# GECKO Optical Follow-up Observation of Three Binary Black Hole Merger
Events: GW190408_181802, GW190412, and GW190503_185404

Joonho Kim  Astronomy Program, Department of Physics and Astronomy, Seoul
National University, Seoul 08826, Korea  SNU Astronomy Research Center, Seoul
National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Korea  Myungshin Im
Astronomy Program, Department of Physics and Astronomy, Seoul National
University, Seoul 08826, Korea  SNU Astronomy Research Center, Seoul National
University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Korea  Gregory S. H. Paek
Astronomy Program, Department of Physics and Astronomy, Seoul National
University, Seoul 08826, Korea  SNU Astronomy Research Center, Seoul National
University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Korea  Chung-Uk Lee  Korea
Astronomy and Space Science Institute, Daejeon 34055, Korea  Seung-Lee Kim
Korea Astronomy and Space Science Institute, Daejeon 34055, Korea  Seo-Won
Chang  Astronomy Program, Department of Physics and Astronomy, Seoul National
University, Seoul 08826, Korea  SNU Astronomy Research Center, Seoul National
University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Korea  Changsu Choi
Astronomy Program, Department of Physics and Astronomy, Seoul National
University, Seoul 08826, Korea  SNU Astronomy Research Center, Seoul National
University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Korea  Sungyong Hwang
Astronomy Program, Department of Physics and Astronomy, Seoul National
University, Seoul 08826, Korea  SNU Astronomy Research Center, Seoul National
University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Korea  Wonseok Kang
Deokheung Optical Astronomical Observatory, National Youth Space Center,
Goheung 59567, Korea  Sophia Kim  Astronomy Program, Department of Physics and
Astronomy, Seoul National University, Seoul 08826, Korea  SNU Astronomy
Research Center, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul
08826, Korea  Taewoo Kim  Deokheung Optical Astronomical Observatory, National
Youth Space Center, Goheung 59567, Korea  Hyung Mok Lee  Astronomy Program,
Department of Physics and Astronomy, Seoul National University, Seoul 08826,
Korea  SNU Astronomy Research Center, Seoul National University, 1 Gwanak-ro,
Gwanak-gu, Seoul 08826, Korea  Korea Astronomy and Space Science Institute,
Daejeon 34055, Korea  Gu Lim  Astronomy Program, Department of Physics and
Astronomy, Seoul National University, Seoul 08826, Korea  SNU Astronomy
Research Center, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul
08826, Korea  Jinguk Seo  Astronomy Program, Department of Physics and
Astronomy, Seoul National University, Seoul 08826, Korea  SNU Astronomy
Research Center, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul
08826, Korea  Hyun-Il Sung  Korea Astronomy and Space Science Institute,
Daejeon 34055, Korea

###### Abstract

We present optical follow-up observation results of three binary black hole
merger (BBH) events, GW190408_181802, GW190412, and GW190503_185404, which
were detected by the Advanced LIGO and Virgo gravitational wave (GW)
detectors. Electromagnetic (EM) counterparts are generally not expected for
BBH merger events. However, some theoretical models suggest that EM
counterparts of BBH can possibly arise in special environments, prompting
motivation to search for EM counterparts for such events. We observed high-
credibility regions of the sky for the three BBH merger events with telescopes
of the Gravitational-wave EM Counterpart Korean Observatory (GECKO), including
the KMTNet. Our observation started as soon as 100 minutes after the GW event
alerts and covered 29 - 63 deg2 for each event with a depth of $\sim$ 22.5 mag
in $R$-band within hours of observation. No plausible EM counterparts were
found for these events, but from no detection in the GW190503_185404 event,
for which we covered 69% credibility region, we place the BBH merger EM
counterpart signal to be $M_{g}>-18.0$ AB mag within about 1 day of the GW
event. The comparison of our detection limits with light curves of several
kilonova models suggests that a kilonova event could have been identified
within hours from GW alert with GECKO observations if the compact merger
happened at $<400$ Mpc and the localization accuracy was of order of 50 deg2.
Our result gives a great promise for the GECKO facilities to find EM
counterparts within a few hours from GW detection in future GW observation
runs.

gravitational wave, optical, follow-up, binary black hole, KMTNet, kilonova

††facilities: KMTNet (Kim et al., 2016), WIT, DOAO, SAO, LOAO (Im et al.,
2019)††software: SExtractor (Bertin & Arnouts, 1996), Scamp (Bertin, 2006),
SWarp (Bertin et al., 2002), Astronomy.net (Lang et al., 2010), SkyBoT (v3.0;
Berthier et al. 2006), HOTPANTS (Becker, 2015), LA-Cosmic (van Dokkum, 2001)

##  1 Introduction

Since the first detection of the binary black hole (BBH) merger event by the
Laser Interferometer Gravitational-Wave Observatory (LIGO) Collaboration and
Virgo Collaboration (LVC) in 2015 September 14 (Abbott et al., 2016), the
reported number of BBH events has been increasing fast, from two in the the
Advanced LIGO and Advanced Virgo (LIGO/Virgo) first (O1) observing run, and
three in the second (O2) observing run, and to 20 in the first half (O3a) of
the third (O3) observing run (Abbott et al., 2020a). The gravitational wave
(GW) astronomy is now flourishing with the GW signals providing valuable
information on the compact star mergers such as the mass of the binary compact
objects, and approximate distance and the location in the sky of the event.

The power of GW astronomy was much enhanced when the first electromagnetic-
wave (EM) counterpart of a GW event was identified on 2017 August 17 during
the LIGO/Virgo O2 run. The GW event, GW170817, was found to be due to a binary
neutron star (BNS) merger with 90% localization of 31 deg2 and the luminosity
distance of 40 Mpc (Abbott et al., 2017a). The combined efforts of the GW and
EM observations produced a series of important results, including the first
identification (AT2017gfo) of the exact location and the distance to the GW
event (Abbott et al., 2017b; Arcavi et al., 2017; Coulter et al., 2017;
Lipunov et al., 2017b; Soares-Santos et al., 2017; Tanvir et al., 2017;
Valenti et al., 2017), convincing observational evidence for the link between
a short gamma-ray bursts (GRB) and a BNS merger (Abbott et al., 2017c;
Goldstein et al., 2017; Savchenko et al., 2017), undeniable discovery of a
kilonova and the first detailed characterization of such an event (Andreoni et
al., 2017; Arcavi et al., 2017; Chornock et al., 2017; Cowperthwaite et al.,
2017; Drout et al., 2017; Evans et al., 2017; Haggard et al., 2017; Hallinan
et al., 2017; Kasliwal et al., 2017; Margutti et al., 2017; Nicholl et al.,
2017; Pian et al., 2017; Smartt et al., 2017; Troja et al., 2017; Utsumi et
al., 2017), a glimpse on the BNS merger environment (Im et al., 2017; Tanvir
et al., 2017), and the first application of the GW siren method to measure the
Hubble constant (Abbott et al., 2017d). In short, this was a monumental event
that marked the beginning of multi-messenger astronomy (MMA) using GW and EM
observations. If a similar study can be performed for BBH GW events that
currently occupy the majority of the detected GW events, it would much broaden
the power of MMA and enhance our knowledge on the previously rarely studied
aspect of the universe.

