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Register a new userDetection of Rotation in a Binary Microlens: PLANET Photometry of MACHO 97-BLG-41
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We analyze PLANET collaboration data for MACHO 97-BLG-41, the only microlensing event observed to date in which the source transits two disjoint caustics. The PLANET data, consisting of 46 V-band and 325 I-band observations from five southern observatories, span a period from the initial alert until the end of the event. Our data are incompatible with a static binary lens, but are well fit by a rotating binary lens of mass ratio q=0.34 and angular separation d ~ 0.5 (in units of the Einstein ring radius) in which the binary separation changes in size by delta d = -0.070 +/- 0.009 and in orientation by delta theta = (5.61 +/- 0.36) degrees during the 35.17 days between the separate caustic transits. We use this measurement combined with other observational constraints to derive the first kinematic estimate of the mass, distance, and period of a binary microlens. The relative probability distributions for these parameters peak at a total lens mass M ~ 0.3 solar masses (M-dwarf binary system), lens distance D_L ~ 5.5 kpc, and binary period P ~ 1.5 yr. The robustness of our model is demonstrated by its striking agreement with MACHO/GMAN data that cover several sharp features in the light curve not probed by the PLANET observations, and which did not enter our modeling procedure in any way. Available data sets thus indicate that the light curve of MACHO 97-BLG-41 can be modeled as a source crossing two caustics of a physically-realistic rotating binary so that, contrary to a recent suggestion, the additional effects of a postulated planetary companion to the binary lens are not required.
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We present the PLANET photometric dataset for the binary-lens microlensing event MACHO 97-BLG-28 consisting of 696 I and V-band measurements, and analyze it to determine the radial surface brightness profile of the Galactic bulge source star. The microlensed source, demonstrated to be a K giant by our independent spectroscopy, crossed the central isolated cusp of the lensing binary, generating a sharp peak in the light curve that was well-resolved by dense (3 - 30 minute) and continuous monitoring from PLANET sites in Chile, South Africa, and Australia. Our modeling of these data has produced stellar profiles for the source star in the I and V bands that are in excellent agreement with those predicted by stellar atmospheric models for K giants. The limb-darkening coefficients presented here are the first derived from microlensing, among the first for normal giants by any technique, and the first for any star as distant as the Galactic bulge. Modeling indicates that the lensing binary has a mass ratio q = 0.23 and an (instantaneous) separation in units of the angular Einstein ring radius of d = 0.69 . For a lens in the Galactic bulge, this corresponds to a typical stellar binary with a projected separation between 1 and 2 AU. If the lens lies closer, the separation is smaller, and one or both of the lens objects is in the brown dwarf regime. Assuming that the source is a bulge K2 giant at 8 kpc, the relative lens-source proper motion is mu = 19.4 +/- 2.6 km/s /kpc, consistent with a disk or bulge lens. If the non-lensed blended light is due to a single star, it is likely to be a young white dwarf in the bulge, consistent with the blended light coming from the lens itself.
We present observations of microlensing event MACHO-98-BLG-35 which reached a peak magnification factor of almost 80. These observations by the Microlensing Planet Search (MPS) and the MOA Collaborations place strong constraints on the possible planetary system of the lens star and show intriguing evidence for a low mass planet with a mass fraction $4times 10^{-5} leq epsilon leq 2times 10^{-4}$. A giant planet with $epsilon = 10^{-3}$ is excluded from 95% of the region between 0.4 and 2.5 $R_E$ from the lens star, where $R_E$ is the Einstein ring radius of the lens. This exclusion region is more extensive than the generic lensing zone which is $0.6 - 1.6 R_E$. For smaller mass planets, we can exclude 57% of the lensing zone for $epsilon = 10^{-4}$ and 14% of the lensing zone for $epsilon = 10^{-5}$. The mass fraction $epsilon = 10^{-5}$ corresponds to an Earth mass planet for a lensing star of mass $sim 0.3 msun$. A number of similar events will provide statistically significant constraints on the prevalence of Earth mass planets. In order to put our limits in more familiar terms, we have compared our results to those expected for a Solar System clone averaging over possible lens system distances and orientations. We find that such a system is ruled out at the 90% confidence level. A copy of the Solar System with Jupiter replaced by a second Saturn mass planet can be ruled out at 70% confidence. Our low mass planetary signal (few Earth masses to Neptune mass) is significant at the $4.5sigma$ confidence level. If this planetary interpretation is correct, the MACHO-98-BLG-35 lens system constitutes the first detection of a low mass planet orbiting an ordinary star without gas giant planets.
We report the discovery of a giant planet in the KMT-2016-BLG-1397 microlensing event, which was found by The Korea Microlensing Telescope Network (KMTNet) alone. The time scale of this event is t_E = 40.0 +- 0.5 days and the mass ratio between the lens star and its companion is q = 0.016 +- 0.002. The planetary perturbation in the light curve is a smooth bump, resulting in the classical binary-lens/binary-source (2L1S/1L2S) degeneracy. We measure the V - I color of the (putative) two sources in the 1L2S model, and then effectively rule out the binary source solution. The finite-source effect is marginally detected. Combined with the limits on the blend flux and the probability distribution of the source size normalized by the Einstein radius rho, a Bayesian analysis yields the lens mass M_L = 0.45+0.33-0.28 M_sun, at distance of D_L = 6.60+1.10-1.30 kpc. Thus the companion is a super-Jupiter of a mass m_p = 7.0+5.2-4.3 M_J , at a projected separation r = 5.1+1.5-1.7 AU, indicating that the planet is well beyond the snow line of the host star.
We report the discovery and the analysis of the short (tE < 5 days) planetary microlensing event, OGLE-2015-BLG-1771. The event was discovered by the Optical Gravitational Lensing Experiment (OGLE), and the planetary anomaly (at I ~ 19) was captured by The Korea Microlensing Telescope Network (KMTNet). The event has three surviving planetary models that explain the observed light curves, with planet-host mass ratio q ~ 5.4 * 10^{-3}, 4.5 * 10^{-3} and 4.5 * 10^{-2}, respectively. The first model is the best-fit model, while the second model is disfavored by Deltachi^2 ~ 3. The last model is strongly disfavored by Deltachi^2 ~ 15 but not ruled out. A Bayesian analysis using a Galactic model indicates that the first two models are probably composed of a Saturn-mass planet orbiting a late M dwarf, while the third one could consist of a super-Jovian planet and a mid-mass brown dwarf. The source-lens relative proper motion is mu_rel ~ 9 mas/yr, so the source and lens could be resolved by current adaptive-optics (AO) instruments in 2021 if the lens is luminous.
We report the discovery of KMT-2018-BLG-1292Lb, a super-Jovian $M_{rm planet} = 4.5pm 1.3,M_J$ planet orbiting an F or G dwarf $M_{rm host} = 1.5pm 0.4,M_odot$, which lies physically within ${cal O}(10,pc)$ of the Galactic plane. The source star is a heavily extincted $A_Isim 5.2$ luminous giant that has the lowest Galactic latitude, $b=-0.28^circ$, of any planetary microlensing event. The relatively blue blended light is almost certainly either the host or its binary companion, with the first explanation being substantially more likely. This blend dominates the light at $I$ band and completely dominates at $R$ and $V$ bands. Hence, the lens system can be probed by follow-up observations immediately, i.e., long before the lens system and the source separate due to their relative proper motion. The system is well characterized despite the low cadence $Gamma=0.15$--$0.20,{rm hr^{-1}}$ of observations and short viewing windows near the end of the bulge season. This suggests that optical microlensing planet searches can be extended to the Galactic plane at relatively modest cost.
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