arXiv subjects

K. A. Rybicki

Publications and source records attributed to K. A. Rybicki.

At least 19 recordsLinked to original sources

arXiv2026

Historic microlensing events in the euclid Galactic Bulge Survey

Microlensing campaigns have a long history of observations covering the Galactic bulge, where thousands of detections have been obtained, including many exoplanetary systems. The Euclid Galactic Bulge Survey represents a unique opportunity to revisit a large number of past events and attempt the lens-source resolution of known events falling in the covered area. As the analysis of individual events requires non-negligible efforts, it is important to establish priorities among all possible targets, identifying those candidates with the higher chance for a successful resolution of the lens from the source and with the highest scientific interest. Drawing from the databases of the three main microlensing surveys (OGLE, MOA and KMTNet), we compile the complete catalog of past microlensing events in the Euclid survey footprint up to year 2023, containing 7801 entries. By re-modeling all events and cross-checking with Galactic models, we estimate the relative lens-source proper motions for all events. Taking into account all uncertainties, for each microlensing event we are able to estimate the probability that the lens is separated from the source by more than a given angular distance threshold. Hence, we rank all events by their resolution probability, providing additional useful information that will guide future analyses on the most promising candidates. A particular attention is dedicated to the 51 known planetary microlensing events.

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arXiv2026

Gaia20fnr: A binary-lens microlensing event with full orbital motion revealed by four space telescopes

The microlensing event Gaia20fnr is a long-duration, non-caustic-crossing binary-lens event at high Galactic latitude. Triggered by a photometric rise detected by the Gaia space mission, the event was followed up with observations from multiple ground-based facilities and four space telescopes: Gaia, NEOWISE, Swift, and TESS. We characterize the Gaia20fnr microlensing system by determining the physical and orbital properties of the binary lens, the nature of the luminous source, and the kinematics of both the source and the lens. We employed a binary-lens microlensing model including full Keplerian orbital motion and annual microlens parallax to fit the photometric data. The event is best explained by a K2 giant source at $D_{\rm S} = 3.10 \pm 0.10\,\mathrm{kpc}$ lensed by a stellar binary composed of $M_{\rm L,1} = 0.46 \pm 0.06\,M_\odot$ and $M_{\rm L,2} = 0.52 \pm 0.06\,M_\odot$ at a distance of $D_{\rm L} = 0.54 \pm 0.05\,\mathrm{kpc}$. The light curve exhibits strong signatures of orbital motion and requires a full Keplerian model with a period of $P = 0.67 \pm 0.04\,\mathrm{yr}$ and a radial-velocity semi-amplitude of $K_1 = 16.9 \pm 0.9\,\mathrm{km\,s^{-1}}$. Gaia20fnr is one of the few microlensing events for which a complete Keplerian binary-lens solution has been derived. The model can be tested with follow-up radial-velocity and high-resolution imaging observations as well as forthcoming Gaia DR4 and DR5 astrometric time-series data. Its long duration, multi-peak structure, and extensive coverage make it a benchmark for studying faint nearby low-mass binaries through microlensing.

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arXiv2025

Constraining Lens Masses in Moderately to Highly Magnified Microlensing Events from Gaia

Microlensing events provide a unique way to detect and measure the masses of isolated, non-luminous objects, particularly dark stellar remnants. Under certain conditions, it is possible to measure the mass of these objects using photometry alone, specifically when a microlensing light curve displays a finite source (FS) effect. This effect generally occurs in highly magnified light curves, i.e. when the source and the lens are very well aligned. In this study, we analyse Gaia Alerts and Gaia Data Release 3 datasets, identifying four moderate-to-high-magnification microlensing events without a discernible FS effect. The absence of this effect suggests a large Einstein radius, implying substantial lens masses. In each event, we constrained the FS effect, and therefore established lower limits for the angular Einstein radius and lens mass. Additionally, we used the DarkLensCode software to obtain the mass, distance, and brightness distribution for the lens based on the Galactic model. Our analysis established lower mass limits of $\sim 0.7$ $M_{\odot}$ for one lens and $\sim 0.3-0.5$ $M_{\odot}$ for two others. A DarkLensCode analysis supports these findings, estimating lens masses in the range of $\sim 0.42-1.70$ $M_{\odot}$ and dark lens probabilities exceeding 80%. These results strongly indicate that the lenses are stellar remnants, such as white dwarfs or neutron stars. While further investigations are required to confirm the nature of these lenses, we demonstrate a straightforward yet effective approach to identifying stellar remnant candidates.