As a result, many follow-up observations have been carried out to find EM
counterparts of BBH GW events, yet no EM counterparts have been identified for
the BBH events (Cowperthwaite et al., 2016; Morokuma et al., 2016; Smartt et
al., 2016a, b; Soares-Santos et al., 2016; Lipunov et al., 2017a; Stalder et
al., 2017; Yoshida et al., 2017; Smith et al., 2019; Doctor et al., 2019; Yang
et al., 2019; Herner et al., 2020; Gompertz et al., 2020; Grado et al., 2020;
Turpin et al., 2020). There are several reasons for the non detections. First
of all, the strong gravitational force of black holes do not allow debris from
the merger to escape, thus prohibiting EM-producing physical processes to
occur (Berger, 2014). A few models suggest that an EM counterpart might be
produced if, e.g., the BBH merger preferentially happened in the active
galactic nucleus (AGN) accretion disk, but the expected strength of EM
radiation is weak or very uncertain (Loeb et al., 2016; Perna et al., 2016,
2018; Bartos et al., 2017; de Mink & King, 2017; Stone et al., 2017; McKernan
et al., 2018). For the O3 run, a candidate of optical counterpart for BBH
merger, GW190521 (Abbott et al., 2020a), is reported as a possible AGN related
transient (Graham et al., 2020). Secondly, the early LIGO/Virgo runs have been
hampered by limited detector sensitivities, and as a result, the locations of
the events estimated from the GW signals were very uncertain at a level of
several hundreds to thousands deg2 and their distance estimates were uncertain
as well (Abbott et al., 2019). Despite of these daunting prospects
identification of BBH-induced GW EM counterparts remains attractive due to its
potential for providing unexpected physical processes during the extreme
compact merger and the prospects for using BBH merger EM counterparts for a
number of MMA applications.

The LIGO/Virgo O3 run provided a unique opportunity to improve our limits of
the BBH EM counterpart search. With the improved sensitivity of the GW
detectors, the number of GW events increased (36 BBH events) and the
localization area was improved as well (514 deg2, median of 90% localization
area for BBH events111https://gracedb.ligo.org). Hence we organized an EM
follow-up observation group in Korea, utilizing a network of telescopes
worldwide that were already in use for monitoring nearby galaxies for new
transients such as supernova (Intensive Monitoring Survey of Nearby Galaxies,
IMSNG; Im et al. 2019). Our EM-follow-up effort was named as the
Gravitational-wave EM Counterpart Korean Observatory (GECKO, Im et al. 2020;
Paek et al. 2021 in preparation). Several of the GECKO facilities have wide-
field capabilities (field-of-view $>$ 1 deg2), and suited for searching EM
counterparts over a wide area.

In this paper, we present the GECKO optical follow-up observation for first
three BBH merger events in the O3a run, namely GW190408_181802 (hereafter,
GW190408 for brevity), GW190412, and GW190503_185404 (hereafter, GW190503). We
focus our attention on the observation made by the Korea Microlensing
Telescope Network (KMTNet; Kim et al. 2016), which is the most sensitive wide-
field telescope of GECKO. The description of the KMTNet observation and
analysis results will serve as a reference for future EM follow-up attempts.
However, observations made by other telescopes are also reported to show the
capabilities of GECKO facilities. In Section 2, we summarize three GW events
which are targeted for our optical follow-up observation. Section 3 describes
the telescopes we used and the observation strategy. Data analysis of the
observed images is given in Section 4. We report the observation results and
transient candidates in Section 5. Finally, we discuss the result in Section 6
and conclude the paper in Section 7.

Table 1: Target GW events

Event | Type | Event time | Initial | Initial | GWTC-2 | GWTC-2 | FAR
---|---|---|---|---|---|---|---
|  | [UT] | 90% Area [deg2] | Distance [Mpc] | 90% Area [deg2] | Distance [Mpc] | [yr-1]
GW190408 | BBH | 2019-04-08 18:18:02 | 387 | 1473 $\pm$ 358 | 140 | $1580^{+400}_{-590}$ |  $<$ 7.9 $\times 10^{-5}$
GW190412 | BBH | 2019-04-12 05:30:44 | 156 | 812 $\pm$ 194 | 21 | $740^{+140}_{-170}$ |  $<$ 7.9 $\times 10^{-5}$
GW190503 | BBH | 2019-05-03 18:54:04 | 448 | 412 $\pm$ 105 | 94 | $1520^{+710}_{-660}$ |  $<$ 7.9 $\times 10^{-5}$

††footnotetext: Note. The FAR values are taken from the result of PyCBC BBH
pipeline in the GWTC-2 catalog.

##  2 Gravitational Wave Events

Here we describe the characteristics of three BBH events observed by GECKO in
the early phase of the O3 run. We note that our observations were performed
using the information from the initial alerts (bayestar.fits), and which were
updated later in GWTC-2 catalog (Abbott et al., 2020a).

###  2.1 GW190408

On 2019 April 8 18:18:02 UT, the first event of LIGO/Virgo O3 run, designated
as originally as S190408a (LIGO Scientific Collaboration & VIRGO
Collaboration, 2019a), and later as GW190408 (Abbott et al., 2020a), was
detected by the Advanced LIGO and Advanced Virgo GW detectors. This event was
classified as a BBH merger with $>$ 99% probability. The luminosity distance
and the 90% sky localization area were initially reported as 1473 $\pm$ 358
Mpc and 387 deg2 (Table 1). Later, the further analysis of the GW data shows
that the luminosity distance of $1580^{+400}_{-590}$ Mpc with a redshift of
0.30${}^{+0.06}_{-0.10}$, the 90% credible sky area of 140 deg2, and a false
alarm rate (FAR) of $<$ 7.9 $\times 10^{-5}$ yr-1 (Abbott et al., 2020a). The
BH masses of the compact objects are found to be
24.5${}^{+5.1}_{-3.4}\,M_{\odot}$ and 18.3${}^{+3.2}_{-3.5}\,M_{\odot}$. The
most recent GWTC-2 localization shows that the event happened in the northern
hemisphere near RA = 351.0 deg and Dec = 53.9 deg, but the initial 90%
localization area contained a localized region in the southern hemisphere
(Figure 1). Hence, the southern hemisphere localization area was observed by
the KMTNet.

###  2.2 GW190412

GW190412, originally a candidate super-event S190412m, (LIGO Scientific
Collaboration & VIRGO Collaboration, 2019b), was detected by three GW
detectors on 2019 April 12 05:30:44 UT with $>$ 99% probability of being BBH
merger. Its 90% credibility area on the sky and the luminosity distance were
estimated as 156 deg2 and 812 $\pm$ 194 Mpc (Table 1). These numbers were
updated recently in the GWTC-2 catalog as 21 deg2, $740^{+140}_{-170}$ Mpc,
and FAR of $<$ 7.9 $\times 10^{-5}$ yr-1 (Abbott et al., 2020a). The redshift
is given as 0.15${}^{+0.03}_{-0.03}$ in the catalog. This was an event of
special interest since this was the first BBH merger with definitely
asymmetric component masses of 30.0${}^{+4.7}_{-5.1}\,M_{\odot}$ and
8.3${}^{+1.6}_{-0.9}M_{\odot}$, providing valuable information on the black
hole spin, and revealing evidence for higher harmonics in the signal. The peak
of GW event localization area is now estimated to be RA = 218.5 deg and Dec =
36.4 deg, but the initial estimates of the GW event location included an
extensive region near equator, therefore this field was observed with GECKO
facilities in the southern hemisphere as well (Figure 1).

