peculiar W Vir stars have markedly different spatial distribution than other subclasses of type II Cepheids, which indicates different evolutionary histories of these objects.

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arXiv2018

OGLE-2017-BLG-0537: Microlensing Event with a Resolvable Lens in $\lesssim 5$ years from High-resolution Follow-up Observations

We present the analysis of the binary-lens microlensing event OGLE-2017-BLG-0537. The light curve of the event exhibits two strong caustic-crossing spikes among which the second caustic crossing was resolved by high-cadence surveys. It is found that the lens components with a mass ratio $\sim 0.5$ are separated in projection by $\sim 1.3\thetae$, where $\thetae$ is the angular Einstein radius. Analysis of the caustic-crossing part yields $\thetae=1.77\pm 0.16$~mas and a lens-source relative proper motion of $μ=12.4\pm 1.1~{\rm mas}~{\rm yr}^{-1}$. The measured $μ$ is the third highest value among the events with measured proper motions and $\sim 3$ times higher than the value of typical Galactic bulge events, making the event a strong candidate for follow-up observations to directly image the lens by separating it from the source. From the angular Einstein radius combined with the microlens parallax, it is estimated that the lens is composed of two main-sequence stars with masses $M_1\sim 0.4~M_\odot$ and $M_2\sim 0.2~M_\odot$ located at a distance of $D_{\rm L}\sim 1.2$~kpc. However, the physical lens parameters are not very secure due to the weak microlens-parallax signal, and thus we cross check the parameters by conducting a Bayesian analysis based on the measured Einstein radius and event timescale combined with the blending constraint. From this, we find that the physical parameters estimated from the Bayesian analysis are consistent with those based on the measured microlens parallax. Resolving the lens from the source can be done in about 5 years from high-resolution follow-up observations and this will provide a rare opportunity to test and refine the microlensing model.

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arXiv2018

Impact of distance determinations on Galactic structure. II. Old tracers

Here we review the efforts of a number of recent results that use old tracers to understand the build up of the Galaxy. Details that lead directly to using these old tracers to measure distances are discussed. We concentrate on the following: (1) the structure and evolution of the Galactic bulge and inner Galaxy constrained from the dynamics of individual stars residing therein; (2) the spatial structure of the old Galactic bulge through photometric observations of RR Lyrae-type stars; (3) the three\--dimensional structure, stellar density, mass, chemical composition, and age of the Milky Way bulge as traced by its old stellar populations; (4) an overview of RR Lyrae stars known in the ultra-faint dwarfs and their relation to the Galactic halo; and (5) different approaches for estimating absolute and relative cluster ages.

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arXiv2018

OGLE-2016-BLG-1045: A Test of Cheap Space-Based Microlens Parallaxes

Microlensing is a powerful and unique technique to probe isolated objects in the Galaxy. To study the characteristics of these interesting objects based on the microlensing method, measurement of the microlens parallax is required to determine the properties of the lens. Of the various methods to measure microlens parallax, the most robust way is to make simultaneous ground- and space-based observations, i.e., by measuring the space-based microlens parallax. However, space-based campaigns usually require "expensive" resources. Gould & Yee (2012) proposed an idea called the "cheap space-based microlens parallax" that can measure the lens-parallax using only $2$ or $3$ space-based observations of high-magnification events. This cost-effective observation strategy to measure microlens parallaxes could be used by space-borne telescopes to build a complete sample for studying isolated objects. This would enable a direct measurement of the mass function including both extremely low-mass objects and high-mass stellar remnants. However, to adopt this idea requires a test to check how it would work in actual situations. Thus, we present the first practical test of the idea using the high-magnification microlensing event OGLE-2016-BLG-1045, for which a subset of Spitzer observations fortuitously duplicate the prescription of Gould & Yee (2012). From the test, we confirm that the measurement of the lens-parallax adopting this idea has sufficient accuracy to determine the physical properties of the isolated lens.

