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Przemek Mroz

Publications and source records attributed to Przemek Mroz.

At least 19 recordsLinked to original sources
arXiv2026

Mass Production of 2023 KMTNet Microlensing Planets I: Low Mass Ratio

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, $q<2\times 10^{-4}$. 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 ($q\sim 1.3\times 10^{-4}$), KMT-2023-BLG-1286 ($q\sim 1.9\times 10^{-4}$), and KMT-2023-BLG-1746 ($q\sim 8\times 10^{-5}$). 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 $M\sim 1.0\,M_\odot$ and $D\sim 1.5$ kpc, 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 due to their ambiguous interpretations as possible non-planetary events.

astro-ph.EP↗
arXiv2025

A free-floating-planet microlensing event caused by a Saturn-mass object

A population of free-floating planets is known from gravitational microlensing surveys. None have a directly measured mass, owing to a degeneracy with the distance, but the population statistics indicate that many are less massive than Jupiter. We report a microlensing event -- KMT-2024-BLG-0792/OGLE-2024-BLG-0516, which was observed from both ground- and space-based telescopes -- that breaks the mass-distance degeneracy. The event was caused by an object with 0.219^{+0.075}_{-0.046} Jupiter masses that is either gravitationally unbound or on a very wide orbit. Through comparison with the statistical properties of other observed microlensing events and predictions from simulations, we infer that this object likely formed in a protoplanetary disk (like a planet), not in isolation (like a brown dwarf), and dynamical processes then ejected it from its birth place, producing a free-floating object.

astro-ph.EP↗
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

HST pre-imaging of a free-floating planet candidate microlensing event

High-cadence microlensing observations uncovered a population of very short-timescale microlensing events, which are believed to be caused by the population of free-floating planets (FFP) roaming the Milky Way. Unfortunately, the light curves of such events are indistinguishable from those caused by wide-orbit planets. To properly differentiate both cases, one needs high-resolution observations that would allow resolving a putative luminous companion to the lens long before or after the event. Usually, the baseline between the event and high-resolution observations needs to be quite long ($\sim 10$ yr), hindering potential follow-up efforts. However, there is a chance to use archival data if they exist. Here, we present an analysis of the microlensing event OGLE-2023-BLG-0524, the site of which was captured in 1997 with the Hubble Space Telescope (HST). Hence, we achieve a record-breaking baseline length of 25 years. A very short duration of the event ($t_E = 0.346 \pm 0.008$ d) indicates an FFP as the explanation. We have not detected any potential companion to the lens with the HST data, which is consistent with the FFP origin of the event. Thanks to the available HST data, we are able to reject from 25% to 48% of potential stellar companions depending on the assumed population model. Based on the finite-source effects in the light curve we measure the angular Einstein radius value $θ_E = 4.78 \pm 0.23 μas$, suggesting a super-Earth in the Galactic disk or a sub-Saturn-mass planet in the Galactic bulge. We show that the archival high-resolution images should be available for several microlensing events, providing us with the unprecedented possibility of seeing the lensing system as it was many years before the event.

astro-ph.EP↗
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.

astro-ph.EP↗
arXiv2024

TESS Free-floating Planet Candidate Is Likely a Stellar Flare

The discovery of a terrestrial-mass free-floating planet candidate in the light curve of the star TIC 107150013 observed by the Transiting Exoplanet Survey Satellite (TESS) has recently been announced. A short-duration (~0.5 day), low-amplitude (~0.06 mag) brightening in the TESS light curve was interpreted as a short-timescale gravitational microlensing event. However, the purported event occurred far from the Galactic center and the Galactic plane (l~ 239 deg, b ~ -5 deg), on a relatively nearby (~3.2 kpc) star, making the microlensing interpretation unlikely. Here, we report the archival photometric observations of TIC 107150013 collected by the Optical Gravitational Lensing Experiment (OGLE) from 2018 through 2020. The archival OGLE light curve reveals periodic variability indicative of starspots on the surface of the star. The presence of starspots indicates magnetic activity of the star, which may also manifest as stellar flares. We interpret the brightening of TIC 107150013 seen in the TESS data as the stellar flare. We present similar flaring stars detected in the archival OGLE data, mimicking short-timescale, low-amplitude microlensing events. Such stars may be a source of non-negligible false positive detections in the planned space-based microlensing surveys.