###  2.3 GW190503

GW190503 was detected as a candidate event, S190503bf, on 2019 May 3 18:54:04
UT (LIGO Scientific Collaboration & VIRGO Collaboration, 2019c). With 96%
probability, this event was classified as a BBH merger. Initially, the 90%
localization region covered 448 deg2 of the sky and the luminosity distance
was estimated to be 412 $\pm$ 105 Mpc (Table 1). The most recent estimate in
the GWTC-2 catalog improved the 90% localization estimate to 94 deg2, the
distance estimate to $1520^{+710}_{-660}$ Mpc, a redshift as
0.29${}^{+0.11}_{-0.11}$, and FAR of $<$ 7.9 $\times 10^{-5}$ yr-1 (Abbott et
al., 2020a). We note that the distance estimate changed significantly from the
initial values for this particular event. The BH masses of the binary system
are found to be 42.9${}^{+9.2}_{-7.8}\,M_{\odot}$ and 28.5
${}^{+7.5}_{-7.9}M_{\odot}$ which are rather ordinary among the BBH mergers
detected in the LVC runs. The 90% localization area shrunk by a factor of 5
since the initial estimate, and the most recent estimate only partially
overlaps with the initial area. The initial and the most up-to-date location
estimates are all in the southern hemisphere, and it served as a prime target
for the KMTNet follow-up observation.

Figure 1: The GW localization and the coverage of our EM follow-up tiling
observation for three GW events. Here, we used the GW localization map of the
initial alerts (bayestar.fits), while the blue contours show the boundary of
the 90% localization area in the GWTC-2 catalog. The large black boxes in the
sky map on the left are expanded in the right panels. The small black and
green boxes are the field coverages of the KMTNet and WIT pointings,
respectively. The blue lines indicate the ecliptic plane.

##  3 Observation

###  3.1 Telescopes

For the follow-up observation of the GW events, we mainly used the KMTNet (Kim
et al., 2016) which is a network of three identical 1.6 m telescopes with a
wide field of view ($\sim$ 4 deg2) located at the Cerro Tololo Inter-American
Observatory (CTIO), the South African Astronomical Observatory (SAAO), and the
Siding Spring Observatory (SSO). The large field of view of the KMTNet enables
us to cover the large GW localization area (See Section 2.3). Also, the three-
site operation makes it possible to perform a follow-up observation 24-hrs a
day when the target is observable (e.q. Troja et al. 2017; Kim et al. 2018).
The KMTNet camera contains four 9k by 9k CCD chips, each covering about 1
deg2. There is a gap of about $3\farcm 1$ (east-west) to $6\farcm 2$ (north-
south) between each chip. To fill the CCD gaps, all the KMTNet observations
were carried out by dithering between exposures.

Additionally, we used four telescopes of IMSNG (Im et al., 2019) to observe
the northern hemisphere, especially at high declination regions
($\delta>25^{\circ}$) that the KMTNet cannot reach. The wide-field IFU
telescope (WIT), a wide-field ($\sim$ 5.5 deg2) imager with a 0.25 m telescope
equipped with multiple medium-band and broad-band filters (Hwang et al., 2021)
at the McDonald Observatory (McD) performed tiling observation to cover the
wide GW localization area, but to a rather shallow depth ($\sim$19.0 mag in
$V$-band at 5-$\sigma$ depth with a 7.5 min exposure). The other three are 1.0
m telescopes at the Deokheung Optical Astronomy Observatory (DOAO), the Mt.
Lemmon Optical Astronomy Observatory (LOAO), and the Seoul National University
Astronomical Observatory (SAO). These telescopes have a field of view of
$13\farcm 2$ x $13\farcm 2$, $28\farcm 1$ x $28\farcm 1$, $21\farcm 2$ x
$21\farcm 2$, respectively. These telescopes are suitable for observing
galaxies that are likely to be the host galaxy of the BBH merger (targeted
observation). The 5-$\sigma$ depth of the three 1.0 m telescopes are $\sim$
19.5 mag in $R$-band with a 3 min exposure.

###  3.2 Basic observation strategy

Since there are both wide-field and narrow-field GECKO telescopes, we adopted
two observing strategies. With wide-field telescopes (FoV $>1$ deg2), we
performed tiling observation with an aim of covering the entire 90%
localization area. When the localization area is too large, the limit for the
credibility of the localization is reduced from 90% to a lower value. Also,
when the localization area is several thousands deg2, we switched to the
target observation in the initial phase of GECKO observation as described
below.

Mostly for the narrow-field telescopes but also for the case of the
localization area is too large to cover even with a wide-field telescope, we
made a prioritized list of galaxies that are likely to be host galaxies of the
GW event. Galaxies are drawn from the GLADE catalog (Dálya et al., 2018)
version 2.3, and we compute the likelihood score of a galaxy to be the host
galaxy of the event. For both BNS and BBH events, the likelihood score
implements the 3-D localization probability, and the $K$-band absolute
magnitude. Here, $K$-band is chosen as a way to sort the list, since
theoretical expectation is such that the more massive the host galaxy is, the
more likely it hosts a BBH or BNS event (Mapelli et al., 2018; Artale et al.,
2019) and the $K$-band is known to be a good proxy for galaxy stellar mass
(e.g., Kim & Im 2013).

To create a list of the observation coordinates for both the tiling and target
observations, we established an automatic alert system which creates the list
based on the GW event alert and the localization map. The coordinates are
either handed over to the telescope operators or fed into the automatic
observation planner. Exposure times were chosen to be typically 4 min per a
tile for the KMTNet observation, and about 3 - 7.5 min for other telescopes.
We use the observation of the three initial O3 events as test cases, and
adjusted the exposure time.

More details on the galaxy prioritization and observational strategy will be
presented in Paek et al. (2021, in preparation).

Table 2: Tiling observation of the three GW events

Event | Telescope | Start Time | Time Since | RAaaCentral coordinate of the field. | DecaaCentral coordinate of the field. | Band | 5-$\sigma$ Depth
---|---|---|---|---|---|---|---
|  | [UT] | GW Alert [hours] | [hh:mm:ss] | [dd:mm:ss] |  | [AB mag]
GW190408 | KMTNet | 2019-04-08 19:58:59 | 1.66 | 14:52:14.3 | -41:22:52 | R | 22.5
GW190408 | WIT | 2019-04-09 11:32:26 | 17.2 | 21:43:19.9 | +15:04:05 | V | 17.6
GW190412 | KMTNet | 2019-04-12 11:30:06 | 6.00 | 12:49:08.5 | +15:11:24 | R | 21.8
GW190412 | WIT | 2019-04-12 07:17:53 | 1.78 | 12:13:33.8 | -00:57:38 | V | 18.3
GW190503 | KMTNet | 2019-05-03 23:00:35 | 4.09 | 04:50:41.2 | -33:47:28 | R | 22.5

††footnotetext: (This table is published in its entirety in the machine-
readable format. A portion is shown here for guidance regarding its form and
content.)

Table 3: Galaxy observation of GW190412

Telescope | Time | NameaaTarget name come from the GLADE catalog (Dálya et al., 2018). | RA | Dec | Band | 5-$\sigma$ Depth
---|---|---|---|---|---|---
| [UT] |  | [hh:mm:ss] | [dd:mm:ss] |  | [AB mag]
DOAO | 2019-04-12 10:57:48 | PGC1631373 | 13:13:56.2 | +20:35:23 | $R$ | 20.5
DOAO | 2019-04-12 11:08:47 | 2MSX J12252912+0317180 | 12:25:29.1 | +03:17:18 | $R$ | 20.3
SAO | 2019-04-12 11:13:55 | PGC1631373 | 13:13:56.2 | +20:35:23 | $R$ | 18.1
SAO | 2019-04-12 12:08:38 | 2MSX J12305009+0916524 | 12:30:50.1 | +09:16:52 | $R$ | 19.0
LOAO | 2019-04-16 10:48:19 | PGC2053980 | 14:53:17.0 | +34:44:59 | $R$ | 19.4

††footnotetext: (This table is published in its entirety in the machine-
readable format. A portion is shown here for guidance regarding its form and
content.)

