arXiv subjects

Greg Olmschenk

Publications and source records attributed to Greg Olmschenk.

At least 19 recordsLinked to original sources

arXiv2026

MOA-2020-BLG-108Lb: A Giant Planet Beyond the Snow Line of a Low-Mass Lens Near the Lower Boundary of the Mass-Ratio Desert

We present an analysis of the microlensing event MOA-2020-BLG-108, which was discovered in June 2020 by the MOA collaboration toward the Galactic bulge. The observed light curve shows significant deviations from the standard single-lens single-source model. We find two degenerate binary-lens single-source solutions, corresponding to the wide and close configurations, with a companion-to-host mass ratio of $q\sim0.02$ and projected host--companion separations of $s=1.33\pm0.01$ and $s=0.76\pm0.01$, respectively. These solutions improve the fit by $\Delta\chi^2>4430$ compared to the single-lens model. We detected the finite-source effect in the light curve and obtained the angular Einstein radius of $\theta_{\rm E} = 0.7\pm0.1\:\mathrm {mas}$, which provides a mass--distance relation for the lens. We conducted a Bayesian analysis to estimate the physical parameters of the lens system. The results indicate that the lens system consists of a host star with a mass of $M_{\rm L,H} \sim 0.6\:M_\odot$ at a distance of $D_{\rm L}\sim5$ kpc and a giant planet with a mass of $M_{\rm L,C}\sim10\:M_{\rm {Jup}}$ orbiting beyond the snow line. Conventional planet formation theories suggest that giant planets are unlikely to form around low-mass stars. Furthermore, several statistical studies have suggested the existence of a companion-to-host mass-ratio desert in the range $0.02 \lesssim q \lesssim 0.05$, and the companion in the lens system discovered in this work lies near the lower boundary of this desert. Objects near the planet-brown dwarf boundary may form through multiple pathways, and this discovery provides an additional data point for understanding their formation mechanisms.

astro-ph.EP↗

arXiv2026

Multipolar Magnetic-Field Inference for PSR J0740+6620 with Neural-Network-Accelerated NICER Pulse-Profile Modeling

We investigate the multipolar surface magnetic-field structure of the high-mass millisecond pulsar PSR J0740+6620 using the 32-bin bolometric NICER pulse profile of Dittmann et al. (2024). Building on the neural-network surrogate framework of Olmschenk et al. (2025), we model the emitting regions as open-field-line footpoints of an offset dipole plus axisymmetric quadrupole static vacuum field, rather than as prescribed geometric hotspots. We fix the stellar mass, radius, observer inclination, and hotspot temperature ratio to the Dittmann et al. (2024) maximum-likelihood values and explore the resulting 11-dimensional magnetic-field space. To make this feasible, we train convolutional neural-network surrogates on $5.12\times10^7$ synthetic bolometric light curves and use them in a parallel ensemble Markov Chain Monte Carlo calculation on 4000 CPU cores, accelerating likelihood evaluations by a factor of $\gtrsim 400$. We perform independent inferences for two calibrated temperature-weight prescriptions, Tw=1.31 and Tw=1.41, encoding the relative bolometric weight associated with the hotspot temperature difference. The posteriors, posterior-predictive light curves, and maximum-likelihood values are very similar, indicating weak sensitivity to this choice. The offset model reproduces the observed double-peaked profile and yields broad, multimodal posteriors, reflecting both the background-dominated data and degeneracies of the multipolar parameterization. The hotspot-density map shows that pulse phases constrain the approximate azimuthal placement of the emission, while latitude, surface extent, and morphology remain weakly constrained. A restricted zero offset run is disfavored within the adopted field basis. This work extends neural-network-accelerated magnetic-field inference to PSR J0740+6620 and motivates future energy-dependent, force-free, and joint X-ray/$\gamma$-ray extensions.

astro-ph.HE↗

arXiv2026

Systematic KMTNet Planetary Anomaly Search. XIII. Complete Sample of 2021 Prime Field Planets