astro-ph.EP↗
arXiv2023

A rotating white dwarf shows different compositions on its opposite faces

White dwarfs, the extremely dense remnants left behind by most stars after their death, are characterised by a mass comparable to that of the Sun compressed into the size of an Earth-like planet. In the resulting strong gravity, heavy elements sink toward the centre and the upper layer of the atmosphere contains only the lightest element present, usually hydrogen or helium. Several mechanisms compete with gravitational settling to change a white dwarf's surface composition as it cools, and the fraction of white dwarfs with helium atmospheres is known to increase by a factor ~2.5 below a temperature of about 30,000 K; therefore, some white dwarfs that appear to have hydrogen-dominated atmospheres above 30,000 K are bound to transition to be helium-dominated as they cool below it. Here we report observations of ZTF J203349.8+322901.1, a transitioning white dwarf with two faces: one side of its atmosphere is dominated by hydrogen and the other one by helium. This peculiar nature is likely caused by the presence of a small magnetic field, which creates an inhomogeneity in temperature, pressure or mixing strength over the surface. ZTF J203349.8+322901.1 might be the most extreme member of a class of magnetic, transitioning white dwarfs -- together with GD 323, a white dwarf that shows similar but much more subtle variations. This new class could help shed light on the physical mechanisms behind white dwarf spectral evolution.

astro-ph.SR↗
arXiv2023

Roman Early-Definition Astrophysics Survey Opportunity: Galactic Roman Infrared Plane Survey (GRIPS)

A wide-field near-infrared survey of the Galactic disk and bulge/bar(s) is supported by a large representation of the community of Galactic astronomers. The combination of sensitivity, angular resolution and large field of view make Roman uniquely able to study the crowded and highly extincted lines of sight in the Galactic plane. A ~1000 deg2 survey of the bulge and inner Galactic disk would yield an impressive dataset of ~120 billion sources and map the structure of our Galaxy. The effort would foster subsequent expansions in numerous dimensions (spatial, depth, wavelengths, epochs). Importantly, the survey would benefit from early defintion by the community, namely because the Galactic disk is a complex environment, and different science goals will require trade offs.

astro-ph.GA↗
arXiv2022

Systematic errors as a source of mass discrepancy in black hole microlensing event OGLE-2011-BLG-0462

Two independent groups reported the discovery of an isolated dark stellar remnant in the microlensing event OGLE-2011-BLG-0462 based on photometric ground-based observations coupled with astrometric measurements taken with the Hubble Space Telescope. These two analyses yielded discrepant mass measurements, with the first group reporting that the lensing object is a black hole of 7.1 +/- 1.3 solar masses whereas the other concluded that the microlensing event was caused by either a neutron star or a low-mass black hole (1.6-4.4 solar masses). Here, we scrutinize the available photometric and astrometric data and conclude that systematic errors are a cause of the discrepant measurements. We find that the lens is an isolated black hole with a mass of 7.88 +/- 0.82 solar masses located at a distance of 1.49 +/- 0.12 kpc. We also study the impact of blending on the accuracy of astrometric microlensing measurements. We find that low-level blending by source companions is a major, previously unrecognized, challenge to astrometric microlensing measurements of black hole masses.

astro-ph.SR↗
arXiv2022

An Isolated Stellar-Mass Black Hole Detected Through Astrometric Microlensing

We report the first unambiguous detection and mass measurement of an isolated stellar-mass black hole (BH). We used the Hubble Space Telescope (HST) to carry out precise astrometry of the source star of the long-duration (t_E~270 days), high-magnification microlensing event MOA-2011-BLG-191/OGLE-2011-BLG-0462 (hereafter designated as MOA-11-191/OGLE-11-462), in the direction of the Galactic bulge. HST imaging, conducted at eight epochs over an interval of six years, reveals a clear relativistic astrometric deflection of the background star's apparent position. Ground-based photometry of MOA-11-191/OGLE-11-462 shows a parallactic signature of the effect of the Earth's motion on the microlensing light curve. Combining the HST astrometry with the ground-based light curve and the derived parallax, we obtain a lens mass of 7.1 +/- 1.3 Msun and a distance of 1.58 +/- 0.18 kpc. We show that the lens emits no detectable light, which, along with having a mass higher than is possible for a white dwarf or neutron star, confirms its BH nature. Our analysis also provides an absolute proper motion for the BH. The proper motion is offset from the mean motion of Galactic-disk stars at similar distances by an amount corresponding to a transverse space velocity of ~45 km/s, suggesting that the BH received a 'natal kick' from its supernova explosion. Previous mass determinations for stellar-mass BHs have come from radial-velocity measurements of Galactic X-ray binaries, and from gravitational radiation emitted by merging BHs in binary systems in external galaxies. Our mass measurement is the first for an isolated stellar-mass BH using any technique.

