REVIEW ARTICLE
Prospects for observing and localizing gravitational-wave
transients with Advanced LIGO, Advanced Virgo
and KAGRA
Abbott, B. P. et al. (KAGRA Collaboration, LIGO Scientific Collaboration and
Virgo Collaboration)*
Received: 1 October 2019 / Accepted: 27 May 2020
The Author(s) 2020
Abstract
We present our current best estimate of the plausible observing scenarios for the
Advanced LIGO, Advanced Virgo and KAGRA gravitational-wave detectors over
the next several years, with the intention of providing information to facilitate
planning for multi-messenger astronomy with gravitational waves. We estimate the
sensitivity of the network to transient gravitational-wave signals for the third (O3),
fourth (O4) and fifth observing (O5) runs, including the planned upgrades of the
Advanced LIGO and Advanced Virgo detectors. We study the capability of the
network to determine the sky location of the source for gravitational-wave signals
from the inspiral of binary systems of compact objects, that is binary neutron star,
neutron star–black hole, and binary black hole systems. The ability to localize the
sources is given as a sky-area probability, luminosity distance, and comoving
volume. The median sky localization area (90% credible region) is expected to be a
few hundreds of square degrees for all types of binary systems during O3 with the
Advanced LIGO and Virgo (HLV) network. The median sky localization area will
improve to a few tens of square degrees during O4 with the Advanced LIGO, Virgo,
and KAGRA (HLVK) network. During O3, the median localization volume (90%
credible region) is expected to be on the order of 10
5
;
10
6
;
10
7
Mpc
3
for binary
neutron star, neutron star–black hole, and binary black hole systems, respectively.
This article is a revised version of
https://doi.org/10.1007/s41114-018-0012-9
.
Change summary
Major revision, updated and expanded.
Change details
Since publication of the previous version (Abbott et al 2018f), several updates to the
document have been made. The most significant changes are that we now frame our projections in terms
of observing runs, we include final results from O2, and we updated our localization projections to
include KAGRA as a fourth detector. Key differences are outlined in the Appendix.
&
KAGRA Collaboration, LIGO Scientific Collaboration and Virgo Collaboration
kscboard-chair@icrr.u-tokyo.ac.jp; lsc-spokesperson@ligo.org; virgo-spokesperson@ego-gw.it
*The full author list and affiliations are given at the end of paper.
123
Living Reviews in Relativity
https://doi.org/10.1007/s41114-020-00026-9
(0123456789().,-volV)
(0123456789().,-volV)
The localization volume in O4 is expected to be about a factor two smaller than in
O3. We predict a detection count of 1
þ
12
1
(10
þ
52
10
) for binary neutron star mergers, of
0
þ
19
0
(1
þ
91
1
) for neutron star–black hole mergers, and 17
þ
22
11
(79
þ
89
44
) for binary black
hole mergers in a one-calendar-year observing run of the HLV network during O3
(HLVK network during O4). We evaluate sensitivity and localization expectations
for unmodeled signal searches, including the search for intermediate mass black
hole binary mergers.
Keywords
Gravitational waves
Gravitational-wave detectors
Electromagnetic
counterparts
Data analysis
Contents
1
Introduction..................................................................................................................
.............
2
Construction, commissioning and observing phases...............................................................
2.1 O1: aLIGO ....................................................................................................................
...
2.2 O2: aLIGO joined by AdV .............................................................................................
2.3 O3: aLIGO, AdV and KAGRA ......................................................................................
2.4 Commissioning and observing roadmap.........................................................................
2.5 Envisioned observing schedule .........................................................................................
3
Searches and localization of gravitational-wave transients ......................................................
3.1 Detection and false alarm rates.........................................................................................
3.2 Localization................................................................................................................
........
3.2.1
Localization for compact binary coalescences.................................................
3.2.2
Localization for unmodeled signals..................................................................
3.3 The O1 and O2 follow-up program ..................................................................................
4
Public alerts ..................................................................................................................
..............
4.1 O3 false alarm rate threshold for automatic alerts...........................................................
4.2 Alert contents ...............................................................................................................
......
4.3 O3a gravitational-wave candidate alerts ...........................................................................
5
Observing scenarios............................................................................................................
........
5.1 O3: aLIGO 110–130 Mpc, AdV 50 Mpc, KAGRA 8–25 Mpc.......................................
5.2 O4: aLIGO 160–190 Mpc, AdV 90–120 Mpc, KAGRA 25–130 Mpc...........................
5.3 O5: aLIGO (LIGO-India will join in 2025) 330 Mpc, AdV 150–260 Mpc, KAGRA
130
?
Mpc ..........................................................................................................................
6
Conclusions...................................................................................................................
..............
7
Changes between versions ........................................................................................................
.
7.1 Updates to Sect. 2, ‘‘Construction, commissioning and observing phases’’: ..................
7.2 Updates to Sect. 3, ‘‘Searches for gravitational-wave transients’’: .................................
7.3 Section 4, ‘‘Public alerts’’ .................................................................................................
