Abstract
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
M
⊙
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
M
⊙
). 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
M
⊙
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.
ABSTRACT
NGC 2004 #115 is a recently identified black hole (BH) candidate in the Large Magellanic Cloud (LMC) containing a B star orbiting an unseen companion in a 2.9 d orbit and a Be star tertiary. ...We show that the unseen companion is not a 25 M⊙ BH, but a $(2\!-\!3)\, {\rm M}_{\odot }$ luminous star. Analysing the Optical Gravitational Lensing Experiment (OGLE) and MAssive Compact Halo Object (MACHO) light curves of the system, we detect ellipsoidal variability with amplitude 10 times larger than would be expected if the companion were a 25 M⊙ BH, ruling out the low inclination required for a massive companion. The light curve also shows a clear reflection effect that is well modelled with a $2.5\, {\rm M}_{\odot }$ main-sequence secondary, ruling out a lower mass BH or neutron star companion. We consider and reject models in which the system is a binary containing a stripped star orbiting the Be star: only a triple model with an outer Be star can explain both the observed light curve and radial velocities. Our results imply that the B star, whose slow projected rotation velocity and presumed tidal synchronization were interpreted as evidence for a low inclination (and thus a high companion mass), is far from being tidally synchronized: despite being in a 2.9 d orbit that is fully or nearly circularized (e < 0.04), its surface rotation period appears to be at least 20 d. We offer cautionary notes on the interpretation of dormant BH candidates in binaries.
Abstract
Recent detections of extremely short-timescale microlensing events imply the existence of a large population of Earth- to Neptune-mass planets that appear to have no host stars. However, it ...is currently unknown whether these objects are truly free-floating planets or whether they are in wide orbits around a distant host star. Here, we present an analysis of high-resolution imaging observations of five free-floating planet candidates collected with the Keck telescope. If these candidates were actually wide-orbit planets, then the light of the host would appear at a separation of 40–60 mas from the microlensing source star. No such stars are detected. We carry out injection and recovery simulations to estimate the sensitivity to putative host stars at different separations. Depending on the object, the presented observations rule out 11%–36% of potential hosts assuming that the probability of hosting a planet does not depend on the host mass. The results are sensitive to the latter assumption, and the probability of detecting the host star in the analyzed images may be a factor of 1.9 ± 0.1 larger, if the exoplanet hosting probability scales as the first power of the host star mass, as suggested by recent studies of planetary microlensing events. We argue that deeper observations, for example with JWST, are needed to confidently confirm or refute the free-floating planet hypothesis.
Flat rotation curves of spiral galaxies are considered as an evidence for dark matter, but the rotation curve of the Milky Way is difficult to measure. Various objects were used to track the rotation ...curve in the outer parts of the Galaxy, but most studies rely on incomplete kinematical information and inaccurate distances. Here, we use a sample of 773 Classical Cepheids with precise distances based on mid-infrared period-luminosity relations coupled with proper motions and radial velocities from Gaia to construct the accurate rotation curve of the Milky Way up to the distance of ∼20 kpc from the Galactic center. We use a simple model of Galactic rotation to measure the rotation speed of the Sun Θ0 = 233.6 2.8 km s−1, assuming a prior on the distance to the Galactic center R0 = 8.122 0.031 kpc from the GRAVITY Collaboration. The rotation curve at Galactocentric distances 4 R 20 kpc is nearly flat with a small gradient of −1.34 0.21 km s−1 kpc−1. This is the most accurate Galactic rotation curve at distances R > 12 kpc constructed so far.
The number and properties of observed gravitational microlensing events depend on the distribution and kinematics of stars and other compact objects along the line of sight. In particular, precise ...measurements of the microlensing optical depth and event rate toward the Galactic bulge enable strict tests of competing models of the Milky Way. Previous estimates, based on samples of up to a few hundred events, gave larger values than expected from the Galactic models and were difficult to reconcile with other constraints on the Galactic structure. Here we used long-term photometric observations of the Galactic bulge by the Optical Gravitational Lensing Experiment (OGLE) to select a homogeneous sample of 8000 gravitational microlensing events. We created the largest and most accurate microlensing optical depth and event rate maps of the Galactic bulge. The new maps ease the tension between the previous measurements and Galactic models. They are consistent with some earlier calculations based on bright stars and are systematically ∼30% smaller than the other estimates based on "all-source" samples of microlensing events. The difference is caused by the careful estimation of the source star population. The new maps agree well with predictions based on the Besançon model of the Galaxy. Apart from testing the Milky Way models, our maps may have numerous other applications, such as the measurement of the initial mass function or constraining the dark matter content in the Milky Way center. The new maps will also inform the planning of future space-based microlensing experiments by revising the expected number of events.
