arXiv subjects

Natasha S. Abrams

Publications and source records attributed to Natasha S. Abrams.

18 recordsLinked to original sources

arXiv2026

Black hole astrometric binaries in the Roman Galactic Bulge Time Domain Survey

The Nancy Grace Roman Space Telescope (Roman), NASA's next flagship mission, is currently scheduled to launch in August 2026. As part of its mission, Roman will conduct the Galactic Bulge Time Domain Survey (GBTDS), which will generate $\sim$50,000 epochs of high-precision photometric and astrometric data for $\sim 10^8$ sources across 1.7 deg$^2$ in the Galactic Bulge. Roman GBTDS astrometry is comparable to Gaia Data Release 4 in terms of number of stars, astrometric precision, and time baseline, and is highly complementary in terms of wavelength and sky location. In this paper, we investigate a synthetic population of GBTDS sources to characterize the detectability of unresolved astrometric binaries, in particular those with compact object companions. Assuming the occurrence rate of black holes (BHs) and neutron stars (NSs) in AU-scale orbits around stars is $10^{-7}$ and $10^{-6}$, respectively, and that Roman achieves an astrometric precision of $1\%$ of a pixel, $\mathcal{O}(10)$ BH+star and $\mathcal{O}(10)$ NS+star detached binaries will be detectable. The BHs will have median mass measurement uncertainties of $\sim 25\%$, increasing the existing sample of detached astrometric BH binaries by a factor of three. Together with the $\mathcal{O}(10^2)$ isolated BHs expected to be discovered by microlensing in the Roman GBTDS and an additional $\mathcal{O}(10)$ detached BH binaries in Gaia DR4, this will provide a representative view of the quiescent Galactic stellar-mass BH population.

astro-ph.SR↗

arXiv2026

Charting the Galactic Underworld I: Comprehensive simulations of the kinematics, rates, and demographics of Milky Way black holes

With upcoming data from Roman, Gaia DR4, and spectroscopic surveys, we will soon have an unprecedented dataset of Milky Way black holes (BHs) to constrain their formation and evolution. To prepare, we simulate the intrinsic population of Milky Way BHs with cogsworth, self-consistently accounting for their binary evolution and trajectories through the Galactic potential. We report the rate, demographics, and kinematics of these BHs, and their sensitivity to 32 variations in binary evolution, supernova physics, and Galactic potentials. In the fiducial model, ~$1.7 \times 10^8$ BHs have formed in the Milky Way (though this total spans an order of magnitude across our variations), where the vast majority (~91%) are currently isolated and ~3% have escaped the Galaxy. Most of the ~$10^7$ BHs in binaries have another BH or a white dwarf companion, but ~$10^5$ retain a luminous stellar companion. BHs are distributed more diffusely than visible stars, with a scale height around ~$2.5\times$ larger. BH masses correlate with present-day location: the most massive BHs are preferentially close to the Galactic plane. This correlation is especially strong for BH-star binaries, which separate into tight, low-mass post-common-envelope systems and wide, high-mass non-interacting ones. The BH mass distribution and kinematics are highly sensitive to the remnant mass prescription and natal kick model, so observations could constrain explodability criteria and BH kicks. Accounting for the time-evolution of the Galactic potential more than doubles the escape fraction and increases the bound population's scale height by ~20%, whilst neglecting binary interactions overestimates it by 30%.

astro-ph.GA↗

arXiv2026

An Updated SynthPop Model for Microlensing Simulations I: Model Description & Evaluation

The optimization and interpretation of microlensing surveys depends on having an accurate model of the Milky Way. However, existing population synthesis Galactic modeling tools often perform poorly in replicating the stellar contents of the inner Galactic bulge region and reproducing microlensing survey results. We present an updated Galactic model implementation within the \synthpop framework that has been tuned for simulating the upcoming {\it Nancy Grace Roman Space Telescope}'s Galactic Bulge Time Domain Survey (RGBTDS). We evaluate the model against stellar catalogs and kinematics from optical and infrared surveys toward the Galactic bulge, finding good agreement in much of the bulge, including the RGBTDS' contiguous lower bulge fields. However, within Galactic latitudes of $b\lesssim0.5^\circ$ of the Galactic plane, some inconsistencies arise which may impact projections for the RGBTDS' Galactic center field. The model over-predicts optical microlensing event rate per star measurements by a $\sim20$\%, but detailed comparisons to near-infrared measurements are hampered by their lack of detection efficiencies. {\it Roman}'s GBTDS and Galactic Plane Survey will be instrumental in resolving the remaining model inconsistencies and improving our understanding of the structure of the central few degrees of our Galaxy.

