Ambiguities in fits of binary lens galactic microlensing events

The Astrophysical Journal, 2000

We present the lightcurves of 21 gravitational microlensing events from the first six years of the MACHO Project gravitational microlensing survey which are likely examples of lensing by binary systems. These events were manually selected from a total sample of ∼ 350 candidate microlensing events which were either detected by the MACHO Alert System or discovered through retrospective analyses of the MACHO database. At least 14 of these 21 events exhibit strong (caustic) features, and 4 of the events are well fit with lensing by large mass ratio (brown dwarf or planetary) systems, although these fits are not necessarily unique. The total binary event rate is roughly consistent with predictions based upon our knowledge of the properties of binary stars, but a precise comparison cannot be made without a determination of our binary lens event detection efficiency. Towards the Galactic bulge, we find a ratio of caustic crossing to non-caustic crossing binary lensing events of 12:4, excluding one event for which we present 2 fits. This suggests significant incompleteness in our ability to detect and characterize non-caustic crossing binary lensing. The distribution of mass ratios, N(q), for these binary lenses appears relatively flat. We are also able to reliably measure source-face crossing times in 4 of the bulge caustic crossing events, and recover from them a distribution of lens proper motions, masses, and distances consistent with a population of Galactic bulge lenses at a distance of 7 ± 1 kpc. This analysis yields 2 systems with companions of ∼ 0.05M .

Nuclear Physics B - Proceedings Supplements, 1996

The MACHO collaboration has recently analyzed 2.1 years of photometric data for about 8.5 million stars in the Large Magellanic Cloud (LMC). This analysis has revealed 8 candidate microlensing events and a total microlensing optical depth of τmeas = 2.9 +1.4 −0.9 × 10 −7 . This significantly exceeds the number of events (1.1) and the microlensing optical depth predicted from known stellar populations: τ back = 5.4 × 10 −8 , but it is consistent with models in which about half of the standard dark halo mass is composed of Machos of mass ∼ 0.5M⊙. One of these 8 events appears to be a binary lensing event with a caustic crossing that is partially resolved, and the measured caustic crossing time allows us to estimate the distance to the lenses. Under the assumption that the source star is a single star and not a short period binary, we show that the lensing objects are very likely to reside in the LMC. However, if we assume that the optical depth for LMC-LMC lensing is large enough to account for our entire lensing signal, then the binary event does not appear to be consistent with lensing of a single LMC source star by a binary residing in the LMC. Thus, while the binary lens may indeed reside in the LMC, there is no indication that most of the lenses reside in the LMC.

2020

Gravitational microlensing is an astronomical phenomenon where the gravity of a foreground massive object bends the rays of light of a background source into images. Effectively, the background source appears to be magnified with respect to time. Since this does not require detection of light from the lens, gravitational microlensing can be used to study different populations of objects in the galaxy, even the extra-solar planets. This phenomenon was practically formulated and investigated since the last decade of 20th century by a few observing groups. Today gravitational microlensing is observed and monitored by fourth generation telescopes towards high density stellar fields like the Galactic Bulge, Large and Small Magellanic clouds. With the increased capabilities there are numerous microlensing events that are detected but not yet analysed. Analysing these events, especially formed by a binary lens is not only challenging but tedious task. However with a suitable model that exp...

Astronomy & Astrophysics, 2022

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 a 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 tE = 491.41−84...

Monthly Notices of the Royal Astronomical Society, 2005

Although point caustics harbour a larger potential for measuring the brightness profile of stars during the course of a microlensing event than (line-shaped) fold caustics, the effect of lens binarity significantly limits the achievable accuracy. Therefore, corresponding close-impact events make a less favourable case for limb-darkening measurements than those events that involve fold-caustic passages, from which precision measurements can easily and routinely be obtained. Examples involving later Bulge giants indicate that a ∼10 per cent misestimate on the linear limb-darkening coefficient can result with the assumption of a single-lens model that looks acceptable, unless the precision of the photometric measurements is pushed below the 1 per cent level even for these favourable targets. However, measurement uncertainties on the proper motion between lens and source are dominated by the assessment of the angular radius of the source star, and remain practically unaffected by lens binarity. Rather than judging the goodness of fit by means of a χ 2 test only, run tests provide useful additional information that can lead to the rejection of models and the detection of lens binarity in close-impact microlensing events.

arXiv (Cornell University), 2023

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 ⊙. Based on its low luminosity and mass, we propose that the lens in Gaia19dke event is an isolated white dwarf.

