OGLE-2018-BLG-0567Lb and OGLE-2018-BLG-0962Lb: Two Microlensing Planets
through the Planetary-caustic Channel
Youn Kil Jung
1,2,15
, Cheongho Han
3,15
, Andrzej Udalski
4,16
, Andrew Gould
1,5,6,15
, Jennifer C. Yee
7,15
and
Michael D. Albrow
8
, Sun-Ju Chung
1,2
, Kyu-Ha Hwang
1
, Yoon-Hyun Ryu
1
, In-Gu Shin
1
, Yossi Shvartzvald
9
,
Wei Zhu
10
, Weicheng Zang
11
, Sang-Mok Cha
1,12
, Dong-Jin Kim
1
, Hyoun-Woo Kim
1
, Seung-Lee Kim
1,2
,
Chung-Uk Lee
1,2
, Dong-Joo Lee
1
, Yongseok Lee
1,12
, Byeong-Gon Park
1,2
, Richard W. Pogge
5
(The KMTNet Collaboration),
and
Przemek Mróz
4,13
, Michał K. Szymański
4
, Jan Skowron
4
, Radek Poleski
4,5
, Igor Soszyński
4
, Paweł Pietrukowicz
4
,
Szymon Kozłowski
4
, Krzystof Ulaczyk
14
, Krzysztof A. Rybicki
4
, Patryk Iwanek
4
, and Marcin Wrona
4
(The OGLE Collaboration)
1
Korea Astronomy and Space Science Institute, Daejon 34055, Republic of Korea
2
University of Science and Technology, Korea, 217 Gajeong-ro Yuseong-gu, Daejeon 34113, Korea
3
Department of Physics, Chungbuk National University, Cheongju 28644, Republic of Korea
4
Warsaw University Observatory, Al. Ujazdowskie 4, 00-478 Warszawa, Poland
5
Department of Astronomy, Ohio State University, 140 W. 18th Ave., Columbus, OH 43210, USA
6
Max-Planck-Institute for Astronomy, Königstuhl 17, D-69117 Heidelberg, Germany
7
Center for Astrophysics|Harvard & Smithsonian, 60 Garden St., Cambridge, MA 02138, USA
8
University of Canterbury, Department of Physics and Astronomy, Private Bag 4800, Christchurch 8020, New Zealand
9
Department of Particle Physics and Astrophysics, Weizmann Institute of Science, Rehovot 76100, Israel
10
Canadian Institute for Theoretical Astrophysics, University of Toronto, 60 St. George St., Toronto, ON M5S 3H8, Canada
11
Department of Astronomy, Tsinghua University, Beijing 100084, People’s Republic of China
12
School of Space Research, Kyung Hee University, Yongin 17104, Republic of Korea
13
Division of Physics, Mathematics, and Astronomy, California Institute of Technology, Pasadena, CA 91125, USA
14
Department of Physics, University of Warwick, Gibbet Hill Road, Coventry, CV4 7AL, UK
Received 2021 January 31; revised 2021 March 23; accepted 2021 April 14; published 2021 June 3
Abstract
We present the analyses of two microlensing events, OGLE-2018-BLG-0567 and OGLE-2018-BLG-0962. In both
events, the short-lasting anomalies were densely and continuously covered by two high-cadence surveys. The light-
curve modeling indicates that the anomalies are generated by source crossings over the planetary caustics induced by
planetary companions to the hosts. The estimated planet/host separation (scaled to the angular Einstein radius θ
E
) and
mass ratio are (s, q × 10
3
) = (1.81 ± 0.02, 1.24 ± 0.07) and (s, q × 10
3
) = (1.25 ± 0.03, 2.38 ± 0.08), respectively.
From Bayesian analyses, we estimate the host and planet masses as
=
-
+
-
+
MM M M, 0.25 , 0.32
hp
0.13
0.27
0.17
0.34
J
(
)( )
and
=
-
+
-
+
MM M M, 0.54 , 1.34
hp
0.28
0.33
0.70
0.82
J
(
)( )
, respectively. These planetary systems are located at a distance of
-
+
7
.06 kpc
1.15
0.93
for OGLE-2018-BLG-0567 and
-
+
6.50 kpc
1.75
1.06
for OGLE-2018-BLG-0962, suggesting that they are likely
to be near the Galactic bulge. The two events prove the capability of current high-cadence surveys for finding planets
through the planetary-caustic channel. We find that most published planetary-caustic planets are found in Hollywood
events in which the source size strongly contributes to the anomaly cross-section relative to the size of the caustic.
