Lens Parallax Research Articles

Predicted microlensing events from analysis of Gaia Data Release 2

Context. Astrometric microlensing can be used to make precise measurements of the masses of lens stars that are independent of their assumed internal physics. Such direct mass measurements, obtained purely by observing the gravitational effects of the stars on external objects, are crucial for validating theoretical stellar models. Specifically, astrometric microlensing provides a channel to direct mass measurements of single stars for which so few measurements exist. Microlensing events that also exhibit a detectable photometric signature provide even stronger lens mass constraints. Aims. I use the astrometric solutions and photometric measurements of ~1.7 billion stars provided by Gaia Data Release 2 (GDR2) to predict microlensing events during the nominal Gaia mission and beyond. This will enable astronomers to observe the entirety of each event, including the peak, with appropriate observing resources. The data collected will allow precise lens mass measurements for white dwarfs and low-mass main sequence stars (K and M dwarfs) helping to constrain stellar evolutionary models. Methods. I search for source-lens pairs in GDR2 that could potentially lead to microlensing events between 25th July 2014 and 25th July 2026. I estimate the lens masses using GDR2 photometry and parallaxes, and appropriate model stellar isochrones. Combined with the source and lens parallax measurements from GDR2, this allows the Einstein ring radius to be computed for each source-lens pair. By considering the source and lens paths on the sky, I calculate the microlensing signals that are to be expected. Results. I present a list of 76 predicted microlensing events. Nine and five astrometric events will be caused by the white dwarf stars LAWD 37 and Stein 2051 B, respectively. A further nine events will exhibit detectable photometric and astrometric signatures. Of the remaining events, ten will exhibit astrometric signals with peak amplitudes above 0.5 mas, while the rest are low-amplitude astrometric events with peak amplitudes between 0.131 and 0.5 mas. Five and two events will reach their peaks during 2018 and 2019, respectively. Five of the photometric events have the potential to evolve into high-magnification events, which may also probe for planetary companions to the lenses.

OGLE-2014-BLG-0257L: A MICROLENSING BROWN DWARF ORBITING A LOW-MASS M DWARF

ABSTRACT In this paper, we report the discovery of a binary composed of a brown dwarf (BD) and a low-mass M dwarf from observation of the microlensing event OGLE-2014-BLG-0257. The resolution of the very brief caustic crossing combined with the detection of subtle continuous deviation in the lensing light curve induced by the Earth’s orbital motion enable us to precisely measure both the Einstein radius and the lens parallax , which are the two quantities needed to unambiguously determine the mass and distance to the lens. It is found that the companion is a substellar BD with a mass of ( ) and it is orbiting an M dwarf with a mass of . The binary is located at a distance of 1.25 ± 0.13 kpc toward the Galactic bulge and the projected separation between the binary components is 0.61 ± 0.07 au. The separation scaled by the mass of the host is . Based on the assumption that separations scale with masses, the discovered BD is located in the BD desert. With the growing sample of BDs in various environments, microlensing will provide a powerful probe of BDs in the Galaxy.

OGLE-2013-BLG-0578 L: A MICROLENSING BINARY COMPOSED OF A BROWN DWARF AND AN M DWARF

Determining physical parameters of binary microlenses is hampered by the lack of information about the angular Einstein radius due to the difficulty of resolving caustic crossings. In this paper, we present the analysis of the binary microlensing event OGLE-2013-BLG-0578, for which the caustic exit was precisely predicted in advance from real-time analysis, enabling to densely resolve the caustic crossing and to measure the Einstein radius. From the mass measurement of the lens system based on the Einstein radius combined with the additional information about the lens parallax, we identify that the lens is a binary that is composed of a late-type M-dwarf primary and a substellar brown-dwarf companion. The event demonstrates the capability of current real-time microlensing modeling and the usefulness of microlensing in detecting and characterizing faint or dark objects in the Galaxy.

A SUPER-JUPITER ORBITING A LATE-TYPE STAR: A REFINED ANALYSIS OF MICROLENSING EVENT OGLE-2012-BLG-0406

We present a detailed analysis of survey and follow-up observations of microlensing event OGLE-2012-BLG-0406 based on data obtained from 10 different observatories. Intensive coverage of the lightcurve, especially the perturbation part, allowed us to accurately measure the parallax effect and lens orbital motion. Combining our measurement of the lens parallax with the angular Einstein radius determined from finite-source effects, we estimate the physical parameters of the lens system. We find that the event was caused by a $2.73\pm 0.43\ M_{\rm J}$ planet orbiting a $0.44\pm 0.07\ M_{\odot}$ early M-type star. The distance to the lens is $4.97\pm 0.29$\ kpc and the projected separation between the host star and its planet at the time of the event is $3.45\pm 0.26$ AU. We find that the additional coverage provided by follow-up observations, especially during the planetary perturbation, leads to a more accurate determination of the physical parameters of the lens.