The Astrophysical Journal, 1996

We study different models of dark matter distribution for the halo of our galaxy. In particular, we consider Eddington and King-Michie models, which include anisotropy in the velocity space, and compute in a self-consistent way the amount of dark matter present in the halo. Assuming that the dark matter is in form of Massive Astrophysical Compact Halo Objects (MACHOs), we find for each model the expected number of microlensing events and their average time duration for an experiment monitoring stars in the Large Magellanic Cloud (LMC). The main effect of including anisotropy is to reduce the microlensing rate by about 30% and to increase, but only slightly, the mean event duration, as compared to the standard halo model. Consideration of different luminous models for the visible part of the galaxy also induce variations in the microlensing results by roughly the same amount as mentioned above. The main uncertainty, in order to be able to discriminate between different dark matter distributions and to estimate the fraction of it in form of MACHOs, is due to the poor knowledge of the rotation velocity at large galactocentric distances.

The Astrophysical Journal, 2015

We present results from a comprehensive lensing analysis in Hubble Space Telescope (HST) data, of the complete Cluster Lensing And Supernova survey with Hubble (CLASH) cluster sample. In most galaxy clusters we identify new multiple-images previously undiscovered, allowing improved or, in some cases, first constraints on the cluster inner mass distributions and profiles. We combine these strong-lensing constraints with weak lensing shape measurements within the HST field-of-view (FOV), to jointly constrain the mass distributions. In nearly all clusters the analysis is performed in two different common parameterizations (one adopts light-traces-mass for both galaxies and dark matter while the other adopts an analytical, elliptical NFW form for the dark matter), to provide a better assessment of the underlying systematics. The characterization of systematics between different lens modeling techniques is most important in the current era of precision cosmology and deep cluster and lensing surveys such as CLASH and the Hubble Frontier Fields, especially when studying high-redshift magnified objects. We find that the typical (median), relative systematic differences throughout the central [4.6 × 4.6 ] analysis FOV, are ∼ 40% in the (dimensionless) mass density, κ, and ∼ 20% in the magnification, µ. We show maps of these differences for each cluster, as well as the mass distributions, critical curves, and 2D integrated mass profiles. For the Einstein radii (z s = 2) we find that all typically agree within 10% between the two models, and Einstein masses agree, typically, within ∼ 15%. At larger radii, the total projected, 2D integrated mass profiles of the two models, within r ∼ 2 , differ by ∼ 30%. Stacking the surface-density profiles of the sample from the two methods together, we obtain an average slope of d log(Σ)/d log(r) ∼ −0.64 ± 0.1, in the radial range [5,350] kpc. Lastly, we examine the behavior of the average magnification, surface density, and shear differences between the two models, as a function of both the radius (normalized to the Einstein radius of each cluster) and the best-fit values of these quantities, uncovering some interesting trends. All mass models and magnification maps are made publicly available for the community.

Mon Notic Roy Astron Soc, 2004

Although point caustics harbour a larger potential for measuring the brightness profile of stars during the course of a microlensing event than (line-shaped) fold caustics, the effect of lens binarity significantly limits the achievable accuracy. Therefore, corresponding close-impact events make a less favourable case for limb-darkening measurements than those events that involve fold-caustic passages, from which precision measurements can easily and routinely be obtained. Examples involving later Bulge giants indicate that a ~ 10 % misestimate on the limb-darkening coefficient can result with the assumption of a single-lens model that looks acceptable, unless the precision of the photometric measurements is pushed below the 1 %-level even for these favourable targets. In contrast, measurement uncertainties on the proper motion between lens and source are dominated by the assessment of the angular radius of the source star and remain practically unaffected by lens binarity. Rather than judging the goodness-of-fit by means of a chi^2 test only, run tests provide useful additional information that can lead to the rejection of models and the detection of lens binarity in close-impact microlensing events.