The Systematic KMTNet Planetary Anomaly Search series was conducted using the KMTNet data archived from $2016$ to $2019$. From this first phase of the series, we reported a total of $50$ planetary systems hidden in the data archive, which represent about $35\%$ of the total microlensing planets discovered from $2016$ to $2019$, demonstrating that this semi-machine-based search is a crucial channel for building a complete microlensing planet sample. We continue this series for $2021$ and beyond to expand the microlensing planet sample. In this work for the $2021$ KMTNet high-cadence fields (Prime fields), we find seven hidden planetary systems and three planet candidates. These new planets represent about $33\%$ of the total microlensing planets discovered within the Prime fields observed during the $2021$ bulge season. While the by-eye search is the primary channel for detecting microlensing planets (i.e., two-thirds of microlensing planet discoveries), this work clearly shows that a systematic search series is still necessary for constructing a complete microlensing planet sample. Such a sample is essential for conducting unbiased statistical studies of planet demographics in our Galaxy. Datasets for all the events used for analyses in this work are publicly available: doi:10.5281/zenodo.21472225.

astro-ph.EP↗

arXiv2026

Mass Production of 2023 KMTNet Microlensing Planets. III: Three Planets from the Subprime Field

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 $\log q \sim -1.9$, $-2.0$, 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--2019.

astro-ph.EP↗

arXiv2026

The swept-back multipolar magnetic field of neutron stars: Application to NICER MSP J0030+0451

NICER observations of millisecond pulsars (MSPs) suggest that non-dipolar magnetic fields are required to explain their surface X-ray hotspots. C. Kalapotharakos et al. (2021) modeled the NICER light curve of MSP J0030+0451 (J0030) using a static vacuum offset dipole-plus-quadrupole field and corresponding force-free (FF) solutions to jointly reproduce the X-ray and Fermi-LAT $\gamma$-ray emission. We substitute their static vacuum field model with a more realistic swept-back configuration that accounts for rotational effects. This field more closely resembles the corresponding FF solutions, making it a more physically motivated choice for future multiwavelength modeling. We adopt a centered swept-back vacuum multipolar magnetic field (SVM2F; J. P\'etri 2015), expressed as a complete expansion in vector spherical harmonics, enabling flexible descriptions of arbitrary magnetic field geometries. We introduce a metric to quantify the complexity among different field prescriptions, illustrated for the static offset vacuum field. To efficiently explore parameter space, we train a neural network surrogate (G. Olmschenk et al. 2025) on SVM2F light curves including components up to the octupole, accelerating Markov chain Monte Carlo sampling by $\sim 10^3$ compared to direct physical model evaluations. Applying this framework to J0030, we constrain the field parameter space and find that a centered swept-back multipolar field including terms up to the octupole adequately reproduces the bolometric thermal X-ray light curve. Our study highlights the importance and inherent complexity of prescribing different multipolar magnetic field models for rotating stars, and can be extended to other MSPs to ultimately constrain the masses and radii of neutron stars, and hence their equation of state.

astro-ph.HE↗

arXiv2026

Candidate Microlensing Brown Dwarfs in Binary Lens Systems from the 2023--2025 Observing Seasons

We present detailed light-curve analyses of ten 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.

astro-ph.SR↗

arXiv2026

Mass Production of 2023 KMTNet Microlensing Planets. II: Two Planets and A Brown Dwarf

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 $\log q \sim -3.0$ and $-2.6$, while the third exhibits a brown dwarf mass ratio of $\log q \sim -1.4$. 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.

astro-ph.EP↗

arXiv2026

Three Saturn-mass Microlensing Planets Identified through Signals from Peripheral-caustic Perturbations

We present the discovery and analysis of three microlensing planets identified through brief positive anomalies on the wings of their light curves. The events, KMT-2021-BLG-0852, KMT-2024-BLG-2005, and KMT-2025-BLG-0481, were detected in high-cadence survey data from the KMTNet, OGLE, MOA, and PRIME collaborations. The anomaly morphologies are consistent with major-image perturbations induced by planetary-mass companions located near the peripheral caustic. A systematic exploration of model degeneracies, including binary-source scenarios, higher mass-ratio binary lenses, and the inner--outer caustic degeneracy, firmly establishes the planetary origin of each signal. Measurements of the angular Einstein radius and event timescale, combined with Bayesian priors from a Galactic model, yield the physical parameters of each system. The hosts are low-mass stars (0.12--0.75~$M_\odot$), while the companions are Saturn-mass planets (0.16--0.59 $M_{\rm J}$) projected at separations of 1.1--7.8 au, placing them beyond the snowline of their hosts. These results demonstrate the capability of microlensing to detect and characterize cold giant planets around low-mass stars at kpc distances, populating the critical transition region between ice giants and gas giants.

astro-ph.EP↗

arXiv2026

Accelerating Posterior Inference from Pulsar Light Curves via Learned Latent Representations and Local Simulator-Guided Optimization