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arXiv2018

OGLE-2015-BLG-1459L: The Challenges of Exo-Moon Microlensing

We show that dense OGLE and KMTNet $I$-band survey data require four bodies (sources plus lenses) to explain the microlensing light curve of OGLE-2015-BLG-1459. However, these can equally well consist of three lenses and one source (3L1S), two lenses and two sources (2L2S) or one lens and three sources (1L3S). In the 3L1S and 2L2S interpretations, the host is a brown dwarf and the dominant companion is a Neptune-class planet, with the third body (in the 3L1S case) being a Mars-class object that could have been a moon of the planet. In the 1L3S solution, the light curve anomalies are explained by a tight (five stellar radii) low-luminosity binary source that is offset from the principal source of the event by $\sim 0.17\,\au$. These degeneracies are resolved in favor of the 1L3S solution by color effects derived from comparison to MOA data, which are taken in a slightly different ($R/I$) passband. To enable current and future ($WFIRST$) surveys to routinely characterize exomoons and distinguish among such exotic systems requires an observing strategy that includes both a cadence faster than 9 min$^{-1}$ and observations in a second band on a similar timescale.

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arXiv2018

Spitzer Opens New Path to Break Classic Degeneracy for Jupiter-Mass Microlensing Planet OGLE-2017-BLG-1140Lb

We analyze the combined Spitzer and ground-based data for OGLE-2017-BLG-1140 and show that the event was generated by a Jupiter-class $(m_p\simeq 1.6\,M_{\rm jup})$ planet orbiting a mid-late M dwarf $(M\simeq 0.2\,M_\odot)$ that lies $D_{LS}\simeq 1.0\,\mathrm{kpc}$ in the foreground of the microlensed, Galactic-bar, source star. The planet-host projected separation is $a_\perp \simeq 1.0\,\mathrm{au}$, i.e., well-beyond the snow line. By measuring the source proper motion ${\mathbfμ}_s$ from ongoing, long-term OGLE imaging, and combining this with the lens-source relative proper motion ${\mathbfμ}_\mathrm{rel}$ derived from the microlensing solution, we show that the lens proper motion ${\mathbfμ}_l={\mathbfμ}_\mathrm{rel} + {\mathbfμ}_s$ is consistent with the lens lying in the Galactic disk, although a bulge lens is not ruled out. We show that while the Spitzer and ground-based data are comparably well fitted by planetary (i.e., binary-lens, 2L1S) models and by binary-source (1L2S) models, the combination of Spitzer and ground-based data decisively favor the planetary model. This is a new channel to resolve the 2L1S/1L2S degeneracy, which can be difficult to break in some cases.

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arXiv2018

The Cluster AgeS Experiment (CASE). Detecting Photometric Variability with the Friends of Friends Algorithm

We adapt the friends of friends algorithm to the analysis of light curves, and show that it can be successfully applied to searches for transient phenomena in large photometric databases. As a test case we search OGLE-III light curves for known dwarf novae. A single combination of control parameters allows to narrow the search to 1% of the data while reaching a $\sim$90% detection efficiency. A search involving $\sim$2% of the data and three combinations of control parameters can be significantly more effective - in our case a 100% efficiency is reached. The method can also quite efficiently detect semi-regular or strictly periodic variability. We report 28 new variables found in the field of the globular cluster M22, which was examined earlier with the help of periodicity-searching algorithms

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arXiv2018

OGLE-2017-BLG-0373Lb: A Jovian Mass-Ratio Planet Exposes A New Accidental Microlensing Degeneracy

We report the discovery of microlensing planet OGLE-2017-BLG-0373Lb. We show that while the planet-host system has an unambiguous microlens topology, there are two geometries within this topology that fit the data equally well, which leads to a factor 2.5 difference in planet-host mass ratio, i.e., $q=1.5\times 10^{-3}$ vs. $q=0.6\times 10^{-3}$. We show that this is an "accidental degeneracy" in the sense that it is due to a gap in the data. We dub it "the caustic-chirality degeneracy". We trace the mathematical origins of this degeneracy, which should enable similar degenerate solutions to be easily located in the future. A Bayesian estimate, based on a Galactic model, yields a host mass $M=0.25^{+0.30}_{-0.15} M_\odot$ at a distance $D_L=5.9^{+1.3}_{-1.95}$ kpc. The lens-source relative proper motion is relatively fast, $μ=9$ mas/yr, which implies that the host mass and distance can be determined by high-resolution imaging after about 10 years. The same observations could in principle resolve the discrete degeneracy in $q$, but this will be more challenging.

