Towards Measuring Microlensing Event Rate in the Galactic Center: I. Events Detection from the UKIRT Microlensing Survey Data

To overcome the high optical extinction, near-infrared observations are needed for probing the microlensing events toward the Galactic center. The 2015-2019 UKIRT microlensing survey toward the Galactic center is the first dedicated precursor near-infrared (NIR) survey for the Nancy Grace Roman Space Telescope. We here analyze the online data from the UKIRT microlensing survey, reaching $l=b=0^\circ$. Using the event-finder algorithm of KMTNet with the $\Delta \chi^2$ threshold of 250, we find 522 clear events, 436 possible events, and 27 possible anomalous events. We fit a point-source point-lens (PSPL) model to all the clear events and derive the PSPL parameters with uncertainties using a Markov chain Monte Carlo method. Assuming perfect detection efficiency, we compute the uncorrected event rates, which should serve as the lower limits on the true event rate. We find that the uncorrected NIR event rates are likely rising toward the Galactic center and higher than the optical event rates.Comment: 16 pages, Accepted for publication at ApJ

Simulations of Triple Microlensing Events I: Detectability of a scaled Sun-Jupiter-Saturn System

Up to date, only 13 firmly established triple microlensing events have been discovered, so the occurrence rates of microlensing two-planet systems and planets in binary systems are still uncertain. With the upcoming space-based microlensing surveys, hundreds of triple microlensing events will be detected. To provide clues for future observations and statistical analyses, we initiate a project to investigate the detectability of triple-lens systems with different configurations and observational setups. As the first step, in this work we develop the simulation software and investigate the detectability of a scaled Sun-Jupiter-Saturn system with the recently proposed microlensing telescope of the ``Earth 2.0 (ET)'' mission. We find that the detectability of the scaled Sun-Jupiter-Saturn analog is about 1%. In addition, the presence of the Jovian planet suppresses the detectability of the Saturn-like planet by $\sim$13% regardless of the adopted detection $\Delta\chi^2$ threshold. This suppression probability could be at the same level as the Poisson noise of future space-based statistical samples of triple-lenses, so it is inappropriate to treat each planet separately during detection efficiency calculations.Comment: 14 pages, 11 figures, submitted to MNRAS, comments welcome

Search full text options here 2 of 4 KMT-2021-BLG-0912Lb: a microlensing super Earth around a K-type star

Aims. The light curve of the microlensing event KMT-2021-BLG-0912 exhibits a very short anomaly relative to a single-lens single-source form. We investigate the light curve for the purpose of identifying the origin of the anomaly. Methods. We model the light curve under various interpretations. From this, we find four solutions, in which three solutions are found under the assumption that the lens is composed of two masses (2L1S models), and the other solution is found under the assumption that the source is comprised of binary stars (1L2S model). The 1L2S model is ruled out based on the contradiction that the faint source companion is bigger than its primary, and one of the 2L1S solutions is excluded from the combination of the poorer fit, blending constraint, and lower overall probability, leaving two surviving solutions with the planet/host mass ratios of q similar to 2.8 x 10(-5) and similar to 1.1 x 10(-5). A subtle central deviation supports the possibility of a tertiary lens component, either a binary companion to the host with a very large or small separation, or a second planet lying near the Einstein ring, but it is difficult to claim a secure detection due to the marginal improvement of the fit, lack of consistency among different data sets, and difficulty in uniquely specifying the nature of the tertiary component. Results. With the observables of the event, it is estimated that the masses of the planet and host are similar to(6.9 M-circle plus, 0.75 M-circle dot) according to one solution and similar to(2.8 M-circle plus, 0.80 M-circle dot) according to the other, indicating that the planet is a super Earth around a K-type star, regardless of the solution. The fact that 16 (including the one reported in this work) out of 19 microlensing planets with M less than or similar to 10 M-circle plus were detected during the last 6 yr nicely demonstrates the importance of high-cadence global surveys in detecting very low-mass planets

Measurement of Source Star Colors with the K2C9-CFHT Multi-color Microlensing Survey

K2 Campaign 9 (K2C9) was the first space-based microlensing parallax survey capable of measuring microlensing parallaxes of free-floating planet candidate microlensing events. Simultaneous to K2C9 observations we conducted the K2C9 Canada-France-Hawaii Telescope Multi-Color Microlensing Survey (K2C9-CFHT MCMS) in order to measure the colors of microlensing source stars to improve the accuracy of K2C9's parallax measurements. We describe the difference imaging photometry analysis of the K2C9-CFHT MCMS observations, and present the project's first data release. This includes instrumental difference flux lightcurves of 217 microlensing events identified by other microlensing surveys, reference image photometry calibrated to PanSTARRS data release 1 photometry, and tools to convert between instrumental and calibrated flux scales. We derive accurate analytic transformations between the PanSTARRS bandpasses and the Kepler bandpass, as well as angular diameter-color relations in the PanSTARRS bandpasses. To demonstrate the use of our data set, we analyze ground-based and K2 data of a short timescale microlensing event, OGLE-2016-BLG-0795. We find the event has a timescale $t_{\rm E}=4.5 \pm 0.1$~days and microlens parallax $\pi_{\rm E}=0.12 \pm 0.03$ or $0.97 \pm 0.04$, subject to the standard satellite parallax degeneracy. We argue that the smaller value of the parallax is more likely, which implies that the lens is likely a stellar-mass object in the Galactic bulge as opposed to a super-Jupiter mass object in the Galactic disk.Comment: Submitted to PAS