Unified Astronomy Thesaurus concepts: Gravitational microlensing (672); Gravitational microlensing exoplanet
detection (2147)
Supporting material: data behind figures
1. Introduction
The signature of a microlensing planet is almost always a
short-lasting anomaly in the smooth and symmetric lensing
light curve produced by the host of the planet. In principle, the
signature can appear at any position of the lensing light curve
(Gaudi 2012). In reality, however, the signatures of planets
detected in the earlier phase of lensing experiments appeared
mainly near the peak of lensing light curves.
The bias toward central anomalies is mostly attributed to the
limitation of early lensing surveys. With a roughly 1-day
cadence of the first-generation survey experiments, e.g., the
Massive Compact Halo Objects (MACHOs; Alcock et al. 1995),
the first phase of Optical Gravitational Lensing Experiment
(OGLE-I; Udalski et al. 1992),andthefirst phase of
Microlensing Observations in Astrophysics (MOA-I; Bond
et al. 2001) surveys, it was difficult to detect planetary signals
lasting of an order of 1 day or less by the survey experiments. To
meet the cadence requirement for planet detections, Gould &
Loeb (1992) proposed an observational mode, in which wide-
field surveys with a low cadence monitor a large area of the sky
mainly to detect lensing events, and follow-up experiments
conduct high-cadence observations for a small number of
lensing events detected by the surveys using a network of
The Astronomical Journal, 161:293 (12pp), 2021 June https://doi.org/10.3847/1538-3881/abf8bd
© 2021. The American Astronomical Society. All rights reserved.
15
The KMTNet Collaboration.
16
The OGLE Collaboration.
1
multiple narrow-field telescopes. However, this mode of
observations had the drawback that only a handful of lensing
events could be monitored by follow-up observations. Combined
with the low probability of planetary perturbations, this implied a
low planet detection rate for this phase of the experiments. In
fact, for the first several years, there were no securely detected
planets using this mode, although there was one tentative
detection in the event MACHO 98-BLG-35 (Rhie et al. 2000).
The first three microlensing detections were found using the
survey+follow-up strategy. In the first event, OGLE 2003-
BLG-235/MOA 2003-BLG-53 (Bond et al. 2004), the planet
was found by the surveys, but the MOA survey carried out
additional follow-up observations in response to the planetary
anomaly. The next two planets, OGLE-2005-BLG-071Lb
(Udalski et al. 2005) and OGLE-2005-BLG-390Lb (Beaulieu
et al. 2006), were both found through extensive follow-up
observations of known microlensing events that were initiated
before the planetary anomaly began. The discovery of OGLE-
2005-BLG-071Lb provided a practical lesson in the value of
high-magnification events (Griest & Safizadeh 1998) for
detecting planets through follow-up observations.
In the following years, the planet detection rate using the
survey+follow-up mode was substantially increased by focus-
ing on events with very high magnifications. Several factors
contributed to the increase of the detection rate. First, the planet
detection efficiency for high-magnification events is high. This
is because a planet located in the lensing zone of its host always
induces a small central caustic near the position of the host, and
during a high-magnification event, the source passes close to
the central caustic. This yields a high probability that a planet
will produce a perturbation and also confines that perturbation
to a short duration of time while the event is highly magnified,
not throughout the whole event. As a result, the time of the
planetary signal, i.e., the peak of the light curve, can be
predicted in advance and enable one to efficiently use resources
for follow-up observations. By contrast, predicting the time of a
planetary signal through other channels is difficult. Finally,
highly magnified source stars are bright enough to be observed
with small-aperture telescopes, down to submeter amateur-class
telescopes, and this enables one to maximize available
telescopes for follow-up observations, e.g., OGLE-2005-
BLG-071 (Udalski et al.
2005; Dong et al. 2009). Thus, the
planets detected from the survey+follow-up experiments were
detected mainly through the high-magni fication channel, and
this led to the bias toward central-caustic perturbations.
The current planetary lensing experiments are in the second
phase, in which lensing events are observed by high-cadence
surveys. The observational cadence of the lensing surveys in
this phase has greatly increased with the employment of large-
format cameras yielding very wide fields of view. The MOA
experiment entered a new phase (MOA-II) by upgrading its
instrument with a new wide-field camera composed of ten
2k × 4k chips yielding a 2.2 deg
2
field of view (Sumi et al.