astro-ph.GA↗

arXiv2026

Modeling Binary Lenses and Sources with the BAGLE Python Package

Gravitational microlensing is a powerful tool that can be used to find and measure the mass of isolated and dark compact objects. In many microlensing events, the lens, the source, or both may be a binary system. In this work, we introduce binary source and lens models into the gravitational lensing formalism encoded in the Bayesian Analysis of Gravitational Lensing Events (BAGLE) Python software package. These new binary models in BAGLE account for Keplerian orbits. We also add binary models with fewer parameters that describe the binary orbital motion as acceleration, linear, or stationary motion of the secondary companion; these are useful when the orbit has a very low eccentricity or the orbital period is much longer than the microlensing timescale. The model parameterizations based on these binary lensing equations enable joint-fitting of photometric and astrometric datasets. These binary models will be used to fit microlensing event data from the Vera C. Rubin Observatory, the Nancy Grace Roman Telescope, and other surveys.

astro-ph.SR↗

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

Hints of an Anomalous Lens Population towards the Galactic Bulge

The dark and dynamic parts of the Galaxy, including the bulk shape and movement of the Galactic Bulge and characteristics of dark compact object populations, such as a hypothetical population of primordial black holes (PBHs), are difficult to study directly by their very nature, but are critical to our understanding of the universe. Fortunately, all of these mysteries can be uniquely studied via gravitational microlensing, a method of astronomical detection that traces mass and dynamics as opposed to light. Using the OGLE-IV microlensing survey bulge fields, we apply a Bayesian hierarchical model to jointly infer properties of the Galaxy, the characteristics of compact objects, and and test PBHs with an extended mass distribution as a test PBHs as a viable explanation of dark matter, extending work focused on the Small and Large Magellanic Clouds, both with much lower event-rates. We infer a preference within the data for a lower patternspeed in the galactic model and a wider mass spectrum for compact objects. When adding a PBH component to the favored astrophysical model from our initial investigations, we find a Bayes factor of $\ln\mathcal{B} = 20.23$ preferring the PBH model. Upon further investigation of these results, we find the critical feature in the PBH model to be the velocity distribution, which is fundamentally different than the velocity distribution of astrophysical objects and uniquely able to explain a large number of low parallax, low timescale microlensing events. Noting that this effect is not unique to PBHs, we consider the implications of these results as applied to a hypothetical population of PBHs and discuss alternative explanations, including a variety of other possible astrophysical and survey or analysis systematics.

astro-ph.GA↗

arXiv2025

The La Silla Schmidt Southern Survey

We present the La Silla Schmidt Southern Survey (LS4), a new wide-field, time-domain survey to be conducted with the 1 m ESO Schmidt telescope. The 268 megapixel LS4 camera mosaics 32 2k$\times$4k fully depleted CCDs, providing a $\sim$20 deg$^2$ field of view with $1''$ pixel$^{-1}$ resolution. The LS4 camera will have excellent performance at longer wavelengths: in a standard 45 s exposure the expected 5$σ$ limiting magnitudes in $g$, $i$, $z$ are $\sim$21.5, $\sim$20.9, and $\sim$20.3 mag (AB), respectively. The telescope design requires a novel filter holder that fixes different bandpasses over each quadrant of the detector. Two quadrants will have $i$ band, while the other two will be $g$ and $z$ band and color information will be obtained by dithering targets across the different quadrants. The majority (90%) of the observing time will be used to conduct a public survey that monitors the extragalactic sky at both moderate (3 d) and high (1 d) cadence, as well as focused observations within the Galactic bulge and plane. Alerts from the public survey will be broadcast to the community via established alert brokers. LS4 will run concurrently with the Vera C. Rubin Observatory's Legacy Survey of Space and Time (LSST). The combination of LS4+LSST will enable detailed holistic monitoring of many nearby transients: high-cadence LS4 observations will resolve the initial rise and peak of the light curve while less-frequent but deeper observations by LSST will characterize the years before and after explosion. Here, we summarize the primary science objectives of LS4 including microlensing events in the Galaxy, extragalactic transients, the search for electromagnetic counterparts to multi-messenger events, and cosmology.