Table 4: Observation summary

Event | Telescope | Observation start | Observation end | Naafootnotemark:  | Covered Area | Band | 5-$\sigma$ Depth [AB mag]
---|---|---|---|---|---|---|---
GW190408 | KMTNet | 2019-04-08 19:58:59 | 2019-04-09 21:54:00 | 8bbfootnotemark:  | 29 deg2 | $BR$ | 21.5 - 23.3
GW190408 | WIT | 2019-04-09 11:32:26 | 2019-04-12 11:34:12 | 6bbfootnotemark:  | 33 deg2 | $V$ | 17.1 - 18.7
GW190412 | SAO | 2019-04-12 11:13:55 | 2019-04-14 19:29:56 | 76ccfootnotemark:  |  | $R$ | 18.0 - 20.7
GW190412 | WIT | 2019-04-12 07:17:53 | 2019-04-14 11:35:23 | 22bbfootnotemark:  | 116 deg2 | $V$ | 17.6 - 19.2
GW190412 | DOAO | 2019-04-12 10:57:48 | 2019-04-17 15:11:07 | 81ccfootnotemark:  |  | $BVRI$ | 15.8 - 21.5
GW190412 | KMTNet | 2019-04-12 11:30:06 | 2019-04-13 13:10:13 | 15bbfootnotemark:  | 63 deg2 | $R$ | 20.2 - 22.9
GW190412 | LOAO | 2019-04-16 10:48:19 | 2019-04-16 11:09:56 | 2ccfootnotemark:  |  | $V$ | 19.3 - 19.4
GW190503 | KMTNet | 2019-05-03 23:00:35 | 2019-05-05 00:17:17 | 13bbfootnotemark:  | 52 deg2 | $R$ | 21.8 - 22.8

††footnotetext: aNumber of the observed targets. bTiling observation. cGalaxy
targeted observation.

###  3.3 Observations

####  3.3.1 GW190408

The first event, GW190408, was localized mostly in the northern hemisphere of
the sky, but the area was too close to the Sun to observe. But, there was a 7
deg2 of the initial 90% localization area in the southern hemisphere around
declination of $\sim$ -40 deg. We targeted this field using the KMTNet with 4
minutes exposure time in $B$ and $R$-bands. We started the observation about
100 minutes after the GW event alert, and we obtained four epochs ($\sim$ 8
hours interval) of observation data from three KMTNet sites. A portion of the
northern hemisphere localization area of initial GW alert was observed with
the WIT for three nights (2019 Apr 09 - 11). The observation covered a 32.6
deg2 of the sky (6 fields) with 7.5 minutes exposure time in $V$-band. No
target observation was performed since the event occurred at a distance that
was too far to be covered by the GLADE catalog. Total probability region
covered by the two telescopes is 2.0% for the initial GW localization, and the
probability region decreased to 0.14% with respect to the GWTC-2 localization
map. The reduction in the covered probability region from the initial to
GWTC-2 localization suggests that EM follow-up observations may miss optical
counterparts due to inaccuracy in the GW event localization. Improving the
accuracy of rapid localization of GW events is important for the success of
future optical follow-up efforts. The KMTNet and WIT observations are
presented in Table 2, along with their 5-$\sigma$ point source detection
limits.

####  3.3.2 GW190412

Most of the initial 90% localization area (156 deg2) of GW190412 lie on the
northern hemisphere of the sky, at declination of $-$3 deg to +38 deg. It can
be divided into two regions one at declination $<$ +25 deg (area1) and another
at declination $>$ +25 deg (area2). The area1 was observable with the KMTNet
and we started the observation at 6 hours after the event alert. We obtained
the KMTNet images for a 63 deg2 on the sky by tiling observation of 15 fields
with 4 minutes exposure time in $R$-band at 2 epochs of 1 day interval. The
high priority regions of both area1 and area2 were observed using the WIT by
tiling 22 fields (116.3 deg2) with a 7.5 min exposure time in $V$-band on
April 12th and 14th, 2019. We also performed the target observation of host
galaxy candidates with the narrow field telescopes. The target galaxies were
chosen as described in Section 3.2. We observed 81, 76, and 2 galaxies with
DOAO 1 m, SAO 1 m, LOAO 1 m telescopes, respectively, with exposure times of 3
- 5 minutes in $R$-band. Overall, we covered a 40% probability region of the
initial GW localization map, and added imaging data for 86 galaxy candidates.
However, the covered probability region reduces to 2.3% with respect to the
GWTC-2 localization map because the 90% region moved to declination $>$ +30
deg where our coverage is deficient (Figure 1). Like GW190408, the reduction
is significant in the covered localization probability area from the initial
to the GWTC-2 credible regions, reiterating the importance of rapid, accurate
GW localization. The list of the tiling observation coordinates and the list
of the observed host galaxy candidates are given in Tables 3. Also given are
5-$\sigma$ point source detection limits of the data.

####  3.3.3 GW190503

We performed follow-up observation for GW190503 only with the KMTNet because
the initial 90% credibility area was located in the southern hemisphere, the
area was extensive (448 deg2) and located in the southern hemisphere.
Furthermore, the KMTNet could reach deepest among our facilities available in
the southern hemisphere where another available GECKO facility was a 0.43 m
Lee Sang Gak Telescope (Im et al., 2015; Choi & Im, 2017). We observed a high
probability area from the initial alert with 13 tiles with 4 minutes exposure
time in $R$-band at two epochs separated by 1 day. The covered area is 52
deg2, which corresponds to a 33% probability region. Since the localization
area has shrunk to 96 deg2 in the GWTC-2 catalog, it turned out that the
KMTNet observation covered the 69% localization probability region of GWTC-2.
Table 2 shows the central coordinates of the KMTNet pointings and their
5-$\sigma$ point source detection limits.

We summarize all of the GECKO observations in Table 4 and the area covered by
the tiling observation is marked as rectangles in Figure 1.

Figure 2: Three transients found in the follow-up observation of GW190412
event. The top, middle, and bottom panels show SN 2019apz, SN 2018imf, and AT
2018jcu, respectively. From left to right, each panel represents the object
(science) image, the reference image, and the residual image after subtracting
the reference image from the object image.

##  4 Data Analysis and Transient Search

All the observed images were processed with basic reduction procedures (bias
subtraction, dark subtraction, and flat fielding). Astrometric calibration is
done using SCAMP (Bertin, 2006) or Astrometry.net (Lang et al., 2010), using
ten to hundreds stars as astrometry references depending on the field of view
and depth. This produces images with astrometric accuracy of rms scatter of
$0\farcs 2-0\farcs 8$ in both RA and Dec directions. For the KMTNet
observation for a given epoch, only two frames were taken, which made it
challenging to remove cosmic-rays during co-addition. Therefore, we removed
the cosmic rays from each single frame using LA-Cosmic software (van Dokkum,
2001). The sequentially observed two images were combined to produce a deeper
image with SWarp (Bertin et al., 2002).

We used Pan-STARRS (PS1, Chambers et al. 2016) images at the closest
wavelength to the observed image filter as reference images (for example, use
PS1 $r$-band image for KMTNet $R$-band image), and the PS1 reference images
were subtracted from observed images are by convolving and re-scaling them to
match the spatial resolution and the flux scale of the observed images using
High Order Transform of PSF ANd Template Subtraction (HOTPANTS, Becker 2015).

