Current Strategy of Search for
Gravitational-Wave Counterpart
by Optical and NIR Observations
Mahito Sasada (Hiroshima University)
on behalf of J-GEM TeamElectro-magnetic counterpart
for GW phenomenon
Gravitational-wave source• Since Nov. 2015, LIGO/Virgo have detected gravitational-wave (GW)
signals from the Universe.
• Those would come from astronomical phenomena, for example mergersof black hole-black hole, black hole-neutron star, and neutron starneutron star pairs.
• The GW-radiated phenomena are expected to radiate electro-magnetic(EM) emission.
• Multi-messenger observation can reveal the physical background of GW
sources.

GW sources discovered by
LIGO/Virgo

GW170817

Credits: LIGO/VIrgo/Northwestern Univ./Frank Elavsky

Abbott +2017

GW170817• LIGO/Virgo detected the GW on
17 Aug. 2017.
• GW170817 was identiﬁed as theEM counterpart in the entire
wavelength.
• Gamma-ray: Detect gamma-ray
emission after 1.74 seconds ofGW detection.
• X-ray: Detect after 9 days of GW
detection.
• Optical and NIR: Identify opticalcounterpart after 10.87 hours.
• Radio: Detect after 17 days.

Neutron-Star Merger
NASA/CXC/M.Weiss•

The obtained GW indicated that the GW was generated
by the binary neutron-star (BNS) merger.

Implication from EM Observation
Optical and IR emissions can be generated
by the radioactive decays of r-process
nuclei. (Kilonova model)• Radio, X-ray and gamma-ray emissions
would come from the relativistic jet which
was generated by the BNS merger.
•Mooley+ 18
Tanaka+ 17

J-GEM Observation for
GW170817
101-5

PubliUtsumi +2017• J-GEM succeeded the observation
for GW170817 by using IRSF,
MOA, B&C and Subaru/HSC.
• Our aim is to observe the opticalcounterpart of GW source before
1 day.
Constrain a physical mechanism
101-3
Publications of the Astronomical Society of Japan (2017), Vol. 69, No. 6of GW counterpart.
2 J-GEM Observations
A broad geometrical distribution of observatories wasrequired to observe SSS17a because it was visible fora limited amount of time after sunset in the northernhemisphere. J-GEM facilities were suitable for observing
this target because they are distributed all over the Earthin terms of the longitude, which included the southernFig.
2. Light where
curvestheandvisibility
color evolutionof SSS17a.
hemisphere
was better.
We usedThethe face color ischangedthe respectivebands.
The Galactic
has been
followingwith
facilities
to perform
follow-up
optical extinction
obsercorrectedby assuming
E(B 8.2− V)m= Subaru/HSC
0.1 mag (Schlafly& Finkbeiner 2011).
vations ofGW170817:(Miyazaki

1 dayFig. 1. Three-color composite images of SSS17a using z-, H-, and Ks -How to Identify Optical Counterpart

Publications of the Astronomic28-4
The Astrophysical Journal Letters, 850:L1 (9pp), 2017 November 20

Lipunov et al.

with a diameter ofArea of GW170817

from 18.3 mag to 2(figure 3 and table
mainly determined
are typically shallow
particular, the depth
August 19 are quite sof detection by a ran
cial point sources w
figure 4). The magni
in time. The large diAugust 19 causes thon the PSF magnitu
Tominaga+ 2018
5σ limiting magnitu
3Figure 4. MASTER-composed discovery image started 2017 August 17 at 23:59:54 UT. We used color
B, R, I,W ﬁlters,MASTER-OAFA,
and MASTER-SAAO
Fig.
1.
Pointing
mapfor
GW170817
overlaidon
theprobability
map
Lipunov+
2017images.
The kilonova position
is marked by white lines on the left part of composed image. The right (large) image is the MASTER main telescope’s usual FOV.
completeness magnit(Abbott et al. 2017d). The white contour represents the 90% credible
As the detected soregion. Circles represent the field-of-view of HSC; their colors changing
Table 2
Possible Kilonova Brightness
withan orderof observation.
Observations have been carried out fromcandidate selection im Max[AB]NameZDL Mpc
DMObs.
Band
MabsFlat spec AB
Flux iso erg sLink
Therearemanystars
andgalaxies
withintheprobableregionofGW170817.
•darker
The dashed curves represent the Galactic graticThis paper
Kilonova NGC 4993 0.0098
42.5
33.14
MASTERW
17.3−16.03 to lighter
10 colors.
Criteria for the dete
GRB 130603
0.35601911.9
41.41
HSTH
25.73
−15.355.50 × 10
Tanvir et al. (2013)ules.
(Color 1.78
online)
GRB 080503
0.561
3290.5
42.59
Gemini/Keckr
25.48−16.62
× 10Perley et al. (2009)• It is not easy to identify an optical transient from the obtained image.
(1) |(S/N)PSF | > 5, (2our wide FOV (Lipunov et al. 2010), in real time (1–2 minutes
the object’s instrumental ﬂux from the difference image. We
correct the obtained stellar magnitudes for the Galactic extinction,2after a charge-coupled device (CCD) readout), and can work in
are the lengths of the28.9 2011).
deg corresponding to the 66.0% cred(Schlaﬂy is& Finkbeiner
based on E (Bobserved- V )=0.1 mag area
alert, inspect, and survey mode, independently of human
The MASTER Global Robotic Net archive contains 126
intervention. This is a unique feature that gives us the ability tosource, respectively,
images of theible
galaxy NGC4993, obtained
from 2015 January 17 (figure 1). Exposures used in theregionof GW170817
detect new objects in large ﬁelds in real time and to study
−1vis

