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Register a new userOGLE-2017-BLG-0329L: A Microlensing Binary Characterized with Dramatically Enhanced Precision Using Data from Space-based Observations
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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$sigma$ 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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We present a combined analysis of the observations of the gravitational microlensing event OGLE-2015-BLG-0479 taken both from the ground and by the {it Spitzer Space Telescope}. The light curves seen from the ground and from space exhibit a time offset of $sim 13$ days between the caustic spikes, indicating that the relative lens-source positions seen from the two places are displaced by parallax effects. From modeling the light curves, we measure the space-based microlens parallax. Combined with the angular Einstein radius measured by analyzing the caustic crossings, we determine the mass and distance of the lens. We find that the lens is a binary composed of two G-type stars with masses $sim 1.0 M_odot$ and $sim 0.9 M_odot$ located at a distance $sim 3$ kpc. In addition, we are able to constrain the complete orbital parameters of the lens thanks to the precise measurement of the microlens parallax derived from the joint analysis. In contrast to the binary event OGLE-2014-BLG-1050, which was also observed by {it Spitzer}, we find that the interpretation of OGLE-2015-BLG-0479 does not suffer from the degeneracy between $(pm,pm)$ and $(pm,mp)$ solutions, confirming that the four-fold parallax degeneracy in single-lens events collapses into the two-fold degeneracy for the general case of binary-lens events. The location of the blend in the color-magnitude diagram is consistent with the lens properties, suggesting that the blend is the lens itself. The blend is bright enough for spectroscopy and thus this possibility can be checked from future follow-up observations.
We present the analysis of the caustic-crossing binary microlensing event OGLE-2017-BLG-0039. Thanks to the very long duration of the event, with an event time scale $t_{rm E}sim 130$ days, the microlens parallax is precisely measured despite its small value of $piesim 0.06$. The analysis of the well-resolved caustic crossings during both the source stars entrance and exit of the caustic yields the angular Einstein radius $thetaesim 0.6$~mas. The measured $pie$ and $thetae$ indicate that the lens is a binary composed of two stars with masses $sim 1.0~M_odot$ and $sim 0.15~M_odot$, and it is located at a distance of $sim 6$ kpc. From the color and brightness of the lens estimated from the determined lens mass and distance, it is expected that $sim 2/3$ of the $I$-band blended flux comes from the lens. Therefore, the event is a rare case of a bright lens event for which high-resolution follow-up observations can confirm the nature of the lens.
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 $sim 1.3thetae$, where $thetae$ is the angular Einstein radius. Analysis of the caustic-crossing part yields $thetae=1.77pm 0.16$~mas and a lens-source relative proper motion of $mu =12.4pm 1.1~{rm mas}~{rm yr}^{-1}$. The measured $mu$ 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 $M_1sim 0.4~M_odot$ and $M_2sim 0.2~M_odot$ 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.
We report a giant exoplanet discovery in the microlensing event OGLE-2017-BLG-1049, which is a planet-host star mass ratio of $q=9.53pm0.39times10^{-3}$ and has a caustic crossing feature in the Korea Microlensing Telescope Network (KMTNet) observations. The caustic crossing feature yields an angular Einstein radius of $theta_{rm E}=0.52 pm 0.11 {rm mas}$. However, the microlens parallax is not measured because of the time scale of the event $t_{rm E}simeq 29 {rm days}$, which is not long enough in this case to determine the microlens parallax. Thus, we perform a Bayesian analysis to estimate physical quantities of the lens system. From this, we find that the lens system has a star with mass $M_{rm h}=0.55^{+0.36}_{-0.29} M_{odot}$ hosting a giant planet with $M_{rm p}=5.53^{+3.62}_{-2.87} M_{rm Jup}$, at a distance of $D_{rm L}=5.67^{+1.11}_{-1.52} {rm kpc}$. The projected star-planet separation in units of the Einstein radius $(theta_{rm E})$ corresponding to the total mass of the lens system is $a_{perp}=3.92^{+1.10}_{-1.32} rm{au}$. This means that the planet is located beyond the snow line of the host. The relative lens-source proper motion is $mu_{rm rel}sim 7 rm{mas yr^{-1}}$, thus the lens and source will be separated from each other within 10 years. Then the flux of the host star can be measured by a 30m class telescope with high-resolution imaging in the future, and thus its mass can be determined.
We report the discovery of a binary composed of two brown dwarfs, based on the analysis of the microlensing event OGLE-2016-BLG-1469. Thanks to detection of both finite-source and microlens-parallax effects, we are able to measure both the masses $M_1sim 0.05 M_odot$, $M_2sim 0.01 M_odot$, and distance $D_{rm L} sim 4.5$ kpc, as well as the projected separation $a_perp sim 0.33$ au. This is the third brown-dwarf binary detected using the microlensing method, demonstrating the usefulness of microlensing in detecting field brown-dwarf binaries with separations less than 1 au.
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