Posterior inference from pulsar observations in the form of light curves is commonly performed using Markov chain Monte Carlo methods, which are accurate but computationally expensive. We introduce a framework that accelerates posterior inference while maintaining accuracy by combining learned latent representations with local simulator-guided optimization. A masked U-Net is first pretrained to reconstruct complete light curves from partial observations and to produce informative latent embeddings. Given a query light curve, we identify similar simulated light curves from the simulation bank by measuring similarity in the learned embedding space produced by pretrained U-Net encoder, yielding an initial empirical approximation to the posterior over parameters. This initialization is then refined using a local optimization procedure using hill-climbing updates, guided by a forward simulator, progressively shifting the empirical posterior toward higher-likelihood parameter regions. Experiments on the observed light curve of PSR J0030+0451, captured by NASA's Neutron Star Interior Composition Explorer (NICER), show that our method closely matches posterior estimates obtained using traditional MCMC methods while achieving 120 times reduction in inference time (from 24 hours to 12 minutes), demonstrating the effectiveness of learned representations and simulator-guided optimization for accelerated posterior inference.

stat.ML↗

arXiv2026

KMT-2016-BLG-1337L: A Saturn-mass planet orbiting within a binary system of low-mass stars

We report the discovery and characterization of a planetary companion in the microlensing event KMT-2016-BLG-1337, which was produced by a binary system of low-mass stars. The light curve of the event exhibits a short-term anomaly superposed on the profile of a binary-lens single-source (2L1S) model. To investigate the nature of this anomaly, we performed detailed modeling under both the binary-lens binary-source (2L2S) and triple-lens single-source (3L1S) interpretations. The 3L1S model provides a substantially better fit to the data, strongly favoring the presence of a planetary companion in the lens system. Two viable $3L1S$ solutions describe the event nearly equally well. In one solution, the planet has a mass of $M_3 \sim 0.3~M_{\mathrm{J}}$ and lies at a projected separation of $a_{\perp,3} \sim 4~{\rm au}$ from the heavier member of the host binary. In the alternative solution, the planet has a mass of $M_3 \sim 7~M_{\mathrm{J}}$ and a projected separation of $a_{\perp,3} \sim 1.5~{\rm au}$. The host binary consists of early M-type dwarfs with masses of $M_1 \sim 0.54~M_\odot$ and $M_2 \sim 0.40~M_\odot$, separated in projection by $a_{\perp,2} \sim 3.5~{\rm au}$. The system is located at a distance of $D_{\rm L} \sim 7~{\rm kpc}$ toward the Galactic bulge. This event demonstrates the sensitivity of microlensing to planets in dynamically complex stellar environments, including systems beyond the reach of other detection techniques. It thereby contributes to a more comprehensive understanding of planet formation in multiple-star systems.

astro-ph.EP↗

arXiv2025

Extrapolation of Periodic Functions Using Binary Encoding of Continuous Numerical Values

We report the discovery that binary encoding allows neural networks to extrapolate periodic functions beyond their training bounds. We introduce Normalized Base-2 Encoding (NB2E) as a method for encoding continuous numerical values and demonstrate that, using this input encoding, vanilla multi-layer perceptrons (MLP) successfully extrapolate diverse periodic signals without prior knowledge of their functional form. Internal activation analysis reveals that NB2E induces bit-phase representations, enabling MLPs to learn and extrapolate signal structure independently of position.

cs.LG↗

arXiv2025

Four Giant Planets from 2024 KMTNet Microlensing Campaign

In this work, we present analyses of four newly discovered planetary microlensing events from the 2024 KMTNet survey season: KMT-2024-BLG-0176, KMT-2024-BLG-0349, KMT-2024-BLG-1870, and KMT-2024-BLG-2087. In each case, the planetary nature was revealed through distinct types of anomalies in the lensing light curves: a positive bump near the peak for KMT-2024-BLG-0176, an asymmetric peak for KMT-2024-BLG-0349, a short-duration central dip for KMT-2024-BLG-1870, and a caustic-crossing feature for KMT-2024-BLG-2087. Detailed modeling of the light curves confirms that these anomalies are produced by planetary companions with planet-to-host mass ratios in the range of $(1.5\text{--}17.9)\times 10^{-3}$. Despite the diversity of signal morphologies, all planets detected in these events are giant planets with masses comparable to or exceeding that of Jupiter in the Solar System. Each planet orbits a host star less massive than the Sun, emphasizing the strength of microlensing in uncovering planetary systems around low-mass stellar hosts.

