High-magnification gravitational microlensing events provide an important channel of detecting planetary systems with multiple giants located at their birth places. In order to investigate the potential existence of additional planets, we reanalyze the light curves of the eight high-magnification microlensing events for each of which a single planet was previously detected. The analyzed events include OGLE-2005-BLG-071, OGLE-2005-BLG-169, MOA-2007-BLG-400, MOA-2008-BLG-310, MOA-2009-BLG-319, MOA-2009-BLG-387, MOA-2010-BLG-477, and MOA-2011-BLG-293. We find that including an additional planet improves fits with $Δχ^2 < 80$ for seven out of eight analyzed events. For MOA-2009-BLG-319, the improvement is relatively big with $Δχ^2 \sim 143$. From inspection of the fits, we find that the improvement of the fits is attributed to systematics in data. Although no clear evidence of additional planets is found, it is still possible to constrain the existence of additional planets in the parameter space. For this purpose, we construct exclusion diagrams showing the confidence levels excluding the existence of an additional planet as a function of its separation and mass ratio. We also present the exclusion ranges of additional planets with 90\% confidence level for Jupiter, Saturn, and Uranus-mass planets.
A measurement by microlensing of the planetary mass function of planets with masses ranging from 5M_E to 10M_J and orbital radii from 0.5 to 10 AU was reported recently. A strategy for extending the mass range down to (1-3)M_E is proposed here. This entails monitoring the peaks of a few tens of microlensing events with moderately high magnifications with 1-2m class telescopes. Planets of a few Earth masses are found to produce deviations of ~ 5% to the peaks of microlensing light curves with durations ~ (0.7-3)hr in events with magnification ~ 100 if the projected separation of the planet lies…
We analyze the extreme high-magnification microlensing event OGLE-2008-BLG-279, which peaked at a maximum magnification of A ~ 1600 on 30 May 2008. The peak of this event exhibits both finite-source effects and terrestrial parallax, from which we determine the mass of the lens, M_l=0.64 +/- 0.10 M_Sun, and its distance, D_l = 4.0 +/- 0.6. We rule out Jupiter-mass planetary companions to the lens star for projected separations in the range 0.5-20 AU. More generally, we find that this event was sensitive to planets with masses as small as 0.2 M_Earth ~= 2 M_Mars with projected separations near t…
The microlensing effective timescale t_eff=beta*t_E is used frequently in high-magnification (beta << 1) microlensing events, because it is better constrained than either the impact parameter beta or the Einstein timescale t_E separately. It also facilitates intuitive understanding of lightcurves prior to determination of a model. Similar considerations may apply to very low magnification events. I therefore provide a generalization of this quantity to all events: t_eff = beta*t_E*sqrt((1+beta^2/2)(1+beta^2/4)).
Current microlensing follow-up observations focus on high-magnification events because of the high efficiency of planet detection. However, central perturbations of high-magnification events caused by a planet can also be produced by a very close or a very wide binary companion, and the two kinds of central perturbations are not generally distinguished without time consuming detailed modeling (a planet-binary degeneracy). Hence, it is important to resolve the planet-binary degeneracy that occurs in high-magnification events. In this paper, we investigate caustic-crossing high-magnification eve…
The microlensing technique has found 10 exoplanets to date and promises to discover more in the near future. While planetary transit light curves all show a familiar shape, planetary perturbations to microlensing light curves can manifest a wide variety of morphologies. We present a graphical guide that may be useful when understanding microlensing events showing planetary caustic perturbations.