GRAVITATIONAL BINARY-LENS EVENTS WITH PROMINENT EFFECTS OF LENS ORBITAL MOTION

Gravitational microlensing events produced by lenses composed of binary masses are important because they provide a major channel to determine physical parameters of lenses. In this work, we analyze the light curves of two binary-lens events OGLE-2006-BLG-277 and OGLE-2012-BLG-0031 for which the light curves exhibit strong deviations from standard models. From modeling considering various second-order effects, we find that the deviations are mostly explained by the effect of the lens orbital motion. We also find that lens parallax effects can mimic orbital effects to some extent. This implies that modeling light curves of binary-lens events not considering orbital effects can result in lens parallaxes that are substantially different from actual values and thus wrong determinations of physical lens parameters. This demonstrates the importance of routine consideration of orbital effects in interpreting light curves of binary-lens events. It is found that the lens of OGLE-2006-BLG-277 is a binary composed of a low-mass star and a brown dwarf companion.

THE SECOND MULTIPLE-PLANET SYSTEM DISCOVERED BY MICROLENSING: OGLE-2012-BLG-0026Lb, c—A PAIR OF JOVIAN PLANETS BEYOND THE SNOW LINE

We report the discovery of a planetary system from observation of the high-magnification microlensing event OGLE-2012-BLG-0026. The lensing light curve exhibits a complex central perturbation with multiple features. We find that the perturbation was produced by two planets located near the Einstein ring of the planet host star. We identify 4 possible solutions resulting from the well-known close/wide degeneracy. By measuring both the lens parallax and the Einstein radius, we estimate the physical parameters of the planetary system. According to the best-fit model, the two planet masses are ~0.11 M_Jupiter and 0.68 M_Jupiter and they are orbiting a G-type main sequence star with a mass ~0.82 M_Sun. The projected separations of the individual planets are beyond the snow line in all four solutions, being ~3.8 AU and 4.6 AU in the best-fit solution. The deprojected separations are both individually larger and possibly reversed in order. This is the second multi-planet system with both planets beyond the snow line discovered by microlensing. This is the only such a system (other than the Solar System) with measured planet masses without sin(i) degeneracy. The planetary system is located at a distance 4.1 kpc from the Earth toward the Galactic center. It is very likely that extra light from stars other than the lensed star comes from the lens itself. If this is correct, it will be possible to obtain detailed information about the planet-host star from follow-up observation.

MICROLENSING BINARIES WITH CANDIDATE BROWN DWARF COMPANIONS

Brown dwarfs are important objects because they may provide a missing link between stars and planets, two populations that have dramatically different formation history. In this paper, we present the candidate binaries with brown dwarf companions that are found by analyzing binary microlensing events discovered during 2004 - 2011 observation seasons. Based on the low mass ratio criterion of q < 0.2, we found 7 candidate events, including OGLE-2004-BLG-035, OGLE-2004-BLG-039, OGLE-2007-BLG-006, OGLE-2007-BLG-399/MOA-2007-BLG-334, MOA-2011-BLG-104/OGLE-2011-BLG-0172, MOA-2011-BLG-149, and MOA-201-BLG-278/OGLE-2011-BLG-012N. Among them, we are able to confirm that the companions of the lenses of MOA-2011-BLG-104/OGLE-2011-BLG-0172 and MOA-2011-BLG-149 are brown dwarfs by determining the mass of the lens based on the simultaneous measurement of the Einstein radius and the lens parallax. The measured mass of the brown dwarf companions are (0.02 +/- 0.01) M_Sun and (0.019 +/- 0.002) M_Sun for MOA-2011-BLG-104/OGLE-2011-BLG-0172 and MOA-2011-BLG-149, respectively, and both companions are orbiting low mass M dwarf host stars. More microlensing brown dwarfs are expected to be detected as the number of lensing events with well covered light curves increases with new generation searches.