astro-ph.EP↗

arXiv2025

Predictions of the Nancy Grace Roman Space Telescope Galactic Exoplanet Survey. V. Detection Rates of Multiplanetary Systems in High Magnification Microlensing Events

The Nancy Grace Roman Space Telescope will expand the reach of gravitational microlensing surveys by increasing the number of events monitored and the precision of their light curves. We investigate Roman's ability to detect triple-lens microlensing systems, cases where a foreground star with two bound exoplanets produces detectable anomalies in a microlensing event, using its planned high-cadence observations toward the Galactic bulge. We simulate a large set of high-magnification microlensing light curves based on Roman's expected survey characteristics. A detection criterion, based on a required $\chi^2$ improvement for a two-planet model, is applied to determine whether the second planet can be reliably distinguished from a single-planet (binary-lens) model. Our simulations show that the majority of two-planet microlensing events would be detectable with Roman. Events in which both planets are relatively massive (planet-star mass ratios of order $10^{-3}$), or in which the more massive planet occupies a favorable resonant configuration, produce strong central perturbations, resulting in detection efficiencies of roughly 90\%. By contrast, systems with only low-mass planets ($q \sim 10^{-4}$) or with less favorable alignments generate much weaker signals, which often fall below the detection threshold. In general, the planetary mass ratios and the resulting caustic geometry (e.g., central caustic size in resonant versus wide/close orbits) are the dominant factors governing detectability. Taking into account the expected frequency of planetary systems and the fraction of high-magnification events, we estimate that Roman will detect a high-magnification triple-lens event in approximately 4.5\% of multi-planet microlensing events, corresponding to about 64 events over the course of the full survey.

astro-ph.EP↗

arXiv2025

Six binary brown dwarf candidates identified by microlensing

In this study, we analyze microlensing events from the 2023 and 2024 observing seasons to identify cases likely caused by binary systems composed of BDs. By applying criteria that the binary-lens events exhibit well-resolved caustics, short time scales ($t_{\rm E} \lesssim 9$ days), and have small angular Einstein radii ($\theta_{\rm E} \lesssim 0.17$~mas), we identify six candidate binary BD events: MOA-2023-BLG-331, KMT-2023-BLG-2019, KMT-2024-BLG-1005, KMT-2024-BLG-1518, MOA-2024-BLG-181, and KMT-2024-BLG-2486. Analysis of these events leads to models that provide precise estimates for both lensing observables, $t_{\rm E}$ and $\theta_{\rm E}$. We estimate the masses of the binary components through Bayesian analysis, utilizing the constraints from $t_{\rm E}$ and $\theta_{\rm E}$. The results show that for the events KMT-2024-BLG-1005, KMT-2024-BLG-1518, MOA-2024-BLG-181, and KMT-2024-BLG-2486, the probability that both binary components lie within the BD mass range exceeds 50\%, indicating a high likelihood that the lenses of these events are binary BDs. In contrast, for MOA-2023-BLG-331L and KMT-2023-BLG-2019L, the probabilities that the lower-mass components of the binary lenses lie within the BD mass range exceed 50\%, while the probabilities for the heavier components are below 50\%, suggesting that these systems are more likely to consist of a low-mass M dwarf and a BD. The brown-dwarf nature of the binary candidates can ultimately be confirmed by combining the measured lens-source relative proper motions with high-resolution imaging taken at a later time.

astro-ph.SR↗

arXiv2025

Degeneracy-Aware Pulsar Parameter Estimation from Light Curves via Deep Learning and Test-Time Optimization

Probing properties of neutron stars from photometric observations of these objects helps us answer crucial questions at the forefront of multi-messenger astronomy, such as, what is behavior of highest density matter in extreme environments and what is the procedure of generation and evolution of magnetic fields in these astrophysical environments? However, uncertainties and degeneracies-where different parameter sets produce similar light curves-make this task challenging. We propose a deep learning framework for inferring pulsar parameters from observed light curves. Traditional deep learning models are not designed to produce multiple degenerate solutions for a given input. To address this, we introduce a custom loss function that incorporates a light curve emulator as a forward model, along with a dissimilarity loss that encourages the model to capture diverse, degenerate parameter sets for a given light curve. We further introduce a test-time optimization scheme that refines predicted parameters by minimizing the discrepancy between the observed light curve and those reconstructed by the forward model from predicted parameters during inference. The model is trained using a suite of state-of-the-art simulated pulsar light curves. Finally, we demonstrate that the parameter sets predicted by our approach reproduce light curves that are consistent with the true observation.