7.4 Updates to Sect. 5, ‘‘Observing scenarios’’: .....................................................................
References.....................................................................................................................
....................
123
B. P. Abbott et al.
1 Introduction
Advanced LIGO (Aasi et al.
2015a
), Advanced Virgo (Acernese et al.
2015
), and
KAGRA (Somiya
2012
; Aso et al.
2013
) are kilometer-scale gravitational-wave
(GW) detectors that are sensitive to GWs with frequencies of
20–2000 Hz.
1
The
era of GW astronomy began with the detection of GW150914 (Abbott et al.
2016i
),
a signal from the coalescence of a binary black hole (BBH); the first confirmed
multi-messenger counterpart to a GW observation came with GW170817 (Abbott
et al.
2017i
), a signal from a binary neutron star (BNS) coalescence which was
accompanied by detections across the electromagnetic spectrum (Abbott et al.
2017j
). In this article, we describe the schedule, sensitivity, sky-localization
accuracy, and expected detections for the GW-detector network. We discuss the
past, present, and future planned sequence of observing runs and the prospects for
multi-messenger astronomy.
The purpose of this article is to provide information to the astronomy community
to assist in the formulation of plans in the era of GW observations. In particular, we
intend this article to provide the information required for assessing the features of
programs for joint observation of GW events using electromagnetic, neutrino, or
other facilities (e.g., Abbott et al.
2016h
,
2017j
; Adrian-Martinez et al.
2016
; Albert
et al.
2017a
,
b
).
The full science of ground-based GW detectors is broad (Abbott et al.
2018e
),
and is not covered in this article. We concentrate solely on candidate GW transient
signals. We place particular emphasis on the coalescence of binary systems of
compact objects, such as BNS and neutron star–black hole (NSBH) systems, which
are the GW sources for which electromagnetic follow-up is most promising
(Goodman
1986
; Paczynski
1986
; Eichler et al.
1989
; Li and Paczynski
1998
;
Kulkarni
2005
; Rosswog
2005
; Metzger et al.
2010
; Roberts et al.
2011
; Abadie
et al.
2012b
,
c
; Evans et al.
2012
; Metzger and Berger
2012
; Nissanke et al.
2013
;
Kasen et al.
2013
; Barnes and Kasen
2013
; Tanaka and Hotokezaka
2013
; Aasi
et al.
2014a
; Grossman et al.
2014
; Ciolfi and Siegel
2015
; Ghirlanda et al.
2016
;
Paschalidis
2017
; Rosswog et al.
2017
; Foucart et al.
2018
; Barbieri et al.
2019
;
Metzger
2020
), and BBHs, which are the most commonly detected source (Abbott
et al.
2016b
,
2017f
,
2018a
,
c
). No electromagnetic emission is expected for vacuum
BBH mergers (Centrella et al.
2010
), but is possible if there is surrounding material
(Schnittman
2013
), for example, remnants of mass lost from the parent star (Perna
et al.
2016
; Janiuk et al.
2017
) or if the binary was embedded in a common
envelope (Woosley
2016
), or a disk of an active galactic nucleus (Bartos et al.
2017
;
Stone et al.
2017
). Mergers of binary systems of compact objects are absolute
distance indicators, and thus can be used as standard sirens to estimate the Hubble
constant (Schutz
1986
; Holz and Hughes
2005
; Dalal et al.
2006
; Nissanke et al.
2010
; Abbott et al.
2017a
). When an electromagnetic counterpart, and hence a host
galaxy cannot be identified, a statistical approach which uses galaxy catalogs and
1
LIGO is short for Laser Interferometer Gravitational-Wave Observatory. KAGRA is named after the
Japanese word
KAGURA
, which means traditional sacred music and dance for the gods; the name has a
secondary meaning as an abbreviation for KAmioka GRavitational-wave Antenna. Virgo is named for the
Virgo constellation and is not written in capital letters.
123
Prospects for observing and localizing GW transients with aLIGO, AdV and KAGRA
the GW localization volume can be used (Del Pozzo
2012
; Chen et al.
2018
;
Fishbach et al.
2019
; Soares-Santos et al.
2019
). For more general introductory
articles on GW generation, detection and astrophysics, we point readers to Blanchet
(
2014
), Pitkin et al. (
2011
) and Sathyaprakash and Schutz (
2009
).
As the detector network grows and evolves we will release updated versions of
this article: This is the fourth version. The plausible observing scenarios for the
upcoming observing runs includes KAGRA and the upgrades of the Advanced
LIGO (aLIGO) and Advanced Virgo (AdV) detectors, called A
?
and AdV
?
,
respectively. The predicted sky-localization accuracies and detection rates have
been updated and now incorporate the atsrophysical results from the first and second
observing runs (Abbott et al.
2018a
,
c
). Changes with respect to the previous
version (Aasi et al.