astro-ph.SR↗
arXiv2022

An isolated mass gap black hole or neutron star detected with astrometric microlensing

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.

astro-ph.GA↗
arXiv2022

Impact of the SpaceX Starlink Satellites on the Zwicky Transient Facility Survey Observations

There is a growing concern about an impact of low-Earth-orbit (LEO) satellite constellations on ground-based astronomical observations, in particular, on wide-field surveys in the optical and infrared. The Zwicky Transient Facility (ZTF), thanks to the large field of view of its camera, provides an ideal setup to study the effects of LEO megaconstellations - such as SpaceX's Starlink - on astronomical surveys. Here, we analyze the archival ZTF observations collected between 2019 November and 2021 September and find 5301 satellite streaks that can be attributed to Starlink satellites. We find that the number of affected images is increasing with time as SpaceX deploys more and more satellites. Twilight observations are particularly affected - a fraction of streaked images taken during twilight has increased from less than 0.5% in late 2019 to 18% in 2021 August. We estimate that once the size of the Starlink constellation reaches 10,000, essentially all ZTF images taken during twilight may be affected. However, despite the increase in satellite streaks observed during the analyzed period, the current science operations of ZTF are not yet strongly affected. We also find that redesigning Starlink satellites (by installing visors intended to block sunlight from reaching the satellite antennas to prevent reflection) reduces their brightness by a factor of 4.6 +/- 0.1 with respect to the original design in g, r, and i bands.

astro-ph.IM↗
arXiv2021

OGLE-2014-BLG-0319: A Sub-Jupiter-Mass Planetary Event Encountered Degeneracy with Different Mass Ratios and Lens-Source Relative Proper Motions

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 $μ_{\rm rel}\sim1\;{\rm mas/yr}$. 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.

astro-ph.EP↗
arXiv2021

The Remarkable Spin-down and Ultra-fast Outflows of the Highly-Pulsed Supersoft Source of Nova Hercules 2021

Nova Her 2021 (V1674 Her), which erupted on 2021 June 12, reached naked-eye brightness and has been detected from radio to $γ$-rays. An extremely fast optical decline of 2 magnitudes in 1.2 days and strong Ne lines imply a high-mass white dwarf. The optical pre-outburst detection of a 501.42s oscillation suggests a magnetic white dwarf. This is the first time that an oscillation of this magnitude has been detected in a classical nova prior to outburst. We report X-ray outburst observations from {\it Swift} and {\it Chandra} which uniquely show: (1) a very strong modulation of super-soft X-rays at a different period from reported optical periods; (2) strong pulse profile variations and the possible presence of period variations of the order of 0.1-0.3s; and (3) rich grating spectra that vary with modulation phase and show P Cygni-type emission lines with two dominant blue-shifted absorption components at $\sim 3000$ and 9000 km s$^{-1}$ indicating expansion velocities up to 11000 km s$^{-1}$. X-ray oscillations most likely arise from inhomogeneous photospheric emission related to the magnetic field. Period differences between reported pre- and post-outburst optical observations, if not due to other period drift mechanisms, suggest a large ejected mass for such a fast nova, in the range $2\times 10^{-5}$-$2\times 10^{-4} M_\odot$. A difference between the period found in the {\it Chandra} data and a reported contemporaneous post-outburst optical period, as well as the presence of period drifts, could be due to weakly non-rigid photospheric rotation.

astro-ph.HE↗
arXiv2021

A systematic search for outbursting AM CVn systems with the Zwicky Transient Facility

AM CVn systems are a rare type of accreting binary that consists of a white dwarf and a helium-rich, degenerate donor star. Using the Zwicky Transient Facility (ZTF), we searched for new AM CVn systems by focusing on blue, outbursting stars. We first selected outbursting stars using the ZTF alerts. We cross-matched the candidates with $Gaia$ and Pan-STARRS catalogs. The initial selection of candidates based on the $Gaia$ $BP$-$RP$ contains 1751 unknown objects. We used the Pan-STARRS $g$-$r$ and $r$-$i$ color in combination with the $Gaia$ color to identify 59 high-priority candidates. We obtained identification spectra of 35 sources, of which 18 are high priority candidates, and discovered 9 new AM CVn systems and one magnetic CV which shows only He-II lines. Using the outburst recurrence time, we estimate the orbital periods which are in the range of 29 to 50 minutes. We conclude that targeted followup of blue, outbursting sources is an efficient method to find new AM CVn systems, and we plan to followup all candidates we identified to systematically study the population of outbursting AM CVn systems.