We report the discovery of a high mass-ratio planet $q=0.012$, i.e., 13 times higher than the Jupiter/Sun ratio. The host mass is not presently measured but can be determined or strongly constrained from adaptive optics imaging. The planet was discovered in a small archival study of high-magnification events in pure-survey microlensing data, which was unbiased by the presence of anomalies. The fact that it was previously unnoticed may indicate that more such planets lie in archival data and could be discovered by similar systematic study. In order to understand the transition from predominantl…
Microlensing offers a unique opportunity to probe exoplanets that are temperate and beyond the snow line, as small as Jovian satellites, at extragalactic distance, and even free floating exoplanets, regimes where the sensitivity of other methods drops dramatically. This is because microlensing does not depend on the brightness of the planetary host star. The microlensing method thus provides great leverage in studying the exoplanets beyond the snow line, posing tests to the core accretion mechanism, especially on the run-away phase of gas accretion to form giant planets. Here we propose to rob…
We conduct the first microlensing simulation in the context of planet formation model. The planet population is taken from the Ida & Lin core accretion model for $0.3M_\odot$ stars. With $6690$ microlensing events, we find for a simplified Korea Microlensing Telescopes Network (KMTNet) the fraction of planetary events is $2.9\%$ , out of which $5.5\%$ show multiple-planet signatures. The number of super-Earths, super-Neptunes and super-Jupiters detected are expected to be almost equal. Our simulation shows that high-magnification events and massive planets are favored by planet detections, whi…
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, $s = 0.753 \pm 0.004$…
To improve the planet detection efficiency, current planetary microlensing experiments are focused on high-magnification events searching for planetary signals near the peak of lensing light curves. However, it is known that central perturbations can also be produced by binary companions and thus it is important to distinguish planetary signals from those induced by binary companions. In this paper, we analyze the light curves of microlensing events OGLE-2007-BLG-137/MOA-2007-BLG-091, OGLE-2007-BLG-355/MOA-2007-BLG-278, and MOA-2007-BLG-199/OGLE-2007-BLG-419, for all of which exhibit short-ter…
We present Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3) observations of the source and lens stars for planetary microlensing event OGLE-2005-BLG-169, which confirm the relative proper motion prediction due to the planetary light curve signal observed for this event. This (and the companion Keck result) provide the first confirmation of a planetary microlensing signal, for which the deviation was only 2%. The follow-up observations determine the flux of the planetary host star in multiple passbands and remove light curve model ambiguity caused by sparse sampling of part of the light …
I present a previously unpublished method for modeling multiple lens microlensing events that is based on the image centered ray shooting approach of Bennett and Rhie. It has been used to model all a wide variety of binary and triple lens systems, but it is designed to efficiently model high-magnification planetary microlensing events, because these are, by far, the most challenging events to model. It is designed to be efficient enough to handle microlensing events with more than two lens masses and lens orbital motion. This method uses a polar coordinate integration grid with a smaller grid …
In future high-cadence microlensing surveys, planets can be detected through a new channel of an independent event produced by the planet itself. The two populations of planets to be detected through this channel are wide-separation planets and free-floating planets. Although they appear as similar short time-scale events, the two populations of planets are widely different in nature and thus distinguishing them is important. In this paper, we investigate the lensing properties of events produced by planets with moderately wide separations from host stars. We find that the lensing behavior of …
To move one step forward toward a Galactic distribution of planets, we present the first planet sensitivity analysis for microlensing events with simultaneous observations from space and the ground. We present this analysis for two such events, OGLE-2014-BLG-0939 and OGLE-2014-BLG-0124, which both show substantial planet sensitivity even though neither of them reached high magnification. This suggests that an ensemble of low to moderate magnification events can also yield significant planet sensitivity and therefore probability to detect planets. The implications of our results to the ongoing …
Predictions of the Nancy Grace Roman Space Telescope Galactic Exoplanet Survey. V. Detection Rates of Multiplanetary Systems in High Magnification Microlensing Events
A close-in giant planetary (CGP) system has a net polarization signal whose value varies depending on the orbital phase of the planet. This polarization signal is either caused by the stellar occultation or by reflected starlight from the surface of the orbiting planet. When the CGP system is located in the Galactic bulge, its polarization signal becomes too weak to be measured directly. One method for detecting and characterizing these weak polarization signatures due to distant CGP systems is gravitational microlensing. In this work, we focus on potential polarimetric observations of highly-…
Extensive simulations of planetary microlensing are necessary both before and after a survey is conducted: before to design and optimize the survey and after to understand its detection efficiency. The major bottleneck in such computations is the computation of lightcurves. However, for low-mass planets most of these computations are wasteful, as most lightcurves do not contain detectable planetary signatures. In this paper I develop a parameterization of the binary microlens that is conducive to avoiding lightcurve computations. I empirically find analytic expressions describing the limits of…