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arXiv2025

AT2021uey: A planetary microlensing event outside the Galactic bulge

We report the analysis of a planetary microlensing event AT2021uey. The event was observed outside the Galactic bulge and was alerted by both space- (Gaia) and ground-based (ZTF and ASAS-SN) surveys. From the observed data, we find that the lens system is located at a distance of 1 kpc and comprises an M-dwarf host star of about half a solar mass, orbited by a Jupiter-like planet beyond the snowline. The source star could be a metal-poor giant located in the halo according to the spectral analyses and modelling. Hence, AT2021uey is a unique example of the binary-lens event outside the bulge that is offered by a disc-halo lens-source combination.

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arXiv2024

OGLE-2015-BLG-1609Lb: Sub-jovian planet orbiting a low-mass stellar or brown dwarf host

We present a comprehensive analysis of a planetary microlensing event OGLE-2015-BLG-1609. The planetary anomaly was detected by two survey telescopes, OGLE and MOA. Each of these surveys collected enough data over the planetary anomaly to allow for an unambiguous planet detection. Such survey detections of planetary anomalies are needed to build a robust sample of planets that could improve studies on the microlensing planetary occurrence rate by reducing biases and statistical uncertainties. In this work, we examined different methods for modeling microlensing events using individual datasets, particularly we incorporated a Galactic model prior to better constrain poorly defined microlensing parallax. Ultimately, we fitted a comprehensive model to all available data, identifying three potential typologies, with two showing comparably high Bayesian evidence. Our analysis indicates that the host of the planet is a brown dwarf with a probability of 34%, or a low-mass stellar object (M-dwarf) with the probability of 66%.

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arXiv2024

Dark lens candidates from Gaia Data Release 3

Gravitational microlensing is a phenomenon that allows us to observe dark remnants of stellar evolution even if they no longer emit electromagnetic radiation. In particular, it can be useful to observe solitary neutron stars or stellar-mass black holes, providing a unique window through which to understand stellar evolution. Obtaining direct mass measurements with this technique requires precise observations of both the change in brightness and the position of the microlensed star. The European Space Agency's Gaia satellite can provide both. Using publicly available data from different surveys, we analysed events published in the Gaia Data Release 3 (Gaia DR3) microlensing catalogue. Here we describe our selection of candidate dark lenses, where we suspect the lens is a white dwarf (WD), a neutron star (NS), a black hole (BH), or a mass-gap object, with a mass in a range between the heaviest NS and the least massive BH. We estimated the mass of the lenses using information obtained from the best-fitting microlensing models, the source star, the Galactic model and the expected distribution of the parameters. We found eleven candidates for dark remnants: one WDs, three NS, three mass-gap objects, and four BHs.

astro-ph.GA↗

arXiv2024

Gaia22dkvLb: A Microlensing Planet Potentially Accessible to Radial-Velocity Characterization

We report discovering an exoplanet from following up a microlensing event alerted by Gaia. The event Gaia22dkv is toward a disk source rather than the traditional bulge microlensing fields. Our primary analysis yields a Jovian planet with M_p = 0.59^{+0.15}_{-0.05} M_J at a projected orbital separation r_perp = 1.4^{+0.8}_{-0.3} AU, and the host is a ~1.1 M_sun turnoff star at ~1.3 kpc. At r'~14, the host is far brighter than any previously discovered microlensing planet host, opening up the opportunity of testing the microlensing model with radial velocity (RV) observations. RV data can be used to measure the planet's orbital period and eccentricity, and they also enable searching for inner planets of the microlensing cold Jupiter, as expected from the ''inner-outer correlation'' inferred from Kepler and RV discoveries. Furthermore, we show that Gaia astrometric microlensing will not only allow precise measurements of its angular Einstein radius theta_E, but also directly measure the microlens parallax vector and unambiguously break a geometric light-curve degeneracy, leading to definitive characterization of the lens system.

