Krzysztof Ulaczyk

To search for very wide-orbit binary-lens events, we crossmatch microlensing events detected by the Optical Gravitational Lensing Experiment (OGLE)-IV and KMTNet between 2011 and 2024. After removing contaminants from variable stars, we identify OGLE-2016-BLG-0523 as a microlensing event showing two short-lived bumps in its light curve, with the two separated by 8.36 yr. We investigate three scenarios to explain the two bumps, including a binary-lens single-source (2L1S), a single-lens binary-source (1L2S), and two unrelated single-lens single-source events (2×1L1S). The 1L2S interpretation is excluded due to its poorer fit to the data, while the 2×1L1S scenario is disfavored by the low event rate, unphysical total source flux, and unphysical color for one of the two putative sources. For the surviving 2L1S interpretation, the Einstein timescales of the two bumps are 8.343 ± 0.076 days and 3.231 ± 0.047 days, respectively, implying a binary mass ratio of q = 0.15. A Bayesian analysis indicates that the primary lens is most likely a low-mass M dwarf, while the companion is probably a low-mass brown dwarf or super-Jupiter. The inferred projected separation is about 450 au, making the companion the widest low-mass object discovered via microlensing to date. Our discovery highlights both the promise and challenges of identifying very wide, low-mass companions through microlensing. Although upcoming facilities such as Roman and Earth 2.0 space telescopes will improve sensitivity to such events, overcoming degeneracies and distinguishing them from variable phenomena will remain essential for building a robust statistical sample.

The microlensing event OGLE-2011-BLG-0950 exhibits the well-known “Planet/Binary” degeneracy, in which distinct lens configurations produce similar light curves but imply substantially different mass ratios between the lens components. A previous study rejected the then-known planetary model because its predicted lens-source relative proper motion was inconsistent with that measured from high-resolution imaging, thereby favoring the stellar-binary interpretation. In this work, we identify a new planetary model for this event that arises from a newly identified degeneracy, simultaneously reproducing the observed light curve and remaining consistent with the high-resolution imaging constraint, leaving the “Planet/Binary” degeneracy unresolved once again. Under this planetary interpretation, combining constraints from the light-curve modeling and high-resolution observations yields a lens system consisting of a ∼1 M _⊙ host star orbited by a ∼1.5 M _Jup planet, with a projected separation of about 2 or 8 au, subject to the “Close/Wide” degeneracy. Our reanalysis of the color–magnitude diagram further indicates that the source star has unresolved companions that contribute nonnegligible blended light, highlighting the importance of carefully accounting for source and lens companions in future Roman microlensing analyses. Finally, we show that adopting a single mass–luminosity relation significantly underestimates the uncertainties in the inferred lens properties for host masses ≳1 M _⊙ .

To complete the analysis of the 2023 KMTNet subprime-field microlensing planetary events identified by its AlertFinder system, we present the analysis of six events, KMT-2023-BLG-(1810, 0084, 1118, 0584, 1697, 2218). We find that the first three events are securely confirmed as planetary, with inferred mass ratios of log q ∼ − 1.9 , −2.0, and −2.6, respectively. The remaining three events exhibit the well-known degeneracy between binary-lens/single-source (2L1S) and single-lens/binary-source (1L2S) models, and two of these also admit viable stellar binary solutions. A Bayesian analysis indicates that the companions in the confirmed planetary events are likely either super-Jupiters orbiting beyond the snow line of M- or K-dwarf hosts or, for two degenerate solutions of KMT-2023-BLG-1118, Saturn-mass planets orbiting late-type M dwarfs. To date, the 2023 KMTNet sample contains 25 unambiguous planetary events, and its mass-ratio distribution is consistent with that of the KMTNet planetary sample from 2016 to 2019.