The declinations of the KMTNet images for GW190408 and GW190503 are lower than
$-$30 deg, and out of the PS1 coverage. For these images, we observed the same
area of the sky twice with a time separation of about 1 day, and we used the
images observed $\sim$ 24 hours after the first epoch as reference images for
the two events.

Transient candidates were detected by identifying positive signals having
signal-to-noise ratio greater than five in the subtracted images by running
SExtractor (Bertin & Arnouts, 1996). Many of the detected sources in the
residual images were false identification found at over or under-subtracted
bright sources. These imperfect subtraction was often caused by a slight
mismatch in astrometry and/or in the wavelength coverage of the reference and
the observed images, and they appear as positive signals right next to
negative pixels (see the upper part of the residual image of SN 2018imf in
Figure 2). To remove these artifacts, we ran SExtractor on the inverse images
of the residual images. The positive detections in the residual images were
rejected if there are positive detections in the inverse residual image right
next to their position ($<3\arcsec$). Additionally, we removed candidates
matched with non-transient sources in the reference images to exclude variable
sources like variable stars or active galactic nuclei. We also removed
candidates that are on bad pixels or whose central pixels were saturated. The
remaining candidates were visually inspected and classified as new transient
candidates if there were no other artifacts. Figure 2 shows the science,
reference, and residual images of three transients found in the KMTNet images
(see Section 5).

For the images from the other telescopes, we subtracted images using PS1
images and visually inspected the residual images to find transient
candidates.

Table 5: List of detected transients and transient candidates Number | Observation time | RA | Dec |  $R$ mag [AB] | Name
---|---|---|---|---|---
1 | 2019-04-12 11:42:31 | 12:49:11.59 | 14:08:00.25 | 21.34 $\pm$ 0.08 |
2 | 2019-04-12 12:00:53 | 12:52:38.01 | 19:04:04.33 | 20.80 $\pm$ 0.07 |
3 | 2019-04-12 12:57:24 | 12:36:45.09 | 11:43:00.09 | 21.06 $\pm$ 0.09 |
4 | 2019-04-12 12:57:24 | 12:42:19.38 | 01:17:22.02 | 22.05 $\pm$ 0.13 |
5 | 2019-04-12 12:57:24 | 12:45:07.81 | 10:37:10.78 | 21.33 $\pm$ 0.09 |
6 | 2019-04-12 12:57:24 | 12:38:11.48 | 10:19:58.89 | 20.88 $\pm$ 0.05 | SN 2019aqz
7 | 2019-04-12 13:03:38 | 12:37:34.85 | 13:16:49.31 | 21.17 $\pm$ 0.06 |
8 | 2019-04-12 13:03:38 | 12:37:42.61 | 13:20:19.88 | 21.40 $\pm$ 0.09 |
9 | 2019-04-12 13:03:38 | 12:36:24.06 | 13:22:36.69 | 21.87 $\pm$ 0.10 |
10 | 2019-04-12 13:03:38 | 12:41:25.46 | 13:21:30.02 | 21.24 $\pm$ 0.06 |
11 | 2019-04-12 13:03:38 | 12:42:41.39 | 13:15:54.89 | 17.82 $\pm$ 0.004 | SN 2018imf
12 | 2019-04-12 13:03:38 | 12:43:54.16 | 13:12:12.77 | 21.15 $\pm$ 0.06 |
13 | 2019-04-12 13:03:38 | 12:40:05.16 | 13:07:58.24 | 19.56 $\pm$ 0.02 | AT 2018jcu
Table 6: List of moving objects Number | Observation time | RA | Dec |  $R$ mag [AB] | KMTNet Site
---|---|---|---|---|---
1 | 2019-04-12 12:57:24 | 12:42:02.43 | 11:32:13.17 | 20.38 $\pm$ 0.07 | SAAO
| 2019-04-12 13:07:08 | 12:42:02.03 | 11:32:12.56 | 20.55 $\pm$ 0.07 | SAAO
2 | 2019-04-12 12:51:10 | 12:36:30.73 | 14:09:43.95 | 21.68 $\pm$ 0.20 | SAAO
| 2019-04-12 13:03:38 | 12:36:31.07 | 14:09:40.43 | 21.72 $\pm$ 0.17 | SAAO
3 | 2019-04-12 13:03:38 | 12:40:39.09 | 13:46:32.90 | 21.62 $\pm$ 0.16 | SAAO
| 2019-04-12 13:13:15 | 12:40:38.80 | 13:46:33.75 | 21.56 $\pm$ 0.15 | SAAO

##  5 Transient Search Result

After the identification of the credible transient candidates, we matched them
with known solar system objects using SkyBoT (Berthier et al., 2006) version
3.0. SkyBot is a system that provides the location of known solar system
objects at a given epoch. Also we matched our transient candidates with those
that were already reported in the Transient Name Server (TNS222https://wis-
tns.weizmann.ac.il).

For GW190408, we found two transient candidates. They are matched with known
solar system objects.

In the observed areas for the GW190412 event, we detected 275 transient
candidates. As we describe below, the majority of them are found to be the
solar system objects, since the localization area of GW190412 was near the
Ecliptic plane with an ecliptic latitude of about 14 deg. Among these
candidates, 258 objects were found to have matches with solar system objects
within 1 arcmin of their positions (most of the sources are matched within
$5\arcsec$). In addition, we found three moving objects that did not have
matching solar system objects. We consider these to be previously unknown
solar system objects or known solar system objects with high positional
errors. One transient candidate was found to be a fast proper motion star of
$\sim$ $1\arcsec$/year (Gaia Collaboration et al., 2018). Three objects were
found in the vicinity of possible host galaxies and they are identified in the
TNS as SN 2019aqz (Type Ia), SN 2018imf (Type IIP) and AT 2018jcu (Figure 2).
The remaining 10 candidates do not have a possible host galaxy in their
vicinity, and only observed in a single frame. Their nature cannot be judged
from our data alone, but we suspect that most of them are solar system objects
considering the large number of such objects in this field. The observing
strategy at that time was geared toward minimizing the overhead by reducing
the time required to move to another field. However, we realize that efficient
identification of moving objects can be done more easily if subsequent frames
were taken about an hour later. We are now considering such a strategy for
future observations.

No transient candidates were found in the observed areas of GW190503 event.
Also, no transient candidates were found in WIT, DOAO, SAO, and LOAO images in
all the events.

Table 5 shows the three transients and 10 transient candidates with uncertain
nature from the GECKO GW190412 observation. In Table 6, we also report the
epochs and the positions of the solar system objects found from our follow-up
observations.

##  6 Discussion

###  6.1 Limits on BBH EM Counterpart Brightness

As described above, we could not identify any plausible EM counterparts to the
three BBH merger events. This is not very surprising, considering that BBH
merger events are not expected to produce EM counterparts. Also, in light of
the recently updated localization area (See Section 2), the KMTNet observation
missed the updated area for GW190408 and GW190412, so identification of EM
counterparts was not possible from our observation in retrospect. For
GW190503, the KMTNet covered most of the updated 90% credible region. No
transients were found in this data, which places a useful upper limit on the
brightness of the EM counterpart. We note, however, that the updated distance
to GW190503 is $1527\pm 411$ Mpc, so the EM counterpart must have been bright
in order to be detected with the KMTNet observation. Below, we discuss the
feasibility of detecting EM counterparts and upper limits that we could have
imposed on these events if our observations covered all of the localization
area.

