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The PRime-focus Infrared Microlensing Experiment (PRIME) will be the first to conduct a dedicated near infrared (NIR) microlensing survey by using a 1.8m telescope with a wide field of view of 1.45 \\({\\rm deg^{2}}\\) at the South African Astronomical Observatory (SAAO). The major goals of the PRIME microlensing survey are to measure the microlensing event rate in the inner Galactic bulge to help design the observing strategy for the exoplanet microlensing survey by the {\\it Nancy Grace Roman Space Telescope} and to make a first statistical measurement of exoplanet demographics in the central bulge fields where optical observations are very difficult owing to the high extinction in these fields. Here we conduct a simulation of the PRIME microlensing survey to estimate its planet yields and determine the optimal survey strategy, using a Galactic model optimized for the inner Galactic bulge. In order to maximize the number of planet detections and the range of planet mass, we compare the planet yields among four observation strategies. Assuming {the \\citet{2012Natur.481..167C} mass function as modified by \\citet{2019ApJS..241...3P}}, we predict that PRIME will detect planetary signals for \\(42-52\\) planets (\\(1-2\\) planets with \\(M_p \\leq 1 M_\\oplus\\), \\(22-25\\) planets with mass \\(1 M_\\oplus < M_p \\leq 100 M_\\oplus\\), \\(19-25\\) planets \\(100 M_\\oplus < M_p \\leq 10000 M_\\oplus\\)), per year depending on the chosen observation strategy.

We describe the optical alignment method for the Prime-focus Infrared Microlensing Experiment (PRIME) telescope which is a prime-focus near-infrared (NIR) telescope with a wide field of view for the microlensing planet survey toward the Galactic center that is the major task for the PRIME project. There are three steps for the optical alignment: preliminary alignment by a laser tracker, fine alignment by intra- and extra-focal (IFEF) image analysis technique, and complementary and fine alignment by the Hartmann test. We demonstrated that the first two steps work well by the test conducted in the laboratory in Japan. The telescope was installed at the Sutherland Observatory of South African Astronomical Observatory in August, 2022. At the final stage of the installation, we demonstrated that the third method works well and the optical system satisfies the operational requirement.

Recently, there have been reports of various types of degeneracies in the interpretation of planetary signals induced by planetary caustics. In this work, we check whether such degeneracies persist in the case of well-covered signals by analyzing the lensing event KMT-2021-BLG-1150, for which the light curve exhibits a densely and continuously covered short-term anomaly. In order to identify degenerate solutions, we thoroughly investigate the parameter space by conducting dense grid searches for the lensing parameters. We then check the severity of the degeneracy among the identified solutions. We identify a pair of planetary solutions resulting from the well-known inner-outer degeneracy, and find that interpreting the anomaly is not subject to any degeneracy other than the inner-outer degeneracy. The measured parameters of the planet separation (normalized to the Einstein radius) and mass ratio between the lens components are \\((s, q)_{\\rm in}\\sim (1.297, 1.10\\times 10^{-3})\\) for the inner solution and \\((s, q)_{\\rm out}\\sim (1.242, 1.15\\times 10^{-3})\\) for the outer solution. According to a Bayesian estimation, the lens is a planetary system consisting of a planet with a mass \\(M_{\\rm p}=0.88^{+0.38}_{-0.36}~M_{\\rm J}\\) and its host with a mass \\(M_{\\rm h}=0.73^{+0.32}_{-0.30}~M_\\odot\\) lying toward the Galactic center at a distance \\(D_{\\rm L} =3.8^{+1.3}_{-1.2}\\)~kpc. By conducting analyses using mock data sets prepared to mimic those obtained with data gaps and under various observational cadences, it is found that gaps in data can result in various degenerate solutions, while the observational cadence does not pose a serious degeneracy problem as long as the anomaly feature can be delineated.

We inspect the microlensing data of the KMTNet survey collected during the 2018--2020 seasons in order to find lensing events produced by binaries with brown-dwarf companions. In order to pick out binary-lens events with candidate BD lens companions, we conduct systematic analyses of all anomalous lensing events observed during the seasons. By applying the selection criterion with mass ratio between the lens components of \\(0.03\\lesssim q\\lesssim 0.1\\), we identify four binary-lens events with candidate BD companions, including KMT-2018-BLG-0321, KMT-2018-BLG-0885, KMT-2019-BLG-0297, and KMT-2019-BLG-0335. For the individual events, we present the interpretations of the lens systems and measure the observables that can constrain the physical lens parameters. The masses of the lens companions estimated from the Bayesian analyses based on the measured observables indicate that the probabilities for the lens companions to be in the brown-dwarf mass regime are high: 59\\%, 68\\%, 66\\%, and 66\\% for the four events respectively.