astro-ph.IM↗

arXiv2025

Assessing the Impact of Binary Systems on Microlensing Using SPISEA and PopSyCLE Population Simulations

Gravitational microlensing provides a unique opportunity to probe the mass distribution of stars, black holes, and other objects in the Milky Way. Population simulations are necessary to interpret results from microlensing surveys. The contribution from binary objects is often neglected or minimized in analysis of observations and simulations despite the high percentage of binary systems and microlensing's ability to probe binaries. To simulate the population effects we added multiple systems to Stellar Population Interface for Stellar Evolution and Atmospheres (SPISEA), which simulates stellar clusters. We then inject these multiples into Population Synthesis for Compact-object Lensing Events (PopSyCLE), which simulates Milky Way microlensing surveys. When making OGLE observational selection criteria, we find that 55% of observed microlensing events involve a binary system. Specifically, 14.5% of events have a multiple-lens and a single source, 31.7% have a single lens and a multiple-source, and 8.8% have a multiple-lens and a multiple-source. The majority of these events have photometric lightcurves that appear single and are fit well by a single-lens, single-source model. This suggests that binary source and binary lens-binary source models should be included more frequently in event analysis. The mean Einstein crossing time shifts from 19.1 days for single events only to 21.3 days for singles and multiple events, after cutting binary events with multiple peaks. The Einstein crossing time distribution of singles and single-peaked multiple events is better aligned with observed distributions from OGLE (arXiv:1707.07634) than singles alone, indicating that multiple systems are a significant missing piece between simulations and reality.

astro-ph.SR↗

arXiv2024

Microlensing Discovery and Characterization Efficiency in the Vera C. Rubin Legacy Survey of Space and Time

The Vera C. Rubin Legacy Survey of Space and Time will discover thousands of microlensing events across the Milky Way Galaxy, allowing for the study of populations of exoplanets, stars, and compact objects. We evaluate numerous survey strategies simulated in the Rubin Operation Simulations (OpSims) to assess the discovery and characterization efficiencies of microlensing events. We have implemented three metrics in the Rubin Metric Analysis Framework: a discovery metric and two characterization metrics, where one estimates how well the lightcurve is covered and the other quantifies how precisely event parameters can be determined. We also assess the characterizability of microlensing parallax, critical for detection of free-floating black hole lenses. We find that, given Rubin's baseline cadence, the discovery and characterization efficiency will be higher for longer duration and larger parallax events. Microlensing discovery efficiency is dominated by the observing footprint, where more time spent looking at regions of high stellar density including the Galactic bulge, Galactic plane, and Magellanic clouds, leads to higher discovery and characterization rates. However, if the observations are stretched over too wide an area, including low-priority areas of the Galactic plane with fewer stars and higher extinction, event characterization suffers by > 10%. This could impact exoplanet, binary star, and compact object events alike. We find that some rolling strategies (where Rubin focuses on a fraction of the sky in alternating years) in the Galactic bulge can lead to a 15-20% decrease in microlensing parallax characterization, so rolling strategies should be chosen carefully to minimize losses.

astro-ph.IM↗

arXiv2024

On Finding Black Holes in Photometric Microlensing Surveys

There are expected to be millions of isolated black holes in the Galaxy resulting from the death of massive stars. Measuring the abundance and properties of this remnant population would shed light on the end stages of stellar evolution and the evolution paths of black hole systems. Detecting isolated black holes is currently only possible via gravitational microlensing which has so far yielded one definitive detection. The difficulty in finding microlensing black holes lies in having to choose a small subset of events based on characteristics of their lightcurves to allocate expensive and scarce follow-up resources to confirm the identity of the lens. Current methods either rely on simple cuts in parameter space without using the full distribution information or are only effective on a small subsets of events. In this paper we present a new lens classification method. The classifier takes in posterior constraints on lightcurve parameters and combines them with a Galactic simulation to estimate the lens class probability. This method is flexible and can be used with any set of microlensing lightcurve parameters making it applicable to large samples of events. We make this classification framework available via the popclass python package. We apply the classifier to $\sim10,000$ microlensing events from the OGLE survey and find $23$ high-probability black hole candidates. Our classifier also suggests that the only known isolated black hole is an observational outlier according to current Galactic models and allocation of astrometric follow-up on this event was a high-risk strategy.