Since 1995, more than 500 exoplanets have been detected using different techniques, of which 11 were detected with gravitational microlensing. Most of these are gravitationally bound to their host stars. There is some evidence of free-floating planetary mass objects in young star-forming regions, but these objects are limited to massive objects of 3 to 15 Jupiter masses with large uncertainties in photometric mass estimates and their abundance. Here, we report the discovery of a population of unbound or distant Jupiter-mass objects, which are almost twice (1.8_{-0.8}^{+1.7}) as common as main-…
We present the first measurement of planet frequency beyond the "snow line" for planet/star mass-ratios[-4.5<log q<-2]: d^2 N/dlog q/dlog s=(0.36+-0.15)/dex^2 at mean mass ratio q=5e-4, and consistent with being flat in log projected separation, s. Our result is based on a sample of 6 planets detected from intensive follow-up of high-mag (A>200) microlensing events during 2005-8. The sample host stars have typical mass M_host 0.5 Msun, and detection is sensitive to planets over a range of projected separations (R_E/s_max,R_E*s_max), where R_E 3.5 AU sqrt(M_host/Msun) is the Einstein radius and…
To a first approximation, the microlensing phenomenon is achromatic, and great advancement has been achieved in the interpretation of the achromatic signals, which among other achievements has led to the discovery and characterization of well above $100$ new exoplanets. At higher order accuracy in the observations, microlensing has a chromatic component (a color term) which has so far been much less explored. Here, we analyze the chromatic microlensing effect of $4$ different physical phenomena, which have the potential to add important new knowledge about the stellar properties not easily rea…
The phenomenon of microlensing has successfully been used to detect extrasolar planets. By observing characteristic, rare deviations in the gravitational microlensing light curve one can discover that a lens is a star--planet system. In this paper we consider an opposite case where the lens is a single star and the source has a transiting planetary companion. We have studied the light curve of a source star with transiting companion magnified during microlensing event. Our model shows that in dense stellar fields, in which blending is significant, the light drop generated by transits is greate…
The transient event labeled as TCP J05074264+2447555 recently discovered towards the Taurus region was quickly recognized to be an ongoing microlensing event on a source located at distance of only $700-800$ pc from Earth. Here, we show that observations with high sampling rate close to the time of maximum magnification revealed features that imply the presence of a binary lens system with very low mass ratio components. We present a complete description of the binary lens system which hosts an Earth-like planet with most likely mass of $9.2\pm 6.6$ M$_{\oplus}$. Furthermore, the source estima…
We show that for high-magnification (Amax > 100) microlensing events, accurate microlens parallaxes can be obtained from three or fewer photometric measurements from a small telescope on a satellite in solar orbit at ~1 AU from Earth. This is 1--2 orders of magnitude less observing resources than are required for standard space-based parallaxes. Such microlens parallax measurements would yield accurate mass and distance measurements to the lens for all cases in which finite-source effects were observed from the ground over peak. This would include virtually all high-magnification events with d…
We report analysis of high microlensing event MOA-2008-BLG-379, which has a strong microlensing anomaly at its peak, due to a massive planet with a mass ratio of q = 6.9 x 10^{-3}. Because the faint source star crosses the large resonant caustic, the planetary signal dominates the light curve. This is unusual for planetary microlensing events, and as a result, the planetary nature of this light curve was not immediately noticed. The planetary nature of the event was found when the MOA Collaboration conducted a systematic study of binary microlensing events previously identified by the MOA aler…
Context. Gaia DR2 offers unparalleled precision on stars' parallaxes and proper motions. This allows the prediction of microlensing events for which the lens stars (and any planets they possess) are nearby and may be well studied and characterised. Aims. We identify a number of potential microlensing events that will occur before the year 2035.5, 20 years from the Gaia DR2 reference epoch. Methods. We query Gaia DR2 for potential lenses within 100 pc, extract parallaxes and proper motions of the lenses and background sources, and identify potential lensing events. We estimate the lens masses f…
We report new results for the gravitational microlensing target OGLE-2011-BLG-0950 from adaptive optics (AO) images using the Keck observatory. The original analysis by Choi et al. 2012 reports degenerate solutions between planetary and stellar binary lens systems. This is due to a degeneracy in high magnification events where the shape of the light curve peak can be explained by a source approach to two different cusp geometries with different source radius crossing times. This particular case is the most important type of degeneracy for exoplanet demographics, because the distinction between…
We introduce MulensModel, a software package for gravitational microlensing modeling. The package provides a framework for calculating microlensing model magnification curves and goodness-of-fit statistics for microlensing events with single and binary lenses as well as a variety of higher-order effects: extended sources with limb-darkening, annual microlensing parallax, satellite microlensing parallax, and binary lens orbital motion. The software could also be used for analysis of the planned microlensing survey by the NASA flag-ship WFIRST satellite. MulensModel is available at https://githu…