We calculate the optical depth and the number of events due to gravitational microlensing towards the Galactic bulge, the spiral arm directions γ Scutum, β Scutum, γ Normae, ϑ Muscae and some dwarf galaxies in the halo of the Galaxy.

The Astrophysical Journal, 2001

We present photometry and analysis of the microlensing alert MACHO 96-LMC-2 (event LMC-14 in ). This event was initially detected by the MACHO Alert System, and subsequently monitored by the Global Microlensing Alert Network (GMAN). The ∼ 3% photometry provided by the GMAN follow-up effort reveals a periodic modulation in the lightcurve. We attribute this to binarity of the lensed source. Microlensing fits to a rotating binary source magnified by a single lens converge on two minima, separated by ∆χ 2 ∼ 1. The most significant fit X1 predicts a primary which contributes ∼ 100% of the light, a dark secondary, and an orbital period (T ) of ∼ 9.2 days. The second fit X2 yields a binary source with two stars of roughly equal mass and luminosity, and T = 21.2 days.

The Astrophysical Journal, 2012

Astronomy and Astrophysics, 1998

The influence of rotating binary systems on the light curves of galactic microlensing events is studied. Three different rotating binary systems are discussed: a rotating binary lens, a rotating binary source, and the earth's motion around the sun (parallax effect). The most dramatic effects arise from the motion of a binary lens because of the changes of the caustic structure with time. I discuss when the treatment of a microlensing event with a static binary model is appropriate. It is shown that additional constraints on the unknown physical quantities of the lens system arise from a fit with a rotating binary lens as well as from the earth-around-sun motion. For the DUO#2 event, a fit with a rotating binary lens is presented.

The Astrophysical Journal, 2013

We present Hubble Space Telescope (HST) imaging data and CFHT Near IR ground-based images for the final sample of 56 candidate galaxy-scale lenses uncovered in the CFHT Legacy Survey as part of the Strong Lensing in the Legacy Survey (SL2S) project. The new images are used to perform lens modeling, measure surface photometry, and estimate stellar masses of the deflector early-type galaxies. Lens modeling is performed on the HST images (or CFHT when HST is not available) by fitting the spatially extended light distribution of the lensed features assuming a singular isothermal ellipsoid mass profile and by reconstructing the intrinsic source light distribution on a pixelized grid. Based on the analysis of systematic uncertainties and comparison with inference based on different methods we estimate that our Einstein Radii are accurate to ∼ 3%. HST imaging provides a much higher success rate in confirming gravitational lenses and measuring their Einstein radii than CFHT imaging does. Lens modeling with ground-based images however, when successful, yields Einstein radius measurements that are competitive with spaced-based images. Information from the lens models is used together with spectroscopic information from the companion paper IV to classify the systems, resulting in a final sample of 39 confirmed (grade-A) lenses and 17 promising candidates (grade-B,C). This represents an increase of half an order of magnitude in sample size with respect to the sample of confirmed lenses studied in papers I and II. The Einstein radii of the confirmed lenses in our sample span the range 5 − 15 kpc and are typically larger than those of other surveys, probing the mass in regions where the dark matter contribution is more important. Stellar masses are in the range 10 11 − 10 12 M , covering the range of massive ETGs. The redshifts of the main deflector span a range 0.3 ≤ zd ≤ 0.8, which nicely complements low-redshift samples like the SLACS and thus provides an excellent sample for the study of the cosmic evolution of the mass distribution of early-type galaxies over the second half of the history of the Universe.

The Astrophysical …, 2000