astro-ph.SR↗

arXiv2024

Synergies between Roman Galactic Plane Survey and other major surveys

Nancy Grace Roman Space Telescope will revolutionize our understanding of the Galactic Bulge with its Galactic Bulge Time Domain survey. At the same time, Rubin Observatories's Legacy Survey of Space and Time (LSST) will monitor billions of stars in the Milky Way. The proposed Roman survey of the Galactic Plane, with its NIR passbands and exquisite spacial resolution, promises groundbreaking insights for a wide range of time-domain galactic astrophysics. In this white paper, we describe the scientific returns possible from the combination of the Roman Galactic Plane Survey with the data from LSST.

astro-ph.GA↗

arXiv2024

Astrometric Microlensing by Primordial Black Holes with The Roman Space Telescope

Primordial Black Holes (PBHs) could explain some fraction of dark matter and shed light on many areas of early-universe physics. Despite over half a century of research interest, a PBH population has so far eluded detection. The most competitive constraints on the fraction of dark matter comprised of PBHs ($f_{\rm DM}$) in the $(10^{-9}-10)M_{\odot}$ mass-ranges come from photometric microlensing and bound $f_{\rm DM}\lesssim10^{-2}-10^{-1}$. With the advent of the Roman Space Telescope with its sub-milliarcsecond (mas) astrometric capabilities and its planned Galactic Bulge Time Domain Survey (GBTDS), detecting astrometric microlensing signatures will become routine. Compared with photometric microlensing, astrometric microlensing signals are sensitive to different lens masses-distance configurations and contains different information, making it a complimentary lensing probe. At sub-mas astrometric precision, astrometric microlensing signals are typically detectable at larger lens-source separations than photometric signals, suggesting a microlensing detection channel of pure astrometric events. We use a Galactic simulation to predict the number of detectable microlensing events during the GBTDS via this pure astrometric microlensing channel. Assuming an absolute astrometric precision floor for bright stars of 0.1 mas for the GBTDS, we find that the number of detectable events peaks at $\approx 10^{3} f_{\rm DM}$ for a population of $ 1 M_{\odot}$ PBHs and tapers to $\approx 10f_{\rm DM}$ and $\approx 100f_{\rm DM}$ at $10^{-4}M_{\odot}$ and $10^{3}M_{\odot}$, respectively. Accounting for the distinguishability of PBHs from Stellar lenses, we conclude the GBTDS will be sensitive to a PBH population at $f_{\rm DM}$ down to $\approx10^{-1}-10^{-3}$ for $(10^{-1}-10^{2})M_{\odot}$ likely yielding novel PBH constraints.

astro-ph.GA↗

arXiv2023

Disentangling the Black Hole Mass Spectrum with Photometric Microlensing Surveys

From the formation mechanisms of stars and compact objects to nuclear physics, modern astronomy frequently leverages surveys to understand populations of objects to answer fundamental questions. The population of dark and isolated compact objects in the Galaxy contains critical information related to many of these topics, but is only practically accessible via gravitational microlensing. However, photometric microlensing observables are degenerate for different types of lenses, and one can seldom classify an event as involving either a compact object or stellar lens on its own. To address this difficulty, we apply a Bayesian framework that treats lens type probabilistically and jointly with a lens population model. This method allows lens population characteristics to be inferred despite intrinsic uncertainty in the lens-class of any single event. We investigate this method's effectiveness on a simulated ground-based photometric survey in the context of characterizing a hypothetical population of primordial black holes (PBHs) with an average mass of $30 M_{\odot}$. On simulated data, our method outperforms current black hole (BH) lens identification pipelines and characterizes different subpopulations of lenses while jointly constraining the PBH contribution to dark matter to ${\approx}25$\%. Key to robust inference, our method can marginalize over population model uncertainty. We find the lower mass cutoff for stellar origin BHs, a key observable in understanding the BH mass gap, particularly difficult to infer in our simulations. This work lays the foundation for cutting-edge PBH abundance constraints to be extracted from current photometric microlensing surveys.