We investigate the data collected by the high-cadence microlensing surveys during the 2022 season in search for planetary signals appearing in the light curves of microlensing events. From this search, we find that the lensing event MOA-2022-BLG-249 exhibits a brief positive anomaly that lasted for about 1 day with a maximum deviation of \\(\\sim 0.2\\)~mag from a single-source single-lens model. We analyze the light curve under the two interpretations of the anomaly: one originated by a low-mass companion to the lens (planetary model) and the other originated by a faint companion to the source (binary-source model). It is found that the anomaly is better explained by the planetary model than the binary-source model. We identify two solutions rooted in the inner--outer degeneracy, for both of which the estimated planet-to-host mass ratio, \\(q\\sim 8\\times 10^{-5}\\), is very small. With the constraints provided by the microlens parallax and the lower limit on the Einstein radius, as well as the blend-flux constraint, we find that the lens is a planetary system, in which a super-Earth planet, with a mass \\((4.83\\pm 1.44)~M_\\oplus\\), orbits a low-mass host star, with a mass \\((0.18\\pm 0.05)~M_\\odot\\), lying in the Galactic disk at a distance \\((2.00\\pm 0.42)\\)~kpc. The planet detection demonstrates the elevated microlensing sensitivity of the current high-cadence lensing surveys to low-mass planets.

We analyze the MOA-2020-BLG-208 gravitational microlensing event and present the discovery and characterization of a new planet, MOA-2020-BLG-208Lb, with an estimated sub-Saturn mass. With a mass ratio \\(q = 3.17^{+0.28}_{-0.26} \\times 10^{-4}\\) and a separation \\(s = 1.3807^{+0.0018}_{-0.0018}\\), the planet lies near the peak of the mass-ratio function derived by the MOA collaboration (Suzuki et al. 2016), near the edge of expected sample sensitivity. For these estimates we provide results using two mass law priors: one assuming that all stars have an equal planet-hosting probability, and the other assuming that planets are more likely to orbit around more massive stars. In the first scenario, we estimate that the lens system is likely to be a planet of mass \\(m_\\mathrm{planet} = 46^{+42}_{-24} \\; M_\\oplus\\) and a host star of mass \\(M_\\mathrm{host} = 0.43^{+0.39}_{-0.23} \\; M_\\odot\\), located at a distance \\(D_L = 7.49^{+0.99}_{-1.13} \\; \\mathrm{kpc}\\). For the second scenario, we estimate \\(m_\\mathrm{planet} = 69^{+37}_{-34} \\; M_\\oplus\\), \\(M_\\mathrm{host} = 0.66^{+0.35}_{-0.32} \\; M_\\odot\\), and \\(D_L = 7.81^{+0.93}_{-0.93} \\; \\mathrm{kpc}\\). As a cool sub-Saturn-mass planet, this planet adds to a growing collection of evidence for revised planetary formation models and qualifies for inclusion in the extended MOA-II exoplanet microlensing sample.

We present the analysis of five black hole candidates identified from gravitational microlensing surveys. Hubble Space Telescope astrometric data and densely sampled lightcurves from ground-based microlensing surveys are fit with a single-source, single-lens microlensing model in order to measure the mass and luminosity of each lens and determine if it is a black hole. One of the five targets (OGLE-2011-BLG-0462/MOA-2011-BLG-191 or OB110462 for short) shows a significant \\(>1\\) mas coherent astrometric shift, little to no lens flux, and has an inferred lens mass of 1.6 - 4.4 \\(M_\\odot\\). This makes OB110462 the first definitive discovery of a compact object through astrometric microlensing and it is most likely either a neutron star or a low-mass black hole. This compact object lens is relatively nearby (0.70-1.92 kpc) and has a slow transverse motion of \\(<\\)30 km/s. OB110462 shows significant tension between models well-fit to photometry vs. astrometry, making it currently difficult to distinguish between a neutron star and a black hole. Additional observations and modeling with more complex system geometries, such as binary sources are needed to resolve the puzzling nature of this object. For the remaining four candidates, the lens masses are \\(<2 M_\\odot\\) and they are unlikely to be black holes; two of the four are likely white dwarfs or neutron stars. We compare the full sample of five candidates to theoretical expectations on the number of black holes in the Milky Way (\\(\\sim 10^8\\)) and find reasonable agreement given the small sample size.