We report the discovery of four cold giant planets identified through the analysis of microlensing events detected by high-cadence surveys: OGLE-2016-BLG-0261, KMT-2025-BLG-0026, KMT-2025-BLG-0030, and KMT-2025-BLG-2272. The planetary signals appear as short-duration anomalies in the light curves and are well described by binary-lens single-source models with mass ratios between the lens components of order q ∼ 10 ⁻³ . Finite-source effects are securely measured in three out of four events, enabling determinations of the angular Einstein radius. A Bayesian analysis incorporating the measured event timescale and angular Einstein radius yields host masses of ∼0.07–0.6 M _⊙ and companion masses of ∼0.2–2.5 M _Jup , confirming that all companions lie in the giant-planet regime. The projected separations are ∼0.7–6 au, placing all planets at or beyond the snow lines of their host stars. The inferred lens distances span ∼6.6–7.9 kpc, with all systems consistent with bulge lenses. These detections expand the sample of cold giant planets from homogeneous high-cadence surveys and highlight the sensitivity of microlensing to planetary systems beyond the snow line, providing further constraints on the occurrence and properties of giant planets around low-mass stars.

We present observations and analysis of two low planet/host mass-ratio ( q ) microlensing planets discovered in high-magnification events. KMT-2025-BLG-0811Lb has q ∼ 4.5 × 10 ⁻⁵ , and a Bayesian analysis favors a super-Earth/mini-Neptune orbiting an M- or K-dwarf host at a projected separation of ∼3 au. KMT-2025-BLG-0912Lb has q = 2.6 × 10 ⁻⁴ and likely hosts a super-Earth/mini-Neptune around either a low-mass M dwarf or a brown dwarf at ∼1 au. Even with an observing cadence of Γ > 30 hr ⁻¹ during the planetary signal, KMT-2025-BLG-0811 still exhibits the “central-resonant” degeneracy. Reviewing nine such events, we find that the “central-resonant” degeneracy can be divided into two distinct types that occupy separate regions in the plane of q and normalized source radius ( ρ ). Type I events have similar q but substantially different ρ and are more difficult to resolve from the light curves. For Type II events, the “resonant” solutions have relatively lower q and larger ρ . Our review provides guidance for searching for the alternative solution once one solution has been identified.

We present detailed analyses of three anomalous microlensing events—KMT-2021-BLG-0209, KMT-2021-BLG-0901, and OGLE-2025-BLG-0356—identified from a systematic reexamination of KMTNet light curves for which previous modeling attempts failed or left persistent residuals. Although all three events show caustic-related features consistent with binary-lens microlensing, we find that their full light-curve structures can be described by four-body configurations that required four-body configurations involving a binary lens and a binary source (2L2S). In KMT-2021-BLG-0209, weak caustic-exit residuals arise from a faint companion source undergoing an additional caustic interaction. In KMT-2021-BLG-0901, a late-time rebrightening is produced when the secondary source encounters the resonant caustic long after the primary. For OGLE-2025-BLG-0356, we test the degeneracy between system is described by a triple-lens single-source (3L1S) and 2L2S interpretations of a short isolated anomaly and find that the 2L2S model provides a significantly better fit. Source colors and magnitudes indicate binary sources composed of (G8V, M3V), (G8V, K2V), and (G6V, G8V) stars for the three events, respectively. Bayesian inference suggests that the lenses are predominantly low-mass binaries, including one system (KMT-2021-BLG-0901) with a companion consistent with a brown dwarf. These events add to the growing sample of well-characterized 2L2S systems and underscore the importance of systematically testing complex models, particularly in anticipation of the high-precision microlensing data expected from the Roman Space Telescope survey.

To expand the homogeneous microlensing planetary sample of the Korea Microlensing Telescope Network (KMTNet), we investigate six planetary candidates identified by the AnomalyFinder search in the 2023 prime-field data, namely KMT-2023-BLG-1592, OGLE-2023-BLG-0766, KMT-2023-BLG-0332, KMT-2023-BLG-0486, KMT-2023-BLG-0792, and OGLE-2023-BLG-1043. Light-curve modeling indicates that the first two events have planetary mass ratios of log q ∼ − 3.0 and −2.6, while the third exhibits a brown dwarf mass ratio of log q ∼ − 1.4 . The remaining three events show the well-known degeneracy between the binary-lens single-source (2L1S) and single-lens binary-source (1L2S) interpretations. A Bayesian analysis yields companion masses of about 0.6 and 1.2 Jupiter masses for the two planetary systems, likely orbiting beyond the snow lines of M- or K-dwarf hosts. A review of the KMTNet planetary sample shows that candidates discovered by AnomalyFinder are significantly more likely to exhibit the 2L1S/1L2S degeneracy, consistent with the tendency of AnomalyFinder to detect subtler planetary signals.