2013). The OGLE survey is in its fourth phase (OGLE-IV)
using a 1.4 deg
2
camera composed of 32 2k × 4k chips
(Udalski et al. 2015). The Korea Microlensing Telescope
Network (KMTNet) survey, which commenced its full
operation in 2016 (Kim et al. 2016), utilizes three globally
distributed telescopes, each of which has a camera with a
4.0 deg
2
field of view. Being able to cover a large area of sky
from a single exposure, the observational cadence of the
current survey experiments now reaches Γ ∼ 4hr
−1
toward the
dense bulge fields. This enables planet detections without
additional follow-up observations.
With the operation of the high-cadence surveys, the
detection rate of planets is rapidly increasing. One important
reason for the rapid increase of the detection rate is that planets
can be detected not only through the central-caustic channel but
also through the additional planetary-caustic channel. Planets
are detected through the planetary-caustic channel as anomalies
produced by the source’s approach close to the planetary
caustic, which denotes one of the two sets of planet-induced
caustics lying away from the host. The planetary caustic lies at
a position with a separation from the host of s − 1/s, and thus
planetary signals produced by this caustic can appear at any
part of the lensing light curve depending on the planet–host
separation s (normalized to the angular Einstein radius θ
E
). The
planetary caustic is substantially larger than the central caustic,
and thus the probability of a planetary perturbation is higher.
Another importance of detecting planets through the planetary-
caustic channel is that interpreting the planetary signal is
usually not subject to the close–wide degeneracy (Griest &
Safizadeh 1998; Dominik 1999), which causes ambiguity in
estimating the planet–host separations for most planets detected
through the central-caustic channel.
In this paper, we present the analysis of two planetary
microlensing events, OGLE-2018-BLG-0567 and OGLE-
2018-BLG-0962, for which planets are both detected through
a planetary-caustic channel. For both events, the signatures of
the planets were densely and continuously covered by two
high-cadence lensing surveys, and this leads us to unambigu-
ously interpret the planetary signals.
2. Observation
The two planetary events were observed by the two lensing
surveys conducted by the OGLE and KMTNet groups. The
OGLE survey uses the 1.3 m telescope that is located at the Las
Campanas Observatory in Chile. The KMTNet survey utilizes
three 1.6 m telescopes that are located at the Siding Spring
Observatory in Australia (KMTA), the Cerro Tololo Inter-
american Observatory in Chile (KMTC), and the South African
Astronomical Observatory in South Africa (KMTS). The global
distribution of the KMTNet telescopes makes it possible to
continuously monitor the events. In both surveys, observations
were mainly conducted in the I band, and a fraction of the
images were taken in the V band to determine the color of the
microlensed source stars.
OGLE-2018-BLG-0567,
=
R.A ., decl.
J2000
(
)
(17:56:04.42,
−27:59:13.6),or(l, b) = (1°.99, − 1°.49) in Galactic coordi-
nates, was discovered on 2018 April 14 by the OGLE Early
Warning System (EWS; Udalski 2003). The event was
independently found by the KMTNet survey as KMT-2018-
BLG-0890 from its event-finding algorithm (Kim et al. 2018).
The observational cadence for the event is Γ = 1hr
−1
for
OGLE and Γ = 2hr
−1
for KMTNet.
OGLE-2018-BLG-0962 was discovered by the OGLE EWS
on 2018 June 2. It is located at
=R.A ., decl.
J2000
()
(17:52:41.95,
−32:18:33.3) or Galactic coordinates of (l, b) = ( − 2°.11,
− 3°.04). The OGLE cadence for this direction is 3–10 times
per night. The KMTNet collaboration also observed the event,
with designation KMT-2018-BLG-2071, located in their two
overlapping fields (BLG41 and BLG22). In combination, the
KMTNet observations have a frequency of Γ = 3hr
−1
for
KMTC and Γ = 2.25 hr
−1
for KMTS and KMTA.
2
The Astronomical Journal, 161:293 (12pp), 2021 June Jung et al.