Spitzer Microlensing parallax reveals two isolated stars in the Galactic bulge

We report the mass and distance measurements of two single-lens events from the 2017 Spitzer microlensing campaign. The ground-based observations yield the detection of finite-source effects, and the microlens parallaxes are derived from the joint analysis of ground-based observations and Spitzer observations. We find that the lens of OGLE-2017-BLG-1254 is a 0.60±0.03M⊙ star with D_(LS) = 0.53±0.11 kpc, where D_(LS) is the distance between the lens and the source. The second event, OGLE-2017-BLG-1161, is subject to the known satellite parallax degeneracy, and thus is either a 0.51^(+0.12)_(−0.10)M⊙ star with D_(LS) = 0.40±0.12 kpc or a 0.38^(+0.13)_(−0.12)M⊙ star with D_(LS) = 0.53±0.19 kpc. Both of the lenses are therefore isolated stars in the Galactic bulge. By comparing the mass and distance distributions of the eight published Spitzer finite-source events with the expectations from a Galactic model, we find that the Spitzer sample is in agreement with the probability of finite-source effects occurrence in single lens events

An Earth-Mass Planet In A Time Of Covid-19: Kmt-2020-Blg-0414Lb

We report the discovery of KMT-2020-BLG-0414Lb, with a planet-to-host mass ratio q (2) = 0.9-1.2 x 10(-5) = 3-4 circle plus at 1 sigma, which is the lowest mass-ratio microlensing planet to date. Together with two other recent discoveries (4 less than or similar to q/q (circle plus) less than or similar to 6), it fills out the previous empty sector at the bottom of the triangular (log s, log q) diagram, where s is the planet-host separation in units of the angular Einstein radius theta E. Hence, these discoveries call into question the existence, or at least the strength, of the break in the mass-ratio function that was previously suggested to account for the paucity of very low-q planets. Due to the extreme magnification of the event, A (max) similar to 1450 for the underlying single-lens event, its light curve revealed a second companion with q (3) similar to 0.05 and |log s (3)| similar to 1, i.e., a factor similar to 10 closer to or farther from the host in projection. The measurements of the microlens parallax pi (E) and the angular Einstein radius theta E allow estimates of the host, planet and second companion masses, (M (1), M (2), M (3)) similar to (0.3 M (circle dot), 1.0 M (circle plus), 17 M-J ), the planet and second companion projected separations, (a (perpendicular to,2), a (perpendicular to,3)) similar to (1.5, 0.15 or 15) au, and system distance D (L) similar to 1 kpc. The lens could account for most or all of the blended light (I similar to 19.3) and so can be studied immediately with high-resolution photometric and spectroscopic observations that can further clarify the nature of the system. The planet was found as part of a new program of high-cadence follow-up observations of high-magnification events. The detection of this planet, despite the considerable difficulties imposed by COVID-19 (two KMT sites and OGLE were shut down), illustrates the potential utility of this program

Two Candidate KH 15D-like Systems from the Zwicky Transient Facility

KH 15D contains a circumbinary disk that is tilted relative to the orbital plane of the central binary. The precession of the disk and the orbital motion of the binary together produce rich phenomena in the photometric light curve. In this work, we present the discovery and preliminary analysis of two objects that resemble the key features of KH 15D from the Zwicky Transient Facility. These new objects, Bernhard-1 and Bernhard-2, show large-amplitude ($>1.5\,$mag), long-duration (more than tens of days), and periodic dimming events. A one-sided screen model is developed to model the photometric behaviour of these objects, the physical interpretation of which is a tilted, warped circumbinary disk occulting the inner binary. Changes in the object light curves suggest potential precession periods over timescales longer than 10 years. Additional photometric and spectroscopic observations are encouraged to better understand the nature of these interesting systems.Comment: 10 pages, 5 figures, 2 tables, accepted to ApJ Letter

KMT-2016-BLG-1107: A New Hollywood-Planet Close/Wide Degeneracy

We show that microlensing event KMT-2016-BLG-1107 displays a new type of degeneracy between wide-binary and close-binary Hollywood events in which a giant-star source envelops the planetary caustic. The planetary anomaly takes the form of a smooth, two-day "bump" far out on the falling wing of the light curve, which can be interpreted either as the source completely enveloping a minor-image caustic due to a close companion with mass ratio $q=0.036$, or partially enveloping a major-image caustic due to a wide companion with $q=0.004$. The best estimates of the companion masses are both in the planetary regime ($3.3^{+3.5}_{-1.8}\,M_{\rm jup}$ and $0.090^{+0.096}_{-0.037}\,M_{\rm jup}$) but differ by an even larger factor than the mass ratios due to different inferred host masses. We show that the two solutions can be distinguished by high-resolution imaging at first light on next-generation ("30m") telescopes. We provide analytic guidance to understand the conditions under which this new type of degeneracy can appear.Comment: 23 pages, 7 figures, accepted for publication in A