2016
) are listed in
Appendix A
. Throughout the paper we
assume a flat cosmology with Hubble parameter H
0
¼
67
:
9kms
1
Mpc
1
, and
density parameters
X
m
¼
0
:
3065 and
X
K
¼
0
:
6935 (Ade et al.
2016
).
2 Construction, commissioning and observing phases
We divide the development of the GW observatories into three phases:
Construction:
includes the installation and testing of the detectors. This phase
ends with
acceptance
of the detectors. Acceptance means that the interferometers
can lock for periods of hours: light is resonant in the arms of the interferometer
with
no guaranteed GW sensitivity.
Construction incorporates several short
engineering runs
with no astrophysical output as the detectors progress towards
acceptance. The aLIGO construction project ended in March 2015. The
construction of AdV was completed in early 2017. Construction of KAGRA
will be completed by mid-late 2019.
Commissioning:
improves the detectors’ performance with the goal of reaching
design sensitivity. Engineering runs in the commissioning phase allow us to
understand our detectors and analyses in an observational mode; these are not
intended to produce astrophysical results, but that does not preclude the
possibility of this happening.
2
Rather than proceeding directly to design
sensitivity before making astrophysical observations, commissioning is inter-
weaved with
observing runs
.
Observing:
begins when the detectors have reached (and can stably maintain) a
significantly improved sensitivity compared with previous operation. Observing
runs produce astrophysical results such as direct detections from certain GW
sources and upper limits on the rates or energetics of others. During the first two
observing runs (O1 and O2) a Memorandum Of Understanding (MOU) governed
the exchange of GW candidates between astronomical partners and the LIGO and
Virgo Collaborations. From the start of the third observing run (O3) GW event
candidates identified in low-latency are released immediately to the full
2
The detection of GW150914 occurred in the engineering run ER8 immediately preceding the formal
start of O1.
123
B. P. Abbott et al.
astronomical community (see Sect.
4
for details). KAGRA will become a part of
the global network with full data sharing in the latter half of O3.
Commissioning is a complex process which involves both scheduled improve-
ments to the detectors and tackling unexpected new problems. While our experience
makes us cautiously optimistic regarding the schedule for the advanced detectors, it
is not possible to make concrete predictions for sensitivity or duty cycle as a
function of time.
As a standard figure of merit for detector sensitivity, we use the range,
R
,
evaluated for CBCs consisting of representative masses. We define
V
as the
orientation-averaged spacetime volume surveyed per unit detector time, assuming a
matched-filter detection signal-to-noise ratio (SNR) threshold of 8 in a single
detector. The volume
V
corresponds to the comoving volume with the inclusion of a
ð
1
þ
z
Þ
factor to account for time dilation (redshifted volume
V
z
in Chen et al.
2017
). For a population of sources with a constant comoving source-frame rate
density,
V
multiplied by the rate density gives the detection rate of those sources by
the particular detector. The range
R
is obtained as
ð
4
p
=
3
Þ
R
3
¼
V
. For further insight
into the range, and a discussion of additional quantities such as the median and
average distances to sources, see (Chen et al.
2017
).
For unmodeled short-duration (
.
1 s) signals or bursts, we evaluate an approx-
imate sensitive luminosity distance determined by the total energy
E
GW
emitted in
GWs, the central frequency
f
0
of the burst, the detector noise power spectral density
S
ð
f
0
Þ
, and the single-detector SNR threshold
q
det
(Sutton
2013
):
D
’
G
2
p
2
c
3
E
GW
S
ð
f
0
Þ
f
2
0
q
2
det
1
=
2
:
ð
1
Þ
This distance is then corrected by the time dilation cosmology factor to obtain the
surveyed volume
V
, and the range
R
.
2.1 O1: aLIGO
O1 began on 18 September 2015 and ended on 12 January 2016. Data from the
surrounding engineering periods were of sufficient quality to be included in the
analysis, meaning that observational data was collected from 12 September 2015 to
19 January 2016. The run involved the Hanford (H) and Livingston (L) detectors
(Abbott et al.
2016e
; Martynov et al.
2016
). We aimed for a BNS range of
60–80 Mpc for both instruments (see Fig.
1
), and achieved a 80 Mpc range.
The localizations of the three BBH events detected during this run (GW150914,
GW151012,
3
GW151226), exhibit the characteristic broken arc for a two-detector
network (Abbott et al.
2016b
,
h
,
2018c
). GW150914 and GW151226 were shared
with partner astronomers soon after detection. Their poor localization (the 90%
credible regions are given in Table
3
) made the follow-up challenging (Abbott et al.
3
The significance of LVT151012, initially classified as a GW candidate, increased after reanalysis of the
O1 data with improved detection pipelines. It is now considered an astrophysical GW event (Abbott et al.
2018c
).
123
Prospects for observing and localizing GW transients with aLIGO, AdV and KAGRA