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arXiv2024

A close binary lens revealed by the microlensing event Gaia20bof

During the last 25 years, hundreds of binary stars and planets have been discovered towards the Galactic Bulge by microlensing surveys. Thanks to a new generation of large-sky surveys, it is now possible to regularly detect microlensing events across the entire sky. The OMEGA Key Projet at the Las Cumbres Observatory carries out automated follow-up observations of microlensing events alerted by these surveys with the aim of identifying and characterizing exoplanets as well as stellar remnants. In this study, we present the analysis of the binary lens event Gaia20bof. By automatically requesting additional observations, the OMEGA Key Project obtained dense time coverage of an anomaly near the peak of the event, allowing characterization of the lensing system. The observed anomaly in the lightcurve is due to a binary lens. However, several models can explain the observations. Spectroscopic observations indicate that the source is located at $\le2.0$ kpc, in agreement with the parallax measurements from Gaia. While the models are currently degenerate, future observations, especially the Gaia astrometric time series as well as high-resolution imaging, will provide extra constraints to distinguish between them.

astro-ph.EP↗

arXiv2024

Gaia21blx: Complete resolution of a binary microlensing event in the Galactic disk

Context. Gravitational microlensing is a method that is used to discover planet-hosting systems at distances of several kiloparsec in the Galactic disk and bulge. We present the analysis of a microlensing event reported by the Gaia photometric alert team that might have a bright lens. Aims. In order to infer the mass and distance to the lensing system, the parallax measurement at the position of Gaia21blx was used. In this particular case, the source and the lens have comparable magnitudes and we cannot attribute the parallax measured by Gaia to the lens or source alone. Methods. Since the blending flux is important, we assumed that the Gaia parallax is the flux-weighted average of the parallaxes of the lens and source. Combining this assumption with the information from the microlensing models and the finite source effects we were able to resolve all degeneracies and thus obtained the mass, distance, luminosities and projected kinematics of the binary lens and the source. Results. According to the best model, the lens is a binary system at $2.18 \pm 0.07$ kpc from Earth. It is composed of a G star with $0.95\pm 0.17\,M_{\odot}$ and a K star with $0.53 \pm 0.07 \, M_{\odot}$. The source is likely to be an F subgiant star at $2.38 \pm 1.71$ kpc with a mass of $1.10 \pm 0.18 \, M_{\odot}$. Both lenses and the source follow the kinematics of the thin-disk population. We also discuss alternative models, that are disfavored by the data or by prior expectations, however.

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arXiv2024

Early recognition of Microlensing Events from Archival Photometry with Machine Learning Methods

Gravitational microlensing method is a powerful method to detect isolated black holes in the Milky Way. During a microlensing event brightness of the source increases and this feature is used by many photometric surveys to alert on potential events. A typical microlensing event shows a characteristic light curve, however, some outbursting variable stars may show similar light curves to microlensing events especially when the cadence of observations is not dense enough. Our aim is to device a method for distinguishing candidates for microlensing events from any other types of alerts using solely their archival photometric multi-wavelength data. The most common contaminants in the microlensing event search are Classical Be-type stars, Young Stellar Objects and Asymptotic Giant Branch stars such as Miras. We build a training set using thousands of examples for the main classes of alerting stars combining optical to mid-infrared magnitudes from Gaia, 2MASS and AllWISE catalogues. We used supervised machine learning techniques to build models for classification of alerts. We verified our method on 120 microlensing events reported by Gaia Science Alerts which were studied spectroscopically and photometrically. With the use of only archival information at 90% probability threshold we correctly identified one-third of the microlensing events. We also run our classifier on positions of 368 Gaia alerts which were flagged as potential candidates for microlensing events. At the 90% probability threshold we classified 38 microlensing events and 29 other types of variables. The machine learning supported method we developed can be universally used for current and future alerting surveys in order to quickly assess the classification of galactic transients and help decide on further follow-up observations.