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arXiv2018

OGLE-2017-BLG-0482Lb: A Microlensing Super-Earth Orbiting a Low-mass Host Star

We report the discovery of a planetary system in which a super-earth orbits a late M-dwarf host. The planetary system was found from the analysis of the microlensing event OGLE-2017-BLG-0482, wherein the planet signal appears as a short-term anomaly to the smooth lensing light curve produced by the host. Despite its weak signal and short duration, the planetary signal was firmly detected from the dense and continuous coverage by three microlensing surveys. We find a planet/host mass ratio of $q\sim 1.4\times 10^{-4}$. We measure the microlens parallax $π_{\rm E}$ from the long-term deviation in the observed lensing light curve, but the angular Einstein radius $θ_{\rm E}$ cannot be measured because the source trajectory did not cross the planet-induced caustic. Using the measured event timescale and the microlens parallax, we find that the masses of the planet and the host are $M_{\rm p}=9.0_{-4.5}^{+9.0}\ M_\oplus$ and $M_{\rm host}=0.20_{-0.10}^{+0.20}\ M_\odot$, respectively, and the projected separation between them is $a_\perp=1.8_{-0.7}^{+0.6}$ au. The estimated distance to the lens is $D_{\rm L}=5.8_{-2.1}^{+1.8}$ kpc. The discovery of the planetary system demonstrates that microlensing provides an important method to detect low-mass planets orbiting low-mass stars.

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arXiv2018

OGLE-2017-BLG-1522: A giant planet around a brown dwarf located in the Galactic bulge

We report the discovery of a giant planet in the OGLE-2017-BLG-1522 microlensing event. The planetary perturbations were clearly identified by high-cadence survey experiments despite the relatively short event timescale of $t_{\rm E} \sim 7.5$ days. The Einstein radius is unusually small, $θ_{\rm E} = 0.065\,$mas, implying that the lens system either has very low mass or lies much closer to the microlensed source than the Sun, or both. A Bayesian analysis yields component masses $(M_{\rm host}, M_{\rm planet})=(46_{-25}^{+79}, 0.75_{-0.40}^{+1.26})~M_{\rm J}$ and source-lens distance $D_{\rm LS} = 0.99_{-0.54}^{+0.91}~{\rm kpc}$, implying that this is a brown-dwarf/Jupiter system that probably lies in the Galactic bulge, a location that is also consistent with the relatively low lens-source relative proper motion $μ= 3.2 \pm 0.5~{\rm mas}~{\rm yr^{-1}}$. The projected companion-host separation is $0.59_{-0.11}^{+0.12}~{\rm AU}$, indicating that the planet is placed beyond the snow line of the host, i.e., $a_{sl} \sim 0.12~{\rm AU}$. Planet formation scenarios combined with the small companion-host mass ratio $q \sim 0.016$ and separation suggest that the companion could be the first discovery of a giant planet that formed in a protoplanetary disk around a brown dwarf host.

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arXiv2018

A Likely Detection of a Two-Planet System in a Low Magnification Microlensing Event