Figure 3: Comparison of different model light curves of kilonova and the
KMTNet image depths (black triangles) of three events in top three panels. The
bottom panel assumes a kilonova distance of 400Mpc, and the KMTNet image
depths from the observation of the three events are shown together. The blue
and red solid lines indicate the blue and red components of the AT2017gfo-like
kilonova model light curves of Kasen et al. (2017). The black solid, dashed,
and dotted lines are the AT2017gfo-like kilonova model of Tanaka et al.
(2017), and Banerjee et al. (2020). These models assume ejecta masses of 0.03
$M_{\odot}$, 0.03 $M_{\odot}$, and 0.05 $M_{\odot}$ and electron fractions of
0.1 - 0.4, 0.25, and 0.3 - 0.4, respectively. The green and yellow regions are
the range of four NSBH and five BNS model light curves, respectively, from
Tanaka et al. (2013, 2014); Hotokezaka et al. (2013).

From non-detection of transients for GW190503, we place an upper limit on the
brightness of EM optical counterpart for the BBH event. The median 5-$\sigma$
depth of the KMTNet observations is about $R=22.5$ AB mag. Adopting the
distance of 1500 Mpc, we get an upper limit for the EM counterpart brightness
of $M_{R}\sim-18.3$ AB mag without K-correction. Since GW190503 occurred at
$z\sim 0.32$ and the $R$-band has an effective wavelength of $\sim 650$ nm
(e.g., Im et al. 2010), the rest-frame wavelength covered by $R$-band for
GW190503 is $\sim 490$nm, which is approximately the effective wavelength of
$g$-band. Therefore, $g$-band K-correction is dominated by the bandwidth
dilation term of $-2.5\times\rm{log}(1+z)$ (e.g., Taak & Im 2020). Adopting
$-2.5\times\rm{log}(1+z)$ as the K-correction, we get a lower limit on $M_{g}$
(the absolute magnitude in $g$-band) of $M_{g}>-18.0$ AB mag (5-$\sigma$).
This is about 1.5 to 2.5 magnitude brighter than the $g$-band peak magnitudes
of various kilonova model light curves (Tanaka et al., 2013, 2014, 2017;
Hotokezaka et al., 2013; Kasen et al., 2017; Arcavi, 2018; Banerjee et al.,
2020).

###  6.2 Comparison with kilonova light curve models

We compare model light curves of kilonova and the imaging depths to discuss
the possibility of GW EM counterpart detection (Figure 3). One of the models
is the AT2017gfo-like kilonova model of Kasen et al. (2017), hereafter K17.
The model is for the AT2017gfo that combines the blue and red kilonova
components with ejecta masses of 0.025 $M_{\odot}$ and 0.04 $M_{\odot}$,
respectively. We plotted the two kilonova components in Figure 3 and it shows
that the blue kilonova is more dominant than red kilonova in the early phase,
$<$ 2 days. Similarly, the AT2017gfo-like kilonova models of Tanaka et al.
(2017) and Banerjee et al. (2020) are plotted in Figure 3 (with ejecta masses
of 0.03 - 0.05 $M_{\odot}$). We also plot earlier kilonova light curve
predictions for NS-BH (NSBH) binary merger and BNS mergers with ejecta masses
of 0.0007 - 0.07 $M_{\odot}$ (Tanaka et al., 2013, 2014; Hotokezaka et al.,
2013). These models assume a higher mass fraction of lanthanides ($X_{lan}\sim
10^{-1.5}$) than the K17 model ($X_{lan}=10^{-4}$), and consequently predict
fainter light curves than the AT2017gfo-like models. For the distances of
these models for each event, we assumed the values from the GWTC-2 catalog.
Overall, we confirm that these events are too far (700 - 1600 Mpc) for
detecting their EM counterparts with KMTNet at a nominal depth. But if
AT2017gfo-like kilonovae appeared at about 400 Mpc or less, KMTNet could have
detected them (Figure 3).

###  6.3 Prospects for rapid follow-up observation

The early light curve can discern different mechanisms proposed for the
observed blue optical-NIR emission in the early phase of EM counterpart of
GW170817. The cooling of ejecta shock-heated by various mechanisms (e.g.,
cocoon model) would produce the early light curve that peaks very quickly in
about $\sim$ hour, while the early light curve that is dominated by the
radioactive decay would peak at in about a day (Arcavi et al., 2017).
Therefore, a rapid EM follow-up observation is important for constraining
different EM production mechanisms.

For the GW190408 event, we were able to start the follow-up observation after
100 minutes from the GW alert. This demonstrates that fast follow-up
observation is possible with our GECKO network, within about 1 hour from the
GW event alert. Yet, it is necessary to shorten the start time of the follow-
up observation, since it requires a long time even for wide-field telescopes
such as KMTNet to cover the whole localization area of the GW events. The
areas covered by our observations are 29, 63, and 52 deg2 for GW190408,
GW190412, and GW190503, respectively. From these data, we estimate that it
took 1.69 minutes per deg2 on average. For BNS events in the 4th (O4) GW
observing run, the median 90% credibility region is expected to be
$33^{+5}_{-5}$ deg2 (Abbott et al., 2020b). Therefore, it would take about 50
minutes to cover the median 90% localization areas of the O4 BNS events with a
KMTNet tiling observation.

For the GECKO facilities to catch the early light curve of EM counterparts
within an hour time-scale during the future O4 run, it is necessary to start
the observation almost immediately after the initial GW alert. The work is in
progress to shorten the latency of the starting time to 10 minutes or less.

For all the events, we also note that the localization area shrunk
significantly or the peak of the highest probability localization area moved
to another location. Considering a large amount of time needed to cover a wide
localization area, improvements in the accuracy of the initial localization
map is highly desirable. If not, one should cover as much 90% localization
area as possible in order not to miss EM counterparts.

###  6.4 Need for deep reference images

When searching for transients through the image subtraction technique, we
found that the PS1 $r$-band images could serve as acceptable reference images
even if the filter system was slightly different from our $R$-band. However,
during the analysis of the GW190408 and GW190503 optical data, we also
discovered the lack of suitable reference images at Dec $<-30$ deg where there
is no PS1 coverage, and the SkyMapper images (Onken et al., 2019) are rather
shallow. The Dark Energy Survey (DES; Abbott et al. 2018) images are
available, covering 5000 deg2 of the southern hemisphere with a depth of
$\sim$ 24 mag. However the DES images provide only a partial coverage at $-$60
$<$ Dec (deg) $<$ $-$30\. The sky below Dec = $-$60 deg will remain uncovered
by deep imaging observations, until the Legacy Survey of Space and Time (LSST,
Ivezic̀ et al. 2019) starts its full operation. Therefore, we are now
performing a wide imaging survey named the KMTNet Synoptic Survey of Southern
Sky (KS4) to provide reference images for the KMTNet follow-up data to the
depths of $\sim 23.5$ AB mag over 7000 deg2 area at the Dec $<-60$ deg areas
and some of the $-60<\rm{Dec(deg)}<-30$ areas that are not deeply covered by
previous surveys. The KS4 will allow us to quickly identify EM counterparts so
that the EM counterparts can be studied in depth.

##  7 Summary

In this paper, we presented the GECKO follow-up observation of three BBH
events, GW190408, GW190412, and GW190503, that were reported early in the O3a
run. In particular, we reported the KMTNet observations in detail which
started at 100 minutes to 6 hours from the GW detection with a sky coverage of
29 - 63 deg2 and $\sim$ 22.5 mag depth in $R$-band.