For both events, the data sets were reduced based on the
image-subtraction methodology (Tomaney & Crotts 1996;
Alard & Lupton 1998), specifically Albrow et al. (2009) for
KMTNet and Woźniak (2000) for OGLE. The photometric
error bars were then readjusted following the prescription
presented in Yee et al. (2012). We note that for the source
color measurement, we additionally carried out pyDIA (Albrow
2017) reductions for a subset of the KMTNet data, which
simultaneously returns the light curve and field-star photometry
on the same system.
3. Light Curve Analysis
Figures 1 and 2 show the light curves of OGLE-2018-BLG-
0567 and OGLE-2018-BLG-0962, respectively. It is found that
the two events share various characteristics in common. First,
the apparent peak magnifications of the baseline single-lens
single-source (1L1S) light curves are not high: A
peak
∼ 1.6 for
OGLE-2018-BLG-0567 and A
peak
∼ 4.9 for OGLE-2018-BLG-
0962. Second, the light curves of both events exhibit strong
short-term positive anomalies from the baseline 1L1S curves.
Third, the anomalies appear when the 1L1S-model lensing
magnifications are low. All these characteristics strongly
suggest that the anomalies are produced by source crossings
over the planetary caustics induced by planetary companions to
the lenses. We, therefore, start with a binary-lens single-source
(2L1S) modeling of the events under the interpretation that the
events were produced by lenses composed of two masses, M
1
and M
2
.
The standard 2L1S modeling requires one to include seven
fitting parameters to describe an observed light curve. The first
three are the Paczyński (1986) parameters (t
0
, u
0
, t
E
), which are
respectively the time of closest source approach to the lens, the
impact parameter (scaled to θ
E
), and the event timescale. The
next three (s, q, α) describe the binary-lens geometry: the
projected binary separation (scaled to θ
E
), the binary mass ratio
(q = M
2
/M
1
), and the angle between the source trajectory and
the binary axis as measured in a clockwise sense, respectively.
The last describes the source radius ρ = θ
*
/θ
E
, where θ
*
is the
angular source radius. See Figure 6 of Jung et al. (2015) for a
graphical presentation of the parameters.
Figure 1. Light curve of OGLE-2018-BLG-0567. The black solid curve on the data is the best-fit 2L1S solution. The upper panel shows the enlarged view of the
planet-induced anomaly centered on
¢~
H
JD 8270
. The second and fourth panels show the residuals from the solution. The lensing parameters of the solution are
listed in Table 1 and the caustic geometry is shown in Figure 3. Note that we use the V-band data only for the source color measurement.
(The data used to create this figure are available.)
3
The Astronomical Journal, 161:293 (12pp), 2021 June Jung et al.
The modeling is conducted following the procedure
described in Jung et al. (2015). In the first step, we carry out
grid searches for the binary parameters (s, q, α), with
100 × 100 × 21 different grid points. The ranges of the grid
parameters are
-
s1 log 1
,
-
q5 log 0
, and 0
α 2π. At each grid, we fix (s, q) and find the remaining
parameters using Markov Chain Monte Carlo (MCMC) χ
2
minimization. In this modeling, the initial values of the
parameters (t
0
, u
0
, t
E
) are given as the values estimated from
a 1L1S fit for the data excluding the anomaly. The initial value
of the normalized source radius is estimated from the caustic-
crossing timescale, which is related to the event timescale by
t
*
= ρt
E
. Here we use inverse ray shooting (Kayser et al. 1986;
Schneider & Weiss 1987) to compute the finite-source lensing
magnifications. The flux values from the source, f
S,i
, and blend,
f
B,i
, for the data set obtained from the ith observatory are
estimated by f
i
(t) = f
S,i
A(t) + f
B,i
, where f
i
is the observed flux.
Once local solutions are found from the first-round modeling,
we refine the individual locals by releasing all fitting
parameters and allowing them to be free parameters in
an MCMC.
From the modeling, it is found that the observed lensing light
curves of both events are well described by unique 2L1S
models, in which the mass ratios between M
1
and M
2
are in the
planetary regime. In addition, the planetary perturbations in
both light curves are not subject to the close–wide degeneracy.
The estimated binary parameters are (s, q) = (1.81,
1.24 × 10
−3
) for OGLE-2018-BLG-0567 and (s, q) = (1.25,
2.38 × 10
−3
) for OGLE-2018-BLG-0962. The full lensing
parameters and their uncertainties are presented in Table 1. The
model curves of the solutions are drawn over the data points in
Figures 1 and 2 for OGLE-2018-BLG-0567 and OGLE-2018-
BLG-BLG-0962, respectively.