astro-ph.HE↗

arXiv2025

Predictions of the Nancy Grace Roman Space Telescope Galactic Exoplanet Survey. IV. Lens Mass and Distance Measurements

As part of the Galactic Bulge Time Domain Survey (GBTDS), the Nancy Grace Roman Galactic Exoplanet Survey (RGES) will use microlensing to discover cold outer planets and free-floating planets unbound to stars. NASA has established several science requirements for the GBTDS to ensure RGES success. A key advantage of RGES is Roman's high angular resolution, which will allow detection of flux from many host stars. One requirement specifies that Roman must measure the masses and distances of 40% of detected planet hosts with 20% precision or better. To test this, we simulated microlensing events toward the GBTDS fields and used Fisher matrix analysis to estimate light curve parameter uncertainties. Combining these with Roman imaging observables (lens flux, relative lens-source proper motion), we estimated the achievable precision of lens mass and distance measurements. Using pyLIMASS, a publicly available code for estimating lens properties, we applied this analysis to 3,000 simulated events. Assuming the Cassan et al. (2012) exoplanet mass function, we find that >40% of host stars meet the required 20% precision threshold, confirming that the GBTDS can satisfy the mission requirement. We validated our approach by comparing our inferred lens masses and distances to empirical measurements from detailed image-constrained light curve modeling of historical microlensing events with Hubble and Keck follow-up imaging. Our results agree within roughly 1 sigma, demonstrating that both approaches yield consistent and reliable mass and distance estimates, and confirming the robustness of our simulations for Roman-era microlensing science.

astro-ph.EP↗

arXiv2025

Technique-agnostic exoplanet demography for the Roman era -- I. Testing a demography retrieval framework using simulated Kepler-like transit datasets

The Nancy Grace Roman Space Telescope (Roman) will unveil for the first time the full architecture of planetary systems across Galactic distances through the discovery of up to 200,000 cool and hot exoplanets using microlensing and transit detection methods. Roman's huge exoplanet haul, and Galactic reach, will require new methods to leverage the full exoplanet demographic content of the combined microlensing and transit samples, given the different sensitivity bias of the techniques to planet and host properties and Galactic location. We present a framework for technique-agnostic exoplanet demography (TAED) that can allow large, multi-technique exoplanet samples distributed over Galactic distance scales to be combined for demographic studies. Our TAED forward modelling and retrieval framework uses parameterised model exoplanet demographic distributions to embed planetary systems within a stellar population synthesis model of the Galaxy, enabling internally consistent forecasts to be made for all detection methods that are based on spatio-kinematic system properties. In this paper, as a first test of the TAED framework, we apply it to simulated transit datasets based on the Kepler Data Release 25 to assess parameter recovery accuracy and method scalability for a single large homogeneous dataset. We find that optimisation using differential evolution provides a computationally scalable framework that gives a good balance between computational efficiency and accuracy of parameter recovery.

astro-ph.EP↗

arXiv2025

A Comprehensive Analysis of Three Microlensing Planet Candidates with the Planet/Binary Degeneracy

We present observations and analyses of three high-magnification microlensing events: KMT-2022-BLG-0954, KMT-2024-BLG-0697, and MOA-2024-BLG-018. All three exhibit the "Planet/Binary" degeneracy, with planetary solutions corresponding to mass ratios in the range $-3.7 < \log q < -2.2$, while the binary solutions yield $\log q > -2.0$. For KMT-2022-BLG-0954, we identify a previously unrecognized degeneracy among planetary solutions, involving different mass ratios and normalized source radii. In all three cases, single-lens binary-source models are excluded. Bayesian analyses suggest that the planetary solutions correspond to gas giants orbiting M/K dwarfs beyond the snow line, while KMT-2022-BLG-0954 also admits an alternative interpretation as a super-Earth orbiting a late-type M dwarf. The binary solutions imply a diverse set of systems, including M-dwarf pairs and M-dwarf--brown-dwarf binaries. A review of known events subject to the "Planet/Binary" degeneracy shows that in most cases the degeneracy cannot be resolved through follow-up high-resolution imaging, particularly in the presence of the newly identified degeneracy.

astro-ph.EP↗