Living Rev Relativ (2018) 21:3
https://doi.org/10.1007/s41114-018-0012-9
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: 11 September 2017 / Accepted: 7 February 2018
© The Author(s) 2018
Abstract
We present possible observing scenarios for the Advanced LIGO, Advanced
Virgo and KAGRA gravitational-wave detectors over the next decade, with the inten-
tion of providing information to the astronomy community to facilitate planning for
multi-messenger astronomy with gravitational waves. We estimate the sensitivity of the
network to transient gravitational-wave signals, and study the capability of the network
to determine the sky location of the source. We report our findings for gravitational-
wave transients, with particular focus on gravitational-wave signals from the inspiral of
binary neutron star systems, which are the most promising targets for multi-messenger
astronomy. The ability to localize the sources of the detected signals depends on the
geographical distribution of the detectors and their relative sensitivity, and 90% cred-
ible regions can be as large as thousands of square degrees when only two sensitive
detectors are operational. Determining the sky position of a significant fraction of
detected signals to areas of 5–20 deg
2
requires at least three detectors of sensitivity
within a factor of
∼
2 of each other and with a broad frequency bandwidth. When
all detectors, including KAGRA and the third LIGO detector in India, reach design
sensitivity, a significant fraction of gravitational-wave signals will be localized to a
few square degrees by gravitational-wave observations alone.
This article is a revised version of
https://doi.org/10.1007/lrr-2016-1
Change summary
: Major revision, updated and expanded.
Change details
: Several updates to the document have been made. The most significant changes are the
inclusion of details regarding KAGRA, and results from O1 and O2, including GW170817, the first
detection with an unambiguous multi-messenger counterpart. The key differences are outlined in an
appendix.
B
(KAGRA Collaboration, LIGO Scientific Collaboration and Virgo Collaboration)
lvc.publications@ligo.org
*The full author list and affiliations are given at the end of paper.
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3 Page 2 of 57
Abbott, B. P. et al.
Keywords
Gravitational waves
·
Gravitational-wave detectors
·
Electromagnetic
counterparts
·
Data analysis
Contents
1 Introduction
...............................................
2 Commissioning and observing phases
..................................
2.1 Commissioning and observing roadmap
..............................
2.2 Past and envisioned observing schedule
..............................
3 Searches for gravitational-wave transients
................................
3.1 Detection and false alarm rates
...................................
3.2 Localization
.............................................
3.2.1 Localization of binary neutron star coalescences
.......................
3.2.2 Localization of bursts
.....................................
4 Observing scenarios
...........................................
4.1 2015–2016 run (O1): aLIGO
....................................
4.2 2016–2017 run (O2): aLIGO joined by AdV
............................
4.3 2018–2019 run (O3): aLIGO 120–170 Mpc, AdV 65–85 Mpc
...................
4.4 2020+ runs: aLIGO 190 Mpc, AdV 65–125 Mpc
..........................
4.5 2024+ runs: aLIGO (including LIGO-India) 190 Mpc, AdV 125 Mpc, KAGRA 140 Mpc
....
5 Conclusions
...............................................
A Changes between versions
........................................
A.1 Updates to detector commissioning
.................................
A.2 Updates to data analysis
.......................................
References
..................................................
1 Introduction
Advanced LIGO (aLIGO; Harry
2010
;Aasietal.
2015a
), Advanced Virgo (AdV;
Acernese et al.
2009
; Accadia et al.
2012
; 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.
2016j
), a signal from the coales-
cence 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.
2017k
). In this article, we describe the
currently projected schedule, sensitivity, and sky-localization accuracy for the GW-
detector network. We discuss the past and future planned sequence of observing runs
(designated O1, O2, O3, etc.) 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 for forthcoming 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
1
LIGO is short for Laser Interferometer Gravitational-Wave Observatory. KAGRA is named after the
Japanese word for traditional sacred music and dance for the gods
kagura
; the name has a secondary
meaning as an abbreviation for KAmioka GRavitational-wave Antenna. Virgo is not an acronym, and is not
written in all caps.
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Prospects for observing and localizing GW tansients...
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facilities (e.g., Abbott et al.
2016i
; Adrian-Martinez et al.
2016
; Albert et al.
2017c
;
Abbott et al.
2017k
).
The full science of ground-based GW detectors is broad (Abbott et al.
2016j
), and is
not covered in this article. We concentrate solely on candidate GW transient signals. We
place particular emphasis on the coalescence of BNS systems, which are the GW source
for which electromagnetic follow-up is most promising (Metzger and Berger
2012
;
Patricelli et al.
2016
; Paschalidis
2017
; Rosswog et al.
2017
; Metzger
2017
). How-
ever, we also mention BBHs, as they are the most commonly detected source (Abbott
et al.
2016c
,
2017f
). No electromagnetic emission is expected for vacuum BBH merg-
ers (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 circumbinary disc or a common
envelope (Bartos et al.
2017
; Woosley
2016
; Stone et al.