CHARACTERIZING LOW-MASS BINARIES FROM OBSERVATION OF LONG-TIMESCALE CAUSTIC-CROSSING GRAVITATIONAL MICROLENSING EVENTS

Despite astrophysical importance of binary star systems, detections are limited to those located in small ranges of separations, distances, and masses and thus it is necessary to use a variety of observational techniques for a complete view of stellar multiplicity across a broad range of physical parameters. In this paper, we report the detections and measurements of 2 binaries discovered from observations of microlensing events MOA-2011-BLG-090 and OGLE-2011-BLG-0417. Determinations of the binary masses are possible by simultaneously measuring the Einstein radius and the lens parallax. The measured masses of the binary components are 0.43 $M_{\odot}$ and 0.39 $M_{\odot}$ for MOA-2011-BLG-090 and 0.57 $M_{\odot}$ and 0.17 $M_{\odot}$ for OGLE-2011-BLG-0417 and thus both lens components of MOA-2011-BLG-090 and one component of OGLE-2011-BLG-0417 are M dwarfs, demonstrating the usefulness of microlensing in detecting binaries composed of low-mass components. From modeling of the light curves considering full Keplerian motion of the lens, we also measure the orbital parameters of the binaries. The blended light of OGLE-2011-BLG-0417 comes very likely from the lens itself, making it possible to check the microlensing orbital solution by follow-up radial-velocity observation. For both events, the caustic-crossing parts of the light curves, which are critical for determining the physical lens parameters, were resolved by high-cadence survey observations and thus it is expected that the number of microlensing binaries with measured physical parameters will increase in the future.

CHARACTERIZING LENSES AND LENSED STARS OF HIGH-MAGNIFICATION SINGLE-LENS GRAVITATIONAL MICROLENSING EVENTS WITH LENSES PASSING OVER SOURCE STARS

We present the analysis of the light curves of 9 high-magnification single-lens gravitational microlensing events with lenses passing over source stars, including OGLE-2004-BLG-254, MOA-2007-BLG-176, MOA-2007-BLG-233/OGLE-2007-BLG-302, MOA-2009-BLG-174, MOA-2010-BLG-436, MOA-2011-BLG-093, MOA-2011-BLG-274, OGLE-2011-BLG-0990/MOA-2011-BLG-300, and OGLE-2011-BLG-1101/MOA-2011-BLG-325. For all events, we measure the linear limb-darkening coefficients of the surface brightness profile of source stars by measuring the deviation of the light curves near the peak affected by the finite-source effect. For 7 events, we measure the Einstein radii and the lens-source relative proper motions. Among them, 5 events are found to have Einstein radii less than 0.2 mas, making the lenses candidates of very low-mass stars or brown dwarfs. For MOA-2011-BLG-274, especially, the small Einstein radius of $\theta_{\rm E}\sim 0.08$ mas combined with the short time scale of $t_{\rm E}\sim 2.7$ days suggests the possibility that the lens is a free-floating planet. For MOA-2009-BLG-174, we measure the lens parallax and thus uniquely determine the physical parameters of the lens. We also find that the measured lens mass of $\sim 0.84\ M_\odot$ is consistent with that of a star blended with the source, suggesting that the blend is likely to be the lens. Although we find planetary signals for none of events, we provide exclusion diagrams showing the confidence levels excluding the existence of a planet as a function of the separation and mass ratio.

OGLE-2009-BLG-023/MOA-2009-BLG-028: characterization of a binary microlensing event based on survey data

We report the result of the analysis of the light curve of a caustic-crossing binary-lens microlensing event OGLE-2009-BLG-023/MOA-2009-BLG-028. Even though the event was observed solely by survey experiments, we could uniquely determine the mass of the lens and distance to it by simultaneously measuring the Einstein radius and lens parallax. From this, we find that the lens system is composed of M-type dwarfs with masses $(0.50\pm 0.07) \ M_\odot$ and $(0.15\pm 0.02)\ M_\odot$ located in the Galactic disk with a distance of $\sim 1.8$ kpc toward the Galactic bulge direction. The event demonstrates that physical lens parameters of binary-lens events can be routinely determined from future high-cadence lensing surveys and thus microlensing can provide a new way to study Galactic binaries.