From our observation, we identified 13 transient candidates for GW190412 and
hundreds of moving objects such as asteroids. Among these transient
candidates, three transients were already reported (two confirmed as a
supernova) and the remaining 10 are likely to be moving objects due to their
proximity to the field near the ecliptic plane, the lack of a host galaxy near
them, and being observed only once. Therefore, no probable EM counterpart was
found in our observation. The improved localization of these events in the
recently published GWTC-2 catalog reveals that the initial position estimates
were very broad and many of the optical follow-up can point to the sky that
are not in the improved localization area. However, we find that our KMTNet
imaging observation covered a 69% probability region of GWTC-2 localization
map for GW190503, and our non-detection places an upper limit on the rest-
frame $g$-band magnitude of EM counterpart of this BBH merger event to be
$M_{g}>-18.0$ AB mag. Together with this, the comparison between the image
depths of KMTNet data and the theoretical kilonova light-curves show that
brighter AT2017gfo-like kilonova events can be detected when the luminosity
distance is out to $\sim$ 400 Mpc.

Our follow-up observation started as quickly as about 100 minutes after the GW
event alert, and could cover an area of about 50 deg2 in one hour,
demonstrating that the observation of the 90% localization area in the future
O4 run can be done in about two hours. The work is ongoing to improve the
response time. Overall, our results show promises that GECKO can identify
kilonova events out to 400 Mpc within hours from the GW event alert in future
GW observing runs that will improve the detector sensitivity event
localization capability.

We thank the referee for useful comments that helped and improved the
presentation of the paper. We appreciate the support and encouragement from
colleagues of the Korean Gravitational Wave Group. This research has made use
of the KMTNet system operated by the Korea Astronomy and Space Science
Institute (KASI) and the data were obtained at three host sites of CTIO in
Chile, SAAO in South Africa, and SSO in Australia. We also thank the operators
at LOAO and KMTNet for performing the requested observations and the staffs at
the McDonald observatory for their support of WIT. This work was supported by
the National Research Foundation of Korea (NRF) grant, No. 2020R1A2C3011091,
funded by the Korea government (MSIT), and the Korea Astronomy and Space
Science Institute under the R & D program (Project No. 2020-1-600-05)
supervised by the Ministry of Science and ICT.