4241

*42

J-GEM (Japanese collaboration for Gravitational-wave Electro-Magnetic follow-up)

520

Purpose of J-GEM
Purpose• Multi-messenger observation to reveal the physical background ofGW sources• Detect and observe an optical counterpart of GW sourceRequirements• Survey huge area (> 10 sq. degrees), because of a low sky-position
accuracy of GW observatories.
• Identify the optical counterpart as soon as possible to understandan early phase of GW event.

Approach• We do a survey observation by using many Japanese telescopes

Two types of telescope
Normal Telescope (FoV < 1 deg^2)

Telescope having large FoV (FoV > 1 deg^2)• B&C• Kanata • OAO WFC• Saitama SaCRA• IRSF
• MITSuME
Akeno/Okayama• Nayuta• Subaru• MOA• Kiso Tomo-eDiﬀerent strategies depending on telescopes
Identify the EM counterpart using both types of telescopes

Strategy for Normal Telescope
The survey area may be so huge that a normal optical telescope can
not cover the entire survey area.
• Optical counterpart should be associated with a host galaxy.
➡ Observe candidate galaxies, and identify the associatedtransient (Targeted Observation).
GW170817 was discovered by the similar way.
• Survey the huge area➡ List candidate galaxies, and observe with many collaborate
telescopes• Do not duplicate candidate host galaxy to survey eﬃciently.
➡ Share a list of candidate host galaxies, and realtime observing
information

Strategy for Normal Telescope①J-GEMWeb Systemplanner②• Make galaxy list
• Serve realtime info

OAOWFC
Okayama Astrophysical Observatory Wide Field Camera④③
2015/12/07

観測装置技術WS• Observe• Reduction

Image
Server7• Save/show obtained images• Calculate limiting ﬂux• Subtract image

Alert Information System;
planner

Web-base system. Communicate through command line

Sharing Information of Observation

Galaxy ID Probability

Galaxy Info

Obs Teles

Obs InfoFlag

Share information to avoid duplication of observation

J-GEM Ranking on GW170817• NGC 4993 got 11-th rank based on GLADE galaxies
with 3D probability and B-mag

How complete?

• 8223 SN Ia are used to validate GLADE completeness• Targeted observation with planner is promising for GW
events up to 200Mpc

Image Subtraction

Web-base image server system.
Assemble images obtained by each telescope.

Identiﬁcation
Obs Image

Ref ImageSub ImageBlinking gif•Reference image obtained by PanSTARRS•Image subtraction•Compare between obtained and reference images.

Schedule for O3 Run
LIGO-G1801056

LIGO-VIRGO Joint Run Planning Committee

Working schedule for O3(Public document G1801056-v4, based on G1800889-v7)

2018
Sep

L
H1
IG
L1
O
VIRGO
GEO

2019
OctNov

DecJanFeb

ER13: from 8am PT Dec 14
to 6 am PT Dec 18

MarAprMay

JunJul

Aug

ER14: up to four weeks, starting at the earliest March 1st, 2019
O3 to follow

Commissioning

ER13 Commissioning ER14

O3: one calendar year long

Commissioning

ER13 Commissioning ER14

O3: one calendar year long

Commissioning

ER13 Commissioning ER14

O3: one calendar year long~70% observing mode
EngineeringRunObserving Run

Now

Detector operational,commissioning mode
(small fraction of observing mode time)
Detector in observing
mode for a fraction of the time
during Engineering Runs (ERs),

https://dcc.ligo.org/public/0157/G1802174/003/G1802174-v3.pdf

Detector not producing data
(downtime)
24/7 observing mode
(Observing Run,We have done two times of test observations for GW alert.

Coordinated Observation for GW Alert
J-GEM

Dummy Alert

Web System
playground

Including supernova
as transient• Organize obs info
and transient info

OAOWFC
Okayama Astrophysical Observatory Wide Field Camera

Image
Server2015/12/07

観測装置技術WS

7• Observe• Send obtained images• Assemble images• Find transient byeye

Lesson from CoordinatedObservation
Observed 45/137 galaxies using four telescopes in one night.
Obtained Image

Smoothing Image

Reference Image

BUT, we passed over the new transient at that time.
Modify the observation scheme and image subtraction system.

Engineering Run (ER13)•LIGO/Virgo team expects rates of GW events:• Binary neutron stars ; 1/month to 1/year• Binary black holes ; few/week to few/month• Neutron-star black-hole binaries ; uncertain•

Engineering Run (ER13) was conducted at the last week(from 12/15 to 12/18).

There was no alert of GW event during ER13.

Summary• Construct observational systems both for normal and wideﬁeld FoV telescopes to observe an optical counterpart ofGW source.
• Made web-base systems:
1. Share a probability ﬁeld of GW event and observational
information.
2. Assemble observed images and subtract from reference
to identify the transient.
• Did coordinated test observations with dummy alert of GW
event.
• There is no public alert during ER13.

Thank you for your attention