astro-ph.SR↗
arXiv2021

OGLE-2017-BLG-1049: Another giant planet microlensing event

We report a giant exoplanet discovery in the microlensing event OGLE-2017-BLG-1049, which is a planet-host star mass ratio of $q=9.53\pm0.39\times10^{-3}$ and has a caustic crossing feature in the Korea Microlensing Telescope Network (KMTNet) observations. The caustic crossing feature yields an angular Einstein radius of $θ_{\rm E}=0.52 \pm 0.11\ {\rm mas}$. However, the microlens parallax is not measured because of the time scale of the event $t_{\rm E}\simeq 29\ {\rm days}$, which is not long enough in this case to determine the microlens parallax. Thus, we perform a Bayesian analysis to estimate physical quantities of the lens system. From this, we find that the lens system has a star with mass $M_{\rm h}=0.55^{+0.36}_{-0.29} \ M_{\odot}$ hosting a giant planet with $M_{\rm p}=5.53^{+3.62}_{-2.87} \ M_{\rm Jup}$, at a distance of $D_{\rm L}=5.67^{+1.11}_{-1.52}\ {\rm kpc}$. The projected star-planet separation in units of the Einstein radius $(θ_{\rm E})$ corresponding to the total mass of the lens system is $a_{\perp}=3.92^{+1.10}_{-1.32}\ \rm{au}$. This means that the planet is located beyond the snow line of the host. The relative lens-source proper motion is $μ_{\rm rel}\sim 7 \ \rm{mas \ yr^{-1}}$, thus the lens and source will be separated from each other within 10 years. Then the flux of the host star can be measured by a 30m class telescope with high-resolution imaging in the future, and thus its mass can be determined.

astro-ph.EP↗
arXiv2021

HO Puppis: Not a Be Star but a Newly Confirmed IW And-Type Star

HO Puppis (HO Pup) was considered as a Be-star candidate based on its gamma-Cassiopeiae-type light curve, but lacked spectroscopic confirmation. Using distance measured from Gaia Data Release 2 and the spectral-energy-distribution (SED) fit on broadband photometry, the Be-star nature of HO Pup is ruled out. Furthermore, based on the 28,700 photometric data points collected from various time-domain surveys and dedicated intensive-monitoring observations, the light curves of HO Pup closely resemble IW And-type stars (as pointed out in Kimura et al. 2020a), exhibiting characteristics such as quasi-standstill phase, brightening, and dips. The light curve of HO Pup displays various variability timescales, including brightening cycles ranging from 23 to 61 days, variations with periods between 3.9 days and 50 minutes during the quasi-standstill phase, and a semi-regular ~14-day period for the dip events. We have also collected time-series spectra (with various spectral resolutions), in which Balmer emission lines and other expected spectral lines for an IW And-type star were detected (even though some of these lines were also expected to be present for Be stars). We detect Bowen fluorescence near the brightening phase, and that can be used to discriminate between IW And-type stars and Be stars. Finally, despite only observing for four nights, the polarization variation was detected, indicating that HO Pup has significant intrinsic polarization.

astro-ph.SR↗
arXiv2020

A systematic search of Zwicky Transient Facility data for ultracompact binary LISA-detectable gravitational-wave sources

Using photometry collected with the Zwicky Transient Facility (ZTF), we are conducting an ongoing survey for binary systems with short orbital periods ($P_{\rm b}<1\rm \,hr)$ with the goal of identifying new gravitational-wave sources detectable by the upcoming Laser Interferometer Space Antenna (LISA). Here, we present a sample of fifteen binary systems discovered thus far, with orbital periods ranging from $6.91\rm\,min$ to $56.35\rm\,min$. Of the fifteen systems, seven are eclipsing systems which do not show signs of significant mass transfer. Additionally, we have discovered two AM Canum Venaticorum (AM CVn) systems and six systems exhibiting primarily ellipsoidal variations in their light curves. We present follow-up spectroscopy and high-speed photometry confirming the nature of these systems, estimates of their LISA signal-to-noise ratios (SNR), and a discussion of their physical characteristics.

astro-ph.SR↗