In Figures 3 and 4, we present the lens-system configurations
of the individual events, showing the source trajectory with
respect to the lens components and resulting caustics. From the
configurations, it is found that the anomalies of both events are
produced by the source crossing over the planetary caustic of the
lens system. For OGLE-2018-BLG-0567, the source size is
comparable to the caustic size, and thus the detailed caustic-
crossing features, two caustic spikes and a U-shape trough
region between the spikes, were smeared out by finite-source
Figure 2. Light curve of OGLE-2018-BLG-0962. The upper panels show the close-up views of the regions around
¢~
H
JD 8271.5
(left) and
¢~
H
JD 8273.8
(right)
when the planet-induced perturbations occur. The lensing parameters of the 2L1S solution are listed in Table 1 and the caustic geometry is shown in Figure 4.
(The data used to create this figure are available.)
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The Astronomical Journal, 161:293 (12pp), 2021 June Jung et al.
effects. For OGLE-2018-BLG-0962, on the other hand, the
caustic is much bigger than the source size, and thus the detailed
caustic-crossing feature of the anomaly is well delineated. It is
found that the first part of the anomaly, centered at
¢= - ~
H
JD HJD 2,450,000 days 8271.5()
, was produced by
the source passing over the two caustic segments that flank the
inner cusp (on the binary axis) of the planetary caustic, and
the second part, centered at
¢~
H
JD 8273.8
, was generated by
the source passage over the adjacent (off-axis) cusp.
We investigate the possibility of other interpretations of the
events. Especially for OGLE-2018-BLG-0567, the short-term
perturbation might, in principle, be produced by a second
source (1L2S) with a large flux ratio (Gaudi 1998). Hence, we
conduct an additional modeling of the event with the 1L2S
interpretation (Jung et al. 2017). We search for the solution
with eight fitting parameters: 2 × (t
0
, u
0
, ρ) for the two sources,
I-band flux ratio q
F,I
, and a shared timescale t
E
. The results are
listed in Table 2.Wefind that the 1L2S solution is disfavored
by Δχ
2
> 500. In addition, the 1L2S model not only provides a
worse fit to the peak of the perturbation, but also fails to
recover the decrease in magnitude seen before and after the
peak. See Figure 5.
We check the feasibility of constraining the microlens
parallax, π
E
, by conducting an additional modeling of the light
curve. In this modeling, we simultaneously consider the
microlens-parallax and lens-orbital effects (Gould 1992;
Dominik 1998), because the light curve deviations induced
by the parallax effect can be correlated with the deviations
induced by the lens-orbital motion (Batista et al. 2011). For
OGLE-2018-BLG-0567, we find that it is difficult to accurately
determine π
E
not only because the improvement of the fit,
Δχ
2
∼ 9, is very minor, but also because the parallax vector
Figure 3. Caustic geometry of OGLE-2018-BLG-0567. The line with an arrow is the source trajectory relative to the binary-lens axis. The open circles (scaled by the
normalized source radius ρ) on the trajectory are the source positions at the times of observations. The two orange circles are the positions of binary-lens masses (M
1
and M
2
). In each panel, the cuspy closed curve drawn in black represents the caustic. The upper panel shows the enlarged view of the planetary caustic. Lengths are
scaled to the angular Einstein radius of the lens system.
Table 1
Lensing Parameters
Parameters OGLE-2018-BLG-0567 OGLE-2018-BLG-0962
c
tot
2
/dof 8677.3/9252 6892.1/6833
t
0
(
¢H
JD
) 8244.845 ± 0.025 8262.494 ± 0.053
u
0
0.733 ± 0.026 0.207 ± 0.032
t
E
(days) 24.641 ± 1.064 28.739 ± 0.298
s 1.806 ± 0.019 1.246 ± 0.027
q (10
−3
) 1.240 ± 0.068 2.375 ± 0.076
α (rad) 0.623 ± 0.045 0.590 ± 0.044
ρ (10
−3
) 17.675 ± 0.831 1.137 ± 0.048
f
S
0.842 ± 0.035 0.039 ± 0.007
f
B
1.026 ± 0.035 0.274 ± 0.007
5
The Astronomical Journal, 161:293 (12pp), 2021 June Jung et al.