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arXiv2023

Candidates for Transiting Planets in OGLE-IV Galactic Bulge Fields

We present results of a search for transiting exoplanets in 10-yr long photometry with thousands of epochs taken in the direction of the Galactic bulge. This photometry was collected in the fourth phase of the Optical Gravitational Lensing Experiment (OGLE-IV). Our search covered approx. 222 000 stars brighter than I = 15.5 mag. Selected transits were verified using a probabilistic method. The search resulted in 99 high-probability candidates for transiting exoplanets. The estimated distances to these targets are between 0.4 kpc and 5.5 kpc, which is a significantly wider range than for previous transit searches. The planets found are Jupiter-size, with the exception of one (named OGLE-TR-1003b) located in the hot Neptune desert. If the candidate is confirmed, it can be important for studies of highly irradiated intermediate-size planets. The existing long-term, high-cadence photometry of our candidates increases the chances of detecting transit timing variations at long timescales. Selected candidates will be observed by the future NASA flagship mission, the Nancy Grace Roman Space Telescope, in its search for Galactic bulge microlensing events, which will further enhance the photometric coverage of these stars.

astro-ph.EP↗

arXiv2023

Lens mass estimate in the Galactic disk extreme parallax microlensing event Gaia19dke

We present the results of our analysis of Gaia19dke, an extraordinary microlensing event in the Cygnus constellation that was first spotted by the {\gaia} satellite. This event featured a strong microlensing parallax effect, which resulted in multiple peaks in the light curve. We conducted extensive photometric, spectroscopic, and high-resolution imaging follow-up observations to determine the mass and the nature of the invisible lensing object. Using the Milky Way priors on density and velocity of lenses, we found that the dark lens is likely to be located at a distance of $D_L =(3.05^{+4.10}_{-2.42})$kpc, and has a mass of $M_L =(0.51^{+3.07}_{-0.40}) M_\odot$. Based on its low luminosity and mass, we propose that the lens in Gaia19dke event is an isolated white dwarf.

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arXiv2023

MOA-2020-BLG-208Lb: Cool Sub-Saturn Planet Within Predicted Desert

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.

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arXiv2022

Gaia Data Release 3: Microlensing Events from All Over the Sky

Context: One of the rarest types of variability is the phenomenon of gravitational microlensing, a transient brightening of a background star due to an intervening lensing object. Microlensing is a powerful tool in studying the invisible or otherwise undetectable populations in the Milky Way, including planets and black holes. Aims: We describe the first Gaia catalogue of microlensing event candidates, give an overview of its content and discuss its validation. Methods: The catalogue of Gaia microlensing events is composed by analysing the light curves of around 2 billion sources of Gaia Data Release 3 from all over the sky covering 34 months between 2014 and 2017. Results: We present 363 Gaia microlensing events and discuss their properties. Ninety events were never reported before and were not discovered by other surveys. The contamination of the catalogue is assessed to 0.6-1.7%.

astro-ph.SR↗

arXiv2022

The nuclear transient AT 2017gge: a tidal disruption event in a dusty and gas-rich environment and the awakening of a dormant SMBH

We present the results from a dense multi-wavelength (optical/UV, near-infrared (IR), and X-ray) follow-up campaign of the nuclear transient AT2017gge, covering a total of 1698 days from the transient's discovery. The bolometric lightcurve, the black body temperature and radius, the broad H and He I $λ$5876 emission lines and their evolution with time, are all consistent with a tidal disruption event (TDE) nature. A soft X-ray flare is detected with a delay of $\sim$200 days with respect to the optical/UV peak and it is rapidly followed by the emergence of a broad He II $λ$4686 and by a number of long-lasting high ionization coronal emission lines. This indicate a clear connection between a TDE flare and the appearance of extreme coronal line emission (ECLEs). An IR echo, resulting from dust re-radiation of the optical/UV TDE light is observed after the X-ray flare and the associated near-IR spectra show a transient broad feature in correspondence of the He I $λ$10830 and, for the first time in a TDE, a transient high-ionization coronal NIR line (the [Fe XIII] $λ$10798) is also detected. The data are well explained by a scenario in which a TDE occurs in a gas and dust rich environment and its optical/UV, soft X-ray, and IR emission have different origins and locations. The optical emission may be produced by stellar debris stream collisions prior to the accretion disk formation, which is instead responsible for the soft X-ray flare, emitted after the end of the circularization process.