We report on the analysis of a microlensing event OGLE-2014-BLG-1722 that showed two distinct short term anomalies. The best fit model to the observed light curves shows that the two anomalies are explained with two planetary mass ratio companions to the primary lens. Although a binary source model is also able to explain the second anomaly, it is marginally ruled out by 3.1 $σ$. The 2-planet model indicates that the first anomaly was caused by planet "b" with a mass ratio of $q = (4.5_{-0.6}^{+0.7}) \times 10^{-4}$ and projected separation in unit of the Einstein radius, $s = 0.753 \pm 0.004$. The second anomaly reveals planet "c" with a mass ratio of $q_{2} = (7.0_{-1.7}^{+2.3}) \times 10^{-4}$ with $Δχ^{2} \sim 170$ compared to the single planet model. Its separation has a so-called close-wide degeneracy. We estimated the physical parameters of the lens system from Bayesian analysis. This gives that the masses of planet b and c are $m_{\rm b} = 56_{-33}^{+51}\,M_{\oplus}$ and $m_{\rm c} = 85_{-51}^{+86}\,M_{\oplus}$, respectively, and they orbit a late type star with a mass of $M_{\rm host} = 0.40_{-0.24}^{+0.36}\,M_{\odot}$ located at $D_{\rm L} = 6.4_{-1.8}^{+1.3}\,\rm kpc$ from us. If the 2-planet model is true, this is the third multiple planet system detected by using the microlensing method, and the first multiple planet system detected in the low magnification events, which are dominant in the microlensing survey data. The occurrence rate of multiple cold gas giant systems is estimated using the two such detections and a simple extrapolation of the survey sensitivity of 6 year MOA microlensing survey (Suzuki et al. 2016) combined with the 4 year $μ$FUN detection efficiency (Gould et al. 2010). It is estimated that $6 \pm 2\,\%$ of stars host two cold giant planets.

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arXiv2018

OGLE-2017-BLG-1130: The First Binary Gravitational Microlens Detected From Spitzer Only

We analyze the binary gravitational microlensing event OGLE-2017-BLG-1130 (mass ratio q~0.45), the first published case in which the binary anomaly was only detected by the Spitzer Space Telescope. This event provides strong evidence that some binary signals can be missed by observations from the ground alone but detected by Spitzer. We therefore invert the normal procedure, first finding the lens parameters by fitting the space-based data and then measuring the microlensing parallax using ground-based observations. We also show that the normal four-fold space-based degeneracy in the single-lens case can become a weak eight-fold degeneracy in binary-lens events. Although this degeneracy is resolved in event OGLE-2017-BLG-1130, it might persist in other events.

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arXiv2018

OGLE-2017-BLG-0329L: A Microlensing Binary Characterized with Dramatically Enhanced Precision Using Data from Space-based Observations

Mass measurements of gravitational microlenses require one to determine the microlens parallax $\pie$, but precise $\pie$ measurement, in many cases, is hampered due to the subtlety of the microlens-parallax signal combined with the difficulty of distinguishing the signal from those induced by other higher-order effects. In this work, we present the analysis of the binary-lens event OGLE-2017-BLG-0329, for which $\pie$ is measured with a dramatically improved precision using additional data from space-based $Spitzer$ observations. We find that while the parallax model based on the ground-based data cannot be distinguished from a zero-$\pie$ model at 2$σ$ level, the addition of the $Spitzer$ data enables us to identify 2 classes of solutions, each composed of a pair of solutions according to the well-known ecliptic degeneracy. It is found that the space-based data reduce the measurement uncertainties of the north and east components of the microlens-parallax vector $\pivec_{\rm E}$ by factors $\sim 18$ and $\sim 4$, respectively. With the measured microlens parallax combined with the angular Einstein radius measured from the resolved caustic crossings, we find that the lens is composed of a binary with components masses of either $(M_1,M_2)\sim (1.1,0.8)\ M_\odot$ or $\sim (0.4,0.3)\ M_\odot$ according to the two solution classes. The first solution is significantly favored but the second cannot be securely ruled out based on the microlensing data alone. However, the degeneracy can be resolved from adaptive optics observations taken $\sim 10$ years after the event.

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arXiv2018

A Neptune-mass Free-floating Planet Candidate Discovered by Microlensing Surveys

Current microlensing surveys are sensitive to free-floating planets down to Earth-mass objects. All published microlensing events attributed to unbound planets were identified based on their short timescale (below two days), but lacked an angular Einstein radius measurement (and hence lacked a significant constraint on the lens mass). Here, we present the discovery of a Neptune-mass free-floating planet candidate in the ultrashort ($t_{\rm E}=0.320\pm0.003$ days) microlensing event OGLE-2016-BLG-1540. The event exhibited strong finite-source effects, which allowed us to measure its angular Einstein radius of $θ_{\rm E}=9.2\pm0.5\,μ$as. There remains, however, a degeneracy between the lens mass and distance. The combination of the source proper motion and source-lens relative proper motion measurements favors a Neptune-mass lens located in the Galactic disk. However, we cannot rule out that the lens is a Saturn-mass object belonging to the bulge population. We exclude stellar companions up to 15 au.