We present the analysis of five black hole candidates identified from gravitational microlensing surveys. Hubble Space Telescope astrometric data and densely sampled light curves 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⨀. 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 kms-1. OB110462 shows significant tension between models well fit to photometry versus 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 <2M⨀, 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 (∼108 ) and find reasonable agreement given the small sample size.
Abstract We use the spectroscopic data collected by the Magellanic Quasars Survey (MQS) and the photometric V - and I -band data from the Optical Gravitational Lensing Experiment (OGLE) to measure ...the physical parameters for active galactic nuclei (AGNs) located behind the Magellanic Clouds. The flux-uncalibrated MQS spectra were obtained with the 4 m Anglo-Australian Telescope and the AAOmega spectroscope ( R = 1300) in a typical ∼1.5 hr visit. They span a spectral range of 3700–8500 Å and have signal-to-noise ratios in a range of 3–300. We report the discovery and observational properties of 161 AGNs in this footprint, which expands the total number of spectroscopically confirmed AGNs by MQS to 919. After the conversion of the OGLE mean magnitudes to the monochromatic luminosities at 5100, 3000, and 1350 Å, we were able to reliably measure the black hole masses for 165 out of 919 AGNs. The remaining physical parameters we provide are the bolometric luminosities and the Eddington ratios. A fraction of these AGNs have been observed by the OGLE survey since 1997 (all of them since 2001), enabling studies of correlations between the variability and physical parameters of these AGNs.
Abstract
Gravitational lensing is an important prediction of general relativity, providing both its test and a tool to detect faint but amplified sources and to measure masses of lenses. For some ...applications, (e.g., testing the theory), a point source lensed by a point-like lens would be more advantageous. However, until now only one gravitationally lensed star has been resolved. Future telescopes will resolve very small lensing signatures for stars orbiting the supermassive black hole (SMBH) in the center of the Milky Way. The lensing signatures, however, should be easier to detect for background stars. We predict that the Extremely Large Telescope (ELT), Thirty Meter Telescope (TMT), and Giant Magellan Telescope (GMT) will resolve the lensed images of around 100 (60) stars in the disk and 30 (20) stars in the bulge in the background of the SMBH, down to 28 (27) mag (Vega) limits at
K
-band, requiring 5 (1) hr of integration. In order to detect several such stars one needs the limit of at least 24 mag. With decade-long monitoring, one can also detect the rotation of the lensed images. The detection of elongated images will not be possible, because this would require a nearly perfect source-lens alignment. The James Webb Space Telescope (JWST) will likely be limited by the confusion caused by stars near the Galactic center. The detection of such lensed images will provide a very clean test of general relativity, when combined with the SMBH mass measurement from orbital motions of stars, and accurate measurements of the SMBH properties, because both the source and the lens can be considered point-like.
ABSTRACT
Distances and transverse along the Galactic longitude velocities vl of 2640 Gaia EDR3 O–B2 stars of Xu et al. within 2.5 kpc from the Sun and 250 pc from the Galactic plane with relative ...distance and velocity accuracies of <10 and <50 per cent are selected. Under the assumption of spiral density waves, both Galactic differential rotation parameters and parameters of the wave structure in this solar neighbourhood are derived from observed vl. In contrast to all preceding studies, we take into account the effect of small but finite thickness of the disc of the Galaxy on even parity (‘sausage’) gravity perturbations of the kind investigated by Lin and Shu. As previously predicted by the modified theory of 3D density waves, two scales of periodic rarefaction–compression irregularity of the vl velocity field with the radial λ0 ≈ 1.5 kpc and vertical ξ0 ≈ 1.0 kpc wavelengths in the form of a spiral wave propagating in the disc are revealed. The Gaia’s DR2 line-of-sight and EDR3 longitudinal velocities analyses performed in the last papers of a series exhibit consistent findings, thus providing a further evidence to support the Lin–Shu density-wave proposal.
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