astro-ph.HE↗

arXiv2022

Gaia Data Release 3. Summary of the variability processing and analysis

Context. Gaia has been in operations since 2014. The third Gaia data release expands from the early data release (EDR3) in 2020 by providing 34 months of multi-epoch observations that allowed us to probe, characterise and classify systematically celestial variable phenomena. Aims. We present a summary of the variability processing and analysis of the photometric and spectroscopic time series of 1.8 billion sources done for Gaia DR3. Methods. We used statistical and Machine Learning methods to characterise and classify the variable sources. Training sets were built from a global revision of major published variable star catalogues. For a subset of classes, specific detailed studies were conducted to confirm their class membership and to derive parameters that are adapted to the peculiarity of the considered class. Results. In total, 10.5 million objects are identified as variable in Gaia DR3 and have associated time series in G, GBP, and GRP and, in some cases, radial velocity time series. The DR3 variable sources subdivide into 9.5 million variable stars and 1 million Active Galactic Nuclei/Quasars. In addition, supervised classification identified 2.5 million galaxies thanks to spurious variability induced by the extent of these objects. The variability analysis output in the DR3 archive amounts to 17 tables containing a total of 365 parameters. We publish 35 types and sub-types of variable objects. For 11 variable types, additional specific object parameters are published. An overview of the estimated completeness and contamination of most variability classes is provided. Conclusions. Thanks to Gaia we present the largest whole-sky variability analysis based on coherent photometric, astrometric, and spectroscopic data. Later Gaia data releases will more than double the span of time series and the number of observations, thus allowing for an even richer catalogue in the future.

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arXiv2022

Precision measurement of a brown dwarf mass in a binary system in the microlensing event OGLE-2019-BLG-0033/MOA-2019-BLG-035

Context. Brown dwarfs are poorly understood transition objects between stars and planets, with several competing mechanisms having been proposed for their formation. Mass measurements are generally difficult for isolated objects but also for brown dwarfs orbiting low-mass stars, which are often too faint for spectroscopic follow-up. Aims. Microlensing provides an alternative tool for the discovery and investigation of such faint systems. Here we present the analysis of the microlensing event OGLE-2019-BLG-0033/MOA-2019-BLG-035, which is due to a binary system composed of a brown dwarf orbiting a red dwarf. Methods. Thanks to extensive ground observations and the availability of space observations from Spitzer, it has been possible to obtain accurate estimates of all microlensing parameters, including parallax, source radius and orbital motion of the binary lens. Results. After accurate modeling, we find that the lens is composed of a red dwarf with mass $M_1 = 0.149 \pm 0.010M_\odot$ and a brown dwarf with mass $M_2 = 0.0463 \pm 0.0031M_\odot$, at a projected separation of $a_\perp = 0.585$ au. The system has a peculiar velocity that is typical of old metal-poor populations in the thick disk. Percent precision in the mass measurement of brown dwarfs has been achieved only in a few microlensing events up to now, but will likely become common with the Roman space telescope.

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arXiv2022

Lens parameters for Gaia18cbf -- a long gravitational microlensing event in the Galactic plane

Context: The timescale of a microlensing event scales as a square root of a lens mass. Therefore, long-lasting events are important candidates for massive lenses, including black holes. Aims: Here we present the analysis of the Gaia18cbf microlensing event reported by the Gaia Science Alerts system. It exhibited a long timescale and features that are common for the annual microlensing parallax effect. We deduce the parameters of the lens based on the derived best fitting model. Methods: We used photometric data collected by the Gaia satellite as well as the follow-up data gathered by the ground-based observatories. We investigated the range of microlensing models and used them to derive the most probable mass and distance to the lens using a Galactic model as a prior. Using known mass-brightness relation we determined how likely it is that the lens is a main-sequence (MS) star. Results: This event is one of the longest ever detected, with the Einstein timescale of $t_\mathrm{E}=491.41^{+128.31}_{-84.94}$ days for the best solution and $t_\mathrm{E}=453.74^{+178.69}_{-105.74}$ days for the second-best. Assuming Galaxy priors, this translates to the most probable lens mass of $M_\mathrm{L} = 2.65^{+5.09}_{-1.48} M_\odot$ and $M_\mathrm{L} = 1.71^{+3.78}_{-1.06} M_\odot$, respectively. The limits on the blended light suggest that this event was most likely not caused by a MS star, but rather by a dark remnant of stellar evolution.

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