We present detailed light-curve analyses of 10 binary-lens microlensing events observed during the 2023–2025 seasons and selected as candidates for hosting brown dwarf companions. The sample includes OGLE-2023-BLG-0249, KMT-2023-BLG-1246, OGLE-2023-BLG-0079, KMT-2024-BLG-0072, KMT-2024-BLG-0897, KMT-2024-BLG-1876, KMT-2024-BLG-2379, KMT-2025-BLG-0922, KMT-2025-BLG-1056, and KMT-2025-BLG-2427. For each event, we carry out modeling of the light curve, explore relevant degeneracies, and, when finite-source effects are present, determine the angular Einstein radius. For OGLE-2023-BLG-0249, we additionally measure the microlens parallax, which allows a direct determination of the lens masses and distance. For the remaining events, we estimate the physical lens properties via Bayesian analyses incorporating Galactic priors. The resulting posteriors show that the lens companions in all systems have median masses in the brown dwarf regime, and the lenses of two events (KMT-2025-BLG-0922 and KMT-2025-BLG-1056) are consistent with binaries in which both lens components fall within the brown dwarf mass range. Spanning a wide range of projected separations and distances, these detections illustrate the power of high-cadence microlensing surveys to build a census of brown dwarf companions, including faint and distant systems beyond the reach of flux-limited methods.

We initiate the systematic search for planets in the 2023 data of the Korea Microlensing Telescope Network (KMTNet), focusing on those planets found by the KMTNet AnomalyFinder with low preliminary estimates of the mass ratio, q < 2 × 10 ⁻⁴ . The 2023 season is the first for which the photometry of all events was re-reduced prior to the AnomalyFinder search, potentially increasing its sensitivity to planets. We find three strong low- q planet candidates, KMT-2023-BLG-0164 ( q ∼ 1.3 × 10 ⁻⁴ ), KMT-2023-BLG-1286 ( q ∼ 1.9 × 10 ⁻⁴ ), and KMT-2023-BLG-1746 ( q ∼ 8 × 10 ⁻⁵ ). KMT-2023-BLG-0164 is notable in that the source is projected on a very bright ( I = 16.0) foreground star, which is either the planet’s host or (more likely) a companion to the host. We obtain a spectrum, finding that its mass and distance are M ∼ 1.0 M _⊙ and D ∼ 1.5 kpc, respectively, the latter being the distance of the lens ( D _L ) regardless of whether the spectroscopic target is the host or its companion. We also analyze two other candidates, KMT-2023-BLG-0614 and KMT-2023-BLG-1593, which are unlikely to enter the statistical sample owing to their ambiguous interpretations as possible nonplanetary events.

We present archival photometry from the Gravitational-wave Optical Transient Observer (GOTO) for four Galactic novae discovered between 2017 and 2024, spanning some of the faintest ZTF24aaomlxy and PGIR22akgylf (at a marginal near-limit level consistent with the practical limiting magnitude of calibrated L to the brightest V1405 Cas and V1674 Her recent eruptions. For each object, we extract GOTO measurements obtained at or near the pre-eruption state, excluding data points with observational uncertainties exceeding 0.5 mag (except for the faintest PGIR22akgylf). The resulting light curves show that GOTO can detect nova progenitors close to its observable limiting depth at calibrated L magnitudes approaching the survey’s practical limiting magnitude, providing meaningful constraints on quiescent brightness, possibly for systems that were only sparsely monitored using surveys such as ZTF and PGIR. These detections demonstrate that wide-field imaging originally designed for gravitational-wave follow-up can yield meaningful limits on both faint and fast-evolving nova progenitors. Simultaneously, we improve the sky positions of five Galactic novae—ZTF24aaomlxy, V3732 Oph, V2000 Aql, V3666 Oph, and V659 Sct—whose published coordinates are affected by crowding or limited precision. Using high-cadence photometry from ZTF and AAVSO, we identify the actual eruption source in each field and obtain revised coordinates that differ by several arcseconds. These findings highlight the importance of time-domain archives for identifying faint nova progenitors and improving astrometric accuracy across the Galactic nova population.