Microlensing Detection and Characterization of Wide‐Separation Planets

With their excellent photometric precision and dramatic increase in monitoring frequency, future microlensing survey experiments are expected to be sensitive to very short timescale, isolated events caused by free-floating and wide-separation planets with masses as low as a few lunar masses. The scientific value of these detections would be greatly enhanced if their nature (bound or unbound) could be accurately characterized and if the planet masses could be measured. Here we present a comprehensive discussion of the ability of microlensing to detect and characterize wide-separation planets. We estimate the probability of measuring the planetary Einstein radius θE,p for bound and free-floating planets; this is one of the two additional observables required to measure the planet mass. We carry out detailed simulations of the planetary events expected in next-generation surveys and estimate the resulting uncertainty in θE,p for these events. We show that, for main-sequence sources and Jupiter-mass planets, the caustic structure of wide-separation planets with projected separations of 20 AU substantially increases the probability of measuring the dimensionless source size and thus determining θE,p compared to the case of unbound planets. In this limit where the source is much smaller than the caustic, the effective cross section to measure θE,p to 10% is ~25% larger than the full width of the caustic. Measurement of the lens parallax is possible for low-mass planetary events through combined observations from the ground and a satellite located in an L2 orbit; this would complete the mass measurements for such wide-separation planets. Finally, short-duration events caused by bound planets can be routinely distinguished from those caused by free-floating planets for planet-star separations 20 AU from either the deviations due to the planetary caustic or (more often) the low-amplitude bump from the magnification due to the parent star.

Astrometric Parallax Measurements of Gravitational Microlensing Events

The Einstein timescale, which is the only information obtained from current microlensing experiments, results from a combination of the lens parameters that we want to determine. Of the methods for breaking the lens parameter degeneracy, the most promising and generally applicable method is to measure the lens parallax from simultaneous photometric observations of a lensing event from the ground and from a heliocentric satellite. However, the elegant idea of lens parallax measurement, which was proposed to resolve the lens parameter degeneracy, paradoxically suffers from another type of degeneracy, which arises because a given set of light curves can have two possible values of parallax (parallax degeneracy). In this paper we show that one can break the parallax degeneracy either from the simultaneous astrometric observations of an event by using a high-precision ground-based interferometer and a heliocentric satellite such as the Space Interferometry Mission or from a combination of the astrometric observations on one site (either the ground or the satellite) and the photometric observations on a different site. Since the proposed methods can measure both the lens parallax and proper motion at the same time, one can completely break the lens parameter degeneracy, and thus the physical parameters of individual lenses can be uniquely determined.

The Lens Parallax Method: Determining Redshifts of Faint Blue Galaxies through Gravitational Lensing

view Abstract Citations (47) References (44) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Lens Parallax Method: Determining Redshifts of Faint Blue Galaxies through Gravitational Lensing Bartelmann, Matthias ; Narayan, Ramesh Abstract We propose a new technique, which we call the lens parallax method, to determine simultaneously the redshift distribution of the faint blue galaxies and the mass distributions of foreground clusters of galaxies. The method is based on gravitational lensing and makes use of the following: (1) the amplitude of lensing-induced distortions of background galaxies increases with redshift; (2) the surface brightnesses of galaxies decrease steeply with redshift The distortions of galaxy images due to lensing are thus expected to be inversely correlated with surface brightness, allowing us to obtain relative distances to galaxies as a function of surface brightness. If the redshifts of the brightest galaxies are measured, then the relative distance scale can be converted to mean galaxy redshifts as a function of surface brightness. Further, by comparing the angular sizes of lensed galaxies with those of similar galaxies in empty control fields, it is possible to break the so-called mass sheet degeneracy inherent to cluster mass reconstruction techniques that are based purely on image ellipticities. This allows an unambiguous determination of the surface density of a lensing cluster. We describe an iterative algorithm based on these ideas and present numerical simulations which show that the proposed techniques are feasible with a sample of ≈10 rich clusters at moderate redshifts ∼0.3-0.4 and an equal number of control fields. The numerical tests show that the method can be used to determine the redshifts of galaxies with an accuracy of Δz ≈ 0.1-0.2 at z ∼1-1.7, and to measure the masses of lensing clusters to about 5% accuracy. Publication: The Astrophysical Journal Pub Date: September 1995 DOI: 10.1086/176200 arXiv: arXiv:astro-ph/9504086 Bibcode: 1995ApJ...451...60B Keywords: COSMOLOGY: GRAVITATIONAL LENSING; COSMOLOGY: OBSERVATIONS; GALAXIES: CLUSTERS: GENERAL; GALAXIES: DISTANCES AND REDSHIFTS; Astrophysics E-Print: 31 pages, uuencoded compressed postscript file containing 10 figures, to be published in the Sep. 20 issue of ApJ full text sources arXiv | ADS | data products SIMBAD (3)