18 research outputs found
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.Comment: accepted for publication in ApJ
Microlensing optical depth and event rate in the OGLE-IV Galactic plane fields
Searches for gravitational microlensing events are traditionally concentrated
on the central regions of the Galactic bulge but many microlensing events are
expected to occur in the Galactic plane, far from the Galactic Center. Owing to
the difficulty in conducting high-cadence observations of the Galactic plane
over its vast area, which are necessary for the detection of microlensing
events, their global properties were hitherto unknown. Here, we present results
of the first comprehensive search for microlensing events in the Galactic
plane. We searched an area of almost 3000 square degrees along the Galactic
plane (|b|<7, 0<l<50, 190<l<360 deg) observed by the Optical Gravitational
Lensing Experiment (OGLE) during 2013-2019 and detected 630 events. We
demonstrate that the mean Einstein timescales of Galactic plane microlensing
events are on average three times longer than those of Galactic bulge events,
with little dependence on the Galactic longitude. We also measure the
microlensing optical depth and event rate as a function of Galactic longitude
and demonstrate that they exponentially decrease with the angular distance from
the Galactic Center (with the characteristic angular scale length of 32 deg).
The average optical depth decreases from at l=10 deg to
in the Galactic anticenter. We also find that the optical
depth in the longitude range 240<l<330 deg is asymmetric about the Galactic
equator, which we interpret as a signature of the Galactic warp.Comment: ApJS, in pres
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 nearby disk source at ~2.5 kpc
rather than the traditional bulge microlensing fields. Our primary analysis
yields a Jovian planet with M_p = 0.50 +/- 0.05 M_J at a projected orbital
separation r_perp = 1.63 +/- 0.17 AU. The host is a turnoff star with mass 1.24
+/- 0.06 M_sun and distance of 1.35 +/- 0.09 kpc, and at r'~14, it 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
OGLE-2018-BLG-0532Lb: Cold Neptune With Possible Jovian Sibling
We report the discovery of the planet OGLE-2018-BLG-0532Lb, with very obvious
signatures in the light curve that lead to an estimate of the planet-host mass
ratio . Although there are
no obvious systematic residuals to this double-lens/single-source (2L1S) fit,
we find that can be significantly improved by adding either a third
lens (3L1S, ) or second source (2L2S, ) to
the lens-source geometry. After thorough investigation, we conclude that we
cannot decisively distinguish between these two scenarios and therefore focus
on the robustly-detected planet. However, given the possible presence of a
second planet, we investigate to what degree and with what probability such
additional planets may affect seemingly single-planet light curves. Our best
estimates for the properties of the lens star and the secure planet are: a host
mass , system distance kpc and planet mass
with projected separation au.
However, there is a relatively bright (and also relatively blue) star
projected within mas of the lens, and if future high-resolution images
show that this is coincident with the lens, then it is possible that it is the
lens, in which case, the lens would be both more massive and more distant than
the best-estimated values above.Comment: 48 pages, 9 figures, 7 table
A Gas Giant Planet in the OGLE-2006-BLG-284L Stellar Binary System
We present the analysis of microlensing event OGLE-2006-BLG-284, which has a
lens system that consists of two stars and a gas giant planet with a mass ratio
of to the primary. The mass ratio of the
two stars is , and their projected separation is AU, while the projected separation of the planet from the primary
is AU. For this lens system to have stable orbits, the
three-dimensional separation of either the primary and secondary stars or the
planet and primary star must be much larger than that these projected
separations. Since we do not know which is the case, the system could include
either a circumbinary or a circumstellar planet. Because there is no
measurement of the microlensing parallax effect or lens system brightness, we
can only make a rough Bayesian estimate of the lens system masses and
brightness. We find host star and planet masses of , , and
, and the -band magnitude of the combined
brightness of the host stars is . The separation
between the lens and source system will be mas in mid-2020, so it
should be possible to detect the host system with follow-up adaptive optics or
Hubble Space Telescope observations
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.Comment: 45 pages, 11 figure