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arXiv2018

OGLE-2017-BLG-1434Lb: Eighth q < 1 * 10^-4 Mass-Ratio Microlens Planet Confirms Turnover in Planet Mass-Ratio Function

We report the discovery of a cold Super-Earth planet (m_p=4.4 +/- 0.5 M_Earth) orbiting a low-mass (M=0.23 +/- 0.03 M_Sun) M dwarf at projected separation a_perp = 1.18 +/- 0.10 AU, i.e., about 1.9 times the snow line. The system is quite nearby for a microlensing planet, D_Lens = 0.86 +/- 0.09 kpc. Indeed, it was the large lens-source relative parallax pi_rel=1.0 mas (combined with the low mass M) that gave rise to the large, and thus well-measured, "microlens parallax" that enabled these precise measurements. OGLE-2017-BLG-1434Lb is the eighth microlensing planet with planet-host mass ratio q < 1 * 10^-4. We apply a new planet-detection sensitivity method, which is a variant of "V/V_max", to seven of these eight planets to derive the mass-ratio function in this regime. We find dN/d(ln q) ~ q^p, with p = 1.05 (+0.78,-0.68), which confirms the "turnover" in the mass function found by Suzuki et al. relative to the power law of opposite sign n = -0.93 +/- 0.13 at higher mass ratios q >~ 2 * 10^-4. We combine our result with that of Suzuki et al. to obtain p = 0.73 (+0.42,-0.34).

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arXiv2018

A Gravitationally Lensed Quasar Discovered in OGLE

We report the discovery of a new gravitationally lensed quasar (double) from the Optical Gravitational Lensing Experiment (OGLE) identified inside the $\sim$670 sq. deg area encompassing the Magellanic Clouds. The source was selected as one of $\sim$60 "red W1-W2" mid-IR objects from WISE and having a significant amount of variability in OGLE for both two (or more) nearby sources. This is the first detection of a gravitational lens, where the discovery is made "the other way around", meaning we first measured the time delay between the two lensed quasar images of $-132<t_{\rm AB}<-76$ days (90% CL), with the median $t_{\rm AB}\approx-102$ days (in the observer frame), and where the fainter image B lags image A. The system consists of the two quasar images separated by 1.5" on the sky, with $I\approx20.0$ mag and $I\approx19.6$ mag, respectively, and a lensing galaxy that becomes detectable as $I \approx 21.5$ mag source, 1.0" from image A, after subtracting the two lensed images. Both quasar images show clear AGN broad emission lines at $z=2.16$ in the NTT spectra. The SED fitting with the fixed source redshift provided the estimate of the lensing galaxy redshift of $z \approx 0.9 \pm 0.2$ (90% CL), while its type is more likely to be elliptical (the SED-inferred and lens-model stellar mass is more likely present in ellipticals) than spiral (preferred redshift by the lens model).

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arXiv2018

The OGLE Collection of Variable Stars. Classical, Type II, and Anomalous Cepheids toward the Galactic Center

We present a collection of classical, type II, and anomalous Cepheids detected in the OGLE fields toward the Galactic center. The sample contains 87 classical Cepheids pulsating in one, two or three radial modes, 924 type II Cepheids divided into BL Her, W Vir, peculiar W Vir, and RV Tau stars, and 20 anomalous Cepheids - first such objects found in the Galactic bulge. Additionally, we upgrade the OGLE Collection of RR Lyr stars in the Galactic bulge by adding 828 newly identified variables. For all Cepheids and RR Lyr stars, we publish time-series VI photometry obtained during the OGLE-IV project, from 2010 through 2017. We discuss basic properties of our classical pulsators: their spatial distribution, light curve morphology, period-luminosity relations, and position in the Petersen diagram. We present the most interesting individual objects in our collection: a type II Cepheid with additional eclipsing modulation, W Vir stars with the period doubling effect and the RVb phenomenon, a mode-switching RR Lyr star, and a triple-mode anomalous RRd star.

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arXiv2018

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

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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