astro-ph.IM↗

arXiv2023

60 Microlensing Events from the Three Years of Zwicky Transient Facility Phase One

Microlensing events have historically been discovered throughout the Galactic bulge and plane by surveys designed solely for that purpose. We conduct the first multi-year search for microlensing events on the Zwicky Transient Facility (ZTF), an all-sky optical synoptic survey that observes the entire visible Northern sky every few nights. We discover 60 high quality microlensing events in the three years of ZTF-I using the bulk lightcurves in the ZTF Public Data Release 5. 19 of our events are found outside of the Galactic plane ($|b| \geq 15^\circ$), nearly doubling the number of previously discovered events in the stellar halo from surveys pointed toward the Magellanic Clouds and the Andromeda Galaxy. We also record 1,558 ongoing candidate events as potential microlensing that can continue to be observed by ZTF-II for identification. The scalable and computationally efficient methods developed in this work can be applied to future synoptic surveys, such as the Vera C. Rubin Observatory's Legacy Survey of Space and Time and the Nancy Grace Roman Space Telescope, as they attempt to find microlensing events in even larger and deeper datasets.

astro-ph.GA↗

arXiv2022

The Impact of Initial-Final Mass Relations on Black Hole Microlensing

Uncertainty in the initial-final mass relation (IFMR) has long been a problem in understanding the final stages of massive star evolution. One of the major challenges of constraining the IFMR is the difficulty of measuring the mass of non-luminous remnant objects (i.e. neutron stars and black holes). Gravitational wave detectors have opened the possibility of finding large numbers of compact objects in other galaxies, but all in merging binary systems. Gravitational lensing experiments using astrometry and photometry are capable of finding compact objects, both isolated and in binaries, in the Milky Way. In this work we improve the PopSyCLE microlensing simulation code in order to explore the possibility of constraining the IFMR using the Milky Way microlensing population. We predict that the Roman Space Telescope's microlensing survey will likely be able to distinguish different IFMRs based on the differences at the long end of the Einstein crossing time distribution and the small end of the microlensing parallax distribution, assuming the small ($π_E \lesssim 0.02$) microlensing parallaxes characteristic of black hole lenses are able to be measured accurately. We emphasize that future microlensing surveys need to be capable of characterizing events with small microlensing parallaxes in order to place the most meaningful constraints on the IFMR.

astro-ph.HE↗

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↗

arXiv2020

Hunting gravitational wave black holes with microlensing

Gravitational microlensing is a powerful tool to search for a population of invisible black holes (BHs) in the Milky Way (MW), including isolated BHs and binary BHs at wide orbits that are complementary to gravitational wave observations. By monitoring highly populated regions of source stars like the MW bulge region, one can pursue microlensing events due to these BHs. We find that if BHs have a Salpeter-like mass function extended beyond $30M_\odot$ and a similar velocity and spatial structure to stars in the Galactic bulge and disk regions, the BH population is a dominant source of the microlensing events at long timescales of the microlensing light curve $\gtrsim 100~$days. This is due to a boosted sensitivity of the microlensing event rate to lens mass, given as $M^2$, for such long-timescale events. A monitoring observation of $2 \times 10^{10}$ stars in the bulge region over 10 years with the Rubin Observatory Legacy Survey of Space and Time (LSST) would enable one to find about $6\times 10^5$ BH microlensing events. We evaluate the efficiency of potential LSST cadences for characterizing the light curves of BH microlensing and find that nearly all events of long timescales can be detected.

astro-ph.GA↗

arXiv2020

Measured Lightcurves and Rotational Periods of 3122 Florence, 3830 Trelleborg, and (131077) 2000 YH105

We determined the rotational periods of 3122 Florence, 3830 Trelleborg, and (131077) 2000 YH105 with the Harvard Clay Telescope and KeplerCam at the Fred L. Whipple Observatory. We found the rotational periods to be 2.3580 $\pm$ 0.0015 h, 17.059 $\pm$ 0.017 h, and 1.813 $\pm$ 0.00003 h, respectively. Our measurement of 3122 Florence's period agrees with Warner (2016), who reported 2.3580 $\pm$ 0.0002 h.

astro-ph.EP↗