2017
). For more general
introductory articles on GW generation, detection and astrophysics, we point readers
to Blanchet (
2014
), Pitkin et al. (
2011
), Sathyaprakash and Schutz (
2009
).
Although our collaborations have amassed a great deal of experience with GW
detectors and analysis, it is still difficult to make predictions for both improvements
in search methods and for the rate of progress for detectors which are not yet fully
installed or operational.
The scenarios of detector sensitivity evolution and observing
times given here should not be considered as fixed or firm commitments.
As the detectors’ construction and commissioning progress, we intend to release
updated versions of this article. This is the third version of the article, written to
coincide with the close of the second observing run (O2) of the advanced-detector era.
Changes with respect to the previous version (Aasi et al.
2016
) are given in Appendix
A
.
Progress has been made in the commissioning of the detectors. We include projections
for KAGRA for the first time; we also include results from the first observing run (O1)
and currently available results from O2.
2 Commissioning and observing phases
We divide the development of the GW observatories into three components:
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 interferome-
ter 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 con-
struction of AdV was completed at the end of 2016. KAGRA will be operational
in its full configuration by early 2019.
Commissioning improves the detectors’ performance with the goal of reaching
the 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 possi-
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Abbott, B. P. et al.
bility of this happening.
2
Rather than proceeding directly to design sensitivity
before making astrophysical observations, commissioning is interleaved with
observing runs
.
Observing runs begin when the detectors have reached (and can stably maintain) a
significantly improved sensitivity compared with previous operation. Observ-
ing runs will produce astrophysical results, direct detections from some GW
sources and upper limits on the rates or energetics of others. During the first
two observing runs (O1 and O2), exchange of GW candidates with partners
outside the LIGO Scientific Collaboration (LSC) and the Virgo Collaboration
was governed by memoranda of understanding (MOUs; Abadie et al.
2012d
;
Aasi et al.
2013b
). From the start of the third observing run (O3), all GW event
candidates identified with high confidence will be released immediately to the
full astronomical community. KAGRA will become a part of the global network
with full data sharing, once sensitivities comparable with aLIGO and AdV are
achieved.
The progress in sensitivity as a function of time will influence the duration of the
observing runs that we plan at any stage. Commissioning is a complex process which
involves both scheduled improvements to the detectors and tackling unexpected new
problems. While our experience makes us cautiously optimistic regarding the schedule
for the advanced detectors, we are targeting an order of magnitude improvement in
sensitivity relative to the previous generation of detectors over a wider frequency
band. Consequently, it is not possible to make concrete predictions for sensitivity or
duty cycle as a function of time. We can, however, use our experience as a guide to
plausible scenarios for the detector operational states that will allow us to reach the
desired sensitivity. Unexpected problems could slow down the commissioning, but
there is also the possibility that progress may happen faster than predicted here. The
schedule of commissioning phases and observation runs will be driven by a cost–
benefit analysis of the time required to make significant sensitivity improvements.
More information on event rates could also change the schedule and duration of runs.
In Sect.
2.1
, we present the commissioning plans for the aLIGO, AdV and KAGRA
detectors. A summary of expected observing runs is in Sect.
2.2
.
2.1 Commissioning and observing roadmap
The anticipated strain sensitivity evolution for aLIGO, AdV and KAGRA is shown
in Fig.
1
. As a standard figure of merit for detector sensitivity, we use the range, the
volume- and orientation-averaged distance at which a compact binary coalescence
consisting of a particular mass gives a matched filter signal-to-noise ratio (SNR) of
8 in a single detector (Finn and Chernoff
1993
). We define
V
z
as the orientation-
averaged spacetime volume surveyed per unit detector time; for a population with a
constant comoving source-frame rate density,
V
z
multiplied by the rate density gives
the detection rate of those sources by the particular detector. We define the range
R
2
The detection of GW150914 occurred in the engineering run ER8 immediately preceding the formal start
of O1.
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Prospects for observing and localizing GW tansients...
Page 5 of 57 3
Frequency/Hz
Strain noise amplitude/Hz
−
1
/
2
Advanced LIGO
Early (2015– 16, 40 – 80 Mpc)
Mid (2016– 17, 80– 120 Mpc)
Late (2018– 19, 120– 170 Mpc)
Design (2020, 190 Mpc)
BNS-optimized (210 Mpc)
10
1
10
2
10
3
10
−
24
10
−
23
10
−
22
10
−
21
Frequency/Hz
Strain noise amplitude/Hz
−
1
/
2
Advanced Virgo
Early (2017, 20 – 65 Mpc)
Mid (2018– 19, 65 – 85 Mpc)
Late (2020– 21, 65– 115 Mpc)
Design (2021, 125 Mpc)
BNS-optimized (140 Mpc)
10
1
10
2
10
3
10
−
24
10
−
23
10
−
22
10
−
21
Frequency/Hz
Strain noise amplitude/Hz
−
1
/
2
KAGRA