We present the analysis of three more planets from the KMTNet 2021 microlensing season. KMT-2021-BLG-0119Lb is a \\(\\sim 6\\, M_{\\rm Jup}\\) planet orbiting an early M-dwarf or a K-dwarf, KMT-2021-BLG-0192Lb is a \\(\\sim 2\\, M_{\\rm Nep}\\) planet orbiting an M-dwarf, and KMT-2021-BLG-0192Lb is a \\(\\sim 1.25\\, M_{\\rm Nep}\\) planet orbiting a very--low-mass M dwarf or a brown dwarf. These by-eye planet detections provide an important comparison sample to the sample selected with the AnomalyFinder algorithm, and in particular, KMT-2021-BLG-2294, is a case of a planet detected by-eye but not by-algorithm. KMT-2021-BLG-2294Lb is part of a population of microlensing planets around very-low-mass host stars that spans the full range of planet masses, in contrast to the planet population at \\(\\lesssim 0.1\\, \\) au, which shows a strong preference for small planets.

We report the light-curve analysis for the event MOA-2020-BLG-135, which leads to the discovery of a new Neptune-class planet, MOA-2020-BLG-135Lb. With a derived mass ratio of \\(q=1.52_{-0.31}^{+0.39} \\times 10^{-4}\\) and separation \\(s\\approx1\\), the planet lies exactly at the break and likely peak of the exoplanet mass-ratio function derived by the MOA collaboration (Suzuki et al. 2016). We estimate the properties of the lens system based on a Galactic model and considering two different Bayesian priors: one assuming that all stars have an equal planet-hosting probability and the other that planets are more likely to orbit more massive stars. With a uniform host mass prior, we predict that the lens system is likely to be a planet of mass \\(m_\\mathrm{planet}=11.3_{-6.9}^{+19.2} M_\\oplus\\) and a host star of mass \\(M_\\mathrm{host}=0.23_{-0.14}^{+0.39} M_\\odot\\), located at a distance \\(D_L=7.9_{-1.0}^{+1.0}\\;\\mathrm{kpc}\\). With a prior that holds that planet occurrence scales in proportion to the host star mass, the estimated lens system properties are \\(m_\\mathrm{planet}=25_{-15}^{+22} M_\\oplus\\), \\(M_\\mathrm{host}=0.53_{-0.32}^{+0.42} M_\\odot\\), and \\(D_L=8.3_{-1.0}^{+0.9}\\; \\mathrm{kpc}\\). This planet qualifies for inclusion in the extended MOA-II exoplanet microlens sample.

We report the discovery of a sub-Jovian-mass planet, OGLE-2014-BLG-0319Lb. The characteristics of this planet will be added into a future extended statistical analysis of the Microlensing Observations in Astrophysics (MOA) collaboration. The planetary anomaly of the light curve is characterized by MOA and OGLE survey observations and results in three degenerate models with different planetary mass-ratios of \\(q=(10.3,6.6,4.5)\\times10^{-4}\\), respectively. We find that the last two models require unreasonably small lens-source relative proper motions of \\(\\mu_{\\rm rel}\\sim1\\;{\\rm mas/yr}\\). Considering Galactic prior probabilities, we rule out these two models from the final result. We conduct a Bayesian analysis to estimate physical properties of the lens system using a Galactic model and find that the lens system is composed of a \\(0.49^{+0.35}_{-0.27}\\;M_{\\rm Jup}\\) sub-Jovian planet orbiting a \\(0.47^{+0.33}_{-0.25}\\; M_{\\odot}\\) M-dwarf near the Galactic bulge. This analysis demonstrates that Galactic priors are useful to resolve this type of model degeneracy. This is important for estimating the mass ratio function statistically. However, this method would be unlikely successful in shorter timescale events, which are mostly due to low-mass objects, like brown dwarfs or free-floating planets. Therefore, careful treatment is needed for estimating the mass ratio function of the companions around such low-mass hosts which only the microlensing can probe.