Galactic microlensing has been widely used to study the star and planet. The stellar wind plays an important role in the formation, environment and habitability of the planet. In this work we study a binary microlensing system including the stellar wind, i.e. a star with plasma environment plus a planet. Plasma surrounding the main lens causes chromatic deflection of the light rays, in addition to the gravitational one. As a result, such a lensing system can generate complicated caustics which depends on the different lensing parameters. In this work we study the magnification curves for diffe…
It is shown that the linear approximation of the central caustic for the planetary ($q\ll1$) microlensing is valid if $|1-s|\gg q^{1/3}$ (where $q$ is the mass ratio and $s$ is the projected separation in the unit of the Einstein ring radius of the primary). The condition is also consistent with the requirement that the binary separation is far from those in the resonant binary regime resulting in a single six-cusp caustic. Given that the linear approximation of the caustic is invariant under $s\leftrightarrow s^{-1}$, the close/wide binary degeneracy observed under the same condition may be u…
We present the first short-duration candidate microlensing events from the Kepler K2 mission. From late April to early July 2016, Campaign 9 of K2 obtained high temporal cadence observations over a 3.7 square degree region of the Galactic bulge. Its primary objectives were to look for evidence of a free-floating planet (FFP) population using microlensing, and demonstrate the feasibility of space-based planetary microlensing surveys. Though Kepler K2 is far from optimal for microlensing, the recently developed MCPM photometric pipeline enables us to identify and model microlensing events. We de…
We present the analysis of a very high-magnification ($A\sim 900$) microlensing event KMT-2019-BLG-1953. A single-lens single-source (1L1S) model appears to approximately delineate the observed light curve, but the residuals from the model exhibit small but obvious deviations in the peak region. A binary lens (2L1S) model with a mass ratio $q\sim 2\times 10^{-3}$ improves the fits by $Δχ^2=181.8$, indicating that the lens possesses a planetary companion. From additional modeling by introducing an extra planetary lens component (3L1S model) and an extra source companion (2L2S model), it is foun…
TESS's First Bound Microlensing Planet: A Binary Microlensing Event Revealing a Planetary Companion toward the Galactic Plane
University of New Mexico（新墨西哥大学）；Université Marie et Louis Pasteur, CNRS, Institut UTINAM UMR 6213（玛丽和路易斯·巴斯德大学，法国国家科学研究中心，UTINAM研究所）；Texas Tech University（德克萨斯理工大学）；The Ohio State University（俄亥俄州立大学）
Gravitational microlensing provides a unique window on the properties and prevalence of extrasolar planetary systems because of its ability to find low-mass planets at separations of a few AU. The early evidence from microlensing indicates that the most common type of exoplanet yet detected are the so-called "super-Earth" planets of ~10 Earth-masses at a separation of a few AU from their host stars. The detection of two such planets indicates that roughly one third of stars have such planets in the separation range 1.5-4 AU, which is about an order of magnitude larger than the prevalence of ga…
MOA-2006-BLG-074 was selected as one of the most promising planetary candidates in a retrospective analysis of the MOA collaboration: its asymmetric high-magnification peak can be perfectly explained by a source passing across a central caustic deformed by a small planet. However, after a detailed analysis of the residuals, we have realized that a single lens and a source orbiting with a faint companion provides a more satisfactory explanation for all the observed deviations from a Paczynski curve and the only physically acceptable interpretation. Indeed the orbital motion of the source is con…
Gravitational microlensing is one of the methods to detect exoplanets; planets outside our solar system. Here we focus on theoretical modeling of three lens systems and in particular circumbinary systems. Circumbinary systems include two stars and a planet and are estimated to make up a sizable portion of all exoplanets. Extending a method developed for binary lenses to the three lens case, we explore the parameter space of circumbinary systems producing exact magnification maps and light curves.
At present, microlensing light curves from different telescopes and filters are photometrically aligned by fitting them to a common model. We present a second method based on photometry of common field stars. If two spectral responses are similar (or the color of the source is known) then this technique can resolve important ambiguities that frequently arise when predicting the future course of the event, and that occasionally persist even when the event is over. Or if the spectral responses are different, it can be used to derive the color of the source when that is unknown. We present the es…
High-magnification microlensing events provide an important channel to detect planets. Perturbations near the peak of a high-magnification event can be produced either by a planet or a binary companion. It is known that central perturbations induced by both types of companions can be generally distinguished due to the basically different magnification pattern around caustics. In this paper, we present a case of central perturbations for which it is difficult to distinguish the planetary and binary interpretations. The peak of a lensing light curve affected by this perturbation appears to be bl…
University of California, Berkeley（加州大学伯克利分校）；The Ohio State University（俄亥俄州立大学）
While gravitational microlensing by planetary systems provides unique vistas on the properties of exoplanets, observations of a given 2-body microlensing event can often be interpreted with multiple distinct physical configurations. Such ambiguities are typically attributed to the close-wide and inner-outer types of degeneracies that arise from transformation invariances and symmetries of microlensing caustics. However, there remain unexplained inconsistencies between aforementioned theories and observations. Here, leveraging a fast machine learning inference framework, we present the discover…