## References

  * Abbott et al. (2016) Abbott, B. P., Abbott, R., Abbott, T. D., et al. 2016, Phys. Rev. Lett., 116, 061102
  * Abbott et al. (2017a) Abbott, B. P., Abbott, R., Abbott, T. D., et al. 2017, Phys. Rev. Lett., 119, 1101
  * Abbott et al. (2017b) Abbott, B. P., Abbott, R., Abbott, T. D., et al. 2017, ApJ, 848, 12
  * Abbott et al. (2017c) Abbott, B. P., Abbott, R., Abbott, T. D., et al. 2017, ApJ, 848, 13
  * Abbott et al. (2017d) Abbott, B. P., Abbott, R., Abbott, T. D., et al. 2017, Nature, 551, 85
  * Abbott et al. (2018) Abbott, T. M. C., Abdalla, F. B., Allam, S., et al. 2018, ApJS, 239, 18
  * Abbott et al. (2019) Abbott, B. P., Abbott, R., Abbott, T. D., et al. 2019, Phys. Rev. Lett., 9, 1040
  * Abbott et al. (2020a) Abbott, R., Abbott, T. D., Abraham, S. et al. 2020, arXiv:2010.14527
  * Abbott et al. (2020b) Abbott, B. P., Abbott, R., Abbott, T. D., et al. 2020, LRR, 23, 3
  * Andreoni et al. (2017) Andreoni, I., Ackley, K., Cooke, J., et al. 2017, PASA, 34, 69
  * Arcavi et al. (2017) Arcavi, I., Hosseinzadeh, G., Howell, D. A., et al. 2017, Nature, 551, 64
  * Arcavi (2018) Arcavi, I. 2018, ApJ, 855, 23
  * Artale et al. (2019) Artale, M. C., Mapelli, M., Giacobbo, N. et al. 2019, MNRAS, 487, 1675
  * Bartos et al. (2017) Bartos, I., Kocsis, B., Haiman, Z., & Márka, S. 2017, ApJ, 835, 165
  * Banerjee et al. (2020) Banerjee, S., Tanaka, M., Kawaguchi, K., Kato, D., Gaigalas, G. 2020, ApJ, 901, 29
  * Becker (2015) Becker, A. 2015, High Order Transform of PSF ANd Template Subtraction, Astrophysics Source Code Library, ascl:1504.004
  * Berger (2014) Berger, E. 2014, ARA&A, 52, 43
  * Berthier et al. (2006) Berthier, J., Vachier, F., Thuillot, W., et al. 2006, in ASP Conf. Ser., 351, Astronomical Data Analysis Software and Systems XV, ed. C. Gabriel et al. (San Francisco, CA: ASP), 367
  * Bertin & Arnouts (1996) Bertin, E. & Arnouts, S. 1996, A&AS, 117, 393
  * Bertin et al. (2002) Bertin, E., Mellier, Y., Radovich, M., et al. 2002, ASPC, 281, 228
  * Bertin (2006) Bertin, E. 2006, Astronomical Data Analysis Software and Systems XV, 351, 112
  * Choi & Im (2017) Choi, C., & Im, M. 2017, JKAS, 50, 71
  * Chambers et al. (2016) Chambers, K. C., Magnier, E. A., Metcalfe, N. et al. 2016, arXiv:1612.05560
  * Chornock et al. (2017) Chornock, R., Berger, E., Kasen, D., et al. 2017, ApJ, 848, 19
  * Connaughton et al. (2016) Connaughton, V., Burns, E., Goldstein, A., et al. 2016, ApJ, 826, 6
  * Coulter et al. (2017) Coulter, D. A., Foley, R. J., Kilpatrick, C. D., et al. 2017, Science, 358, 1556
  * Cowperthwaite et al. (2016) Cowperthwaite, P. S., Berger, E., Soares-Santos, M., et al. 2016, ApJ, 826, 29
  * Cowperthwaite et al. (2017) Cowperthwaite, P. S., Berger, E., Villar, V. A., et al. 2017, ApJ, 848, 17
  * Dálya et al. (2018) Dálya, G., Galgóczi, G., Dobos, L., et al. 2018, MNRAS, 479, 2374
  * de Mink & King (2017) de Mink, S. E., King, A. 2017, ApJ, 839, 7
  * Doctor et al. (2019) Doctor, Z., Kessler, R., Herner, K., et al. 2019, ApJ, 873, 24
  * Drout et al. (2017) Drout, M. R., Piro, A. L., Shappee, B. J., et al. 2017, Science, 358, 1570
  * Evans et al. (2017) Evans, P. A., Cenko, S. B., Kennea, J. A., et al. 2017, Science, 358, 1565
  * Gaia Collaboration et al. (2018) Gaia Collaboration, Brown, A. G. A., Vallenari, A., et al. 2018, A&A, 616, 1
  * Goldstein et al. (2017) Goldstein, A., Veres, P., Burns, E., et al. 2017, ApJ, 848, 14
  * Gompertz et al. (2020) Gompertz, B. P., Cutter, R., Steeghs, D. et al. 2020, MNRAS, 497, 726
  * Grado et al. (2020) Grado, A., Cappellaro, E., Covino, S. et al. 2020, MNRAS, 492, 1731
  * Graham et al. (2020) Graham, M. J., Ford, K. E. S., McKernan, B., Ross, N. P., Stern, D. et al. 2020, Phys. Rev. Lett., 124, 1102
  * Hwang et al. (2021) Hwang, S., Im, M., Taak, Y. C. et al. 2020, arXiv:2011.14049
  * Haggard et al. (2017) Haggard, D., Nynka, M., Ruan, J. J., et al. 2017, ApJ, 848, 25
  * Hallinan et al. (2017) Hallinan, G., Corsi, A., Mooley, K. P., et al. 2017, Science, 358, 1579
  * Herner et al. (2020) Herner, K., Annis, J., Brout, D., Soares-Santos, M. et al. 2020, A&C, 33, 100425
  * Hotokezaka et al. (2013) Hotokezaka, K., Kyutoku, K., Tanaka, M., et al. 2013, ApJ, 778, 16
  * Im et al. (2010) Im, M., Ko, J., Cho, Y., et al. 2010, JKAS, 43, 75
  * Im et al. (2015) Im, M., Choi, C., & Kim, K. 2015, JKAS, 48, 207
  * Im et al. (2017) Im, M., Yoon, Y., Lee, S. J., et al. 2017, ApJ, 849, 16
  * Im et al. (2019) Im, M., Choi, C., Hwang, S., et al. 2019, JKAS, 52, 11
  * Im et al. (2020) Im, M., Kim, J., Paek, G. S. -H. & GECKO team 2020, Yamada Conference LXXI: Gamma-ray Bursts in the Gravitational Wave Era 2019, ed. T. Sakamoto, M. Serino & S. Sugta. (Yokohama, Japan: Aoyama Gakuin Univ.), 25
  * Ivezic̀ et al. (2019) Ivezic̀, Z̆., Kahn, S, M., Tyson, J. A., et al. 2019, ApJ, 873, 111
  * Kasen et al. (2017) Kasen, D., Metzger, B., Barnes, J., Quataert, E., Ramirez-Ruiz, E. 2017, Nature, 551, 80
  * Kasliwal et al. (2017) Kasliwal, M. M., Nakar, E., Singer, L. P., et al. 2017, Science, 358, 1559
  * Kim & Im (2013) Kim, D., & Im, M. 2013, ApJ, 766, 109
  * Kim et al. (2016) Kim, S.-L., Lee, C.-U., Park, B.-G., et al. 2016, JKAS, 49, 37
  * Kim et al. (2018) Kim, J., Karouzos, M., Im, M., et al. 2018, JKAS, 51, 89
  * Lang et al. (2010) Lang, D., Hogg, D. W., Mierle, K., Blanton, M., & Roweis, S. 2010, AJ, 139, 1782
  * LIGO Scientific Collaboration & VIRGO Collaboration (2019a) Ligo Scientific Collaboration & VIRGO Collaboration 2019a, GCN, 24069, 1
  * LIGO Scientific Collaboration & VIRGO Collaboration (2019b) Ligo Scientific Collaboration & VIRGO Collaboration 2019b, GCN, 24098, 1
  * LIGO Scientific Collaboration & VIRGO Collaboration (2019c) Ligo Scientific Collaboration & VIRGO Collaboration 2019c, GCN, 24377, 1
  * Lipunov et al. (2017a) Lipunov, V. M., Kornilov, V., Gorbovskoy, E., et al. 2017, MNRAS, 465, 3656
  * Lipunov et al. (2017b) Lipunov, V. M., Gorbovskoy, E., Kornilov, V. G., et al. 2017, ApJ, 850, 1
  * Loeb et al. (2016) Loeb, A. 2016, ApJ, 819, 21
  * Mapelli et al. (2018) Mapelli, M., Giacobbo, N., Toffano, M. et al. 2018, MNRAS, 481, 5324
  * Margutti et al. (2017) Margutti, R., Berger, E., Fong, W., et al. 2017, ApJ, 848, 20
  * McKernan et al. (2018) McKernan, B., Ford, K. E. S., Bellovary, J., et al. 2018, ApJ, 866, 66
  * Morokuma et al. (2016) Morokuma, T., Tanaka, M., Asakura, Y., et al. 2016, PASJ, 68, 9
  * Nicholl et al. (2017) Nicholl, M., Berger, E., Kasen, D., et al. 2017, ApJ, 848, 18
  * Perna et al. (2016) Perna, R., Lazzati, D., & Giacomazzo, B. 2016, ApJ, 812, 18
  * Perna et al. (2018) Perna, R., Chruslinska, M., Corsi, A., & Belczynski, K. 2018, ApJ, 477, 4228
  * Pian et al. (2017) Pian, E., D’Avanzo, P., Benetti, S., et al. 2017, Nature, 551, 67
  * Savchenko et al. (2017) Savchenko, V., Ferrigno, C., Kuulkers, E., et al. 2017, ApJ, 848, 15
  * Smartt et al. (2016a) Smartt, S. J., Chambers, K. C., Smith, K. W., et al. 2016, ApJ, 827, 40
  * Smartt et al. (2016b) Smartt, S. J., Chambers, K. C., Smith, K. W., et al. 2016, MNRAS, 462, 4094
  * Smartt et al. (2017) Smartt, S. J., Chen, T. -W., Jerkstrand, A., et al. 2017, Nature, 551, 75
  * Smith et al. (2019) Smith, G. P., Bianconi, M., Jauzac, M., et al. 2019, MNRAS, 485, 5180
  * Soares-Santos et al. (2016) Soares-Santos, M., Kessler, R., Berger, E., et al. 2016, ApJ, 823, 33
  * Soares-Santos et al. (2017) Soares-Santos, M., Holz, D. E., Annis, J., et al. 2017, ApJ, 848, 16
  * Stalder et al. (2017) Stalder, B., Tonry, J., Smartt, S. J., et al. 2017, ApJ, 850, 149
  * Stone et al. (2017) Stone, N. C., Metzger, B. D., & Haiman, Z. 2017, MNRAS, 464, 946
  * Taak & Im (2020) Taak, Y. C., & Im, M. 2020, ApJ, 897, 163
  * Tanaka et al. (2013) Tanaka, M., & Hotokezaka, K. 2013, ApJ, 775, 113
  * Tanaka et al. (2014) Tanaka, M., Hotokezaka, K., Kyutoku, K., et al. 2014, ApJ, 780, 31
  * Tanaka et al. (2017) Tanaka, M., Utsumi, Y., Mazzali, P. A., et al. 2017, PASJ, 69, 102
  * Tanvir et al. (2017) Tanvir, N. R., Levan, A. J., González-Fernández, C., et al. 2017, ApJ, 848, 16
  * Troja et al. (2017) Troja, E., Piro, L., van Eerten, H., et al. 2017, Nature, 551, 71
  * Turpin et al. (2020) Turpin, D., Wu, C., Han, X.-H. et al. 2017, RAA, 20, 13
  * Utsumi et al. (2017) Utsumi, Y., Tanaka, M., Tominaga, N., et al. 2017, PASJ, 69, 101
  * van Dokkum (2001) van Dokkum, P. G. 2001, PASP, 113, 1420
  * Valenti et al. (2017) Valenti, S., Sand, D. J., Yang, S., et al. 2017, ApJ, 848, 24
  * Onken et al. (2019) Onken, C. A., Wolf, C., Bessell, M. S., et al. 2019, PASA, 36, 33
  * Yang et al. (2019) Yang, S., Sand, D. J., Valenti, S., et al. 2019, ApJ, 875, 59
  * Yoshida et al. (2017) Yoshida, M., Utsumi, Y., Tominaga, N., et al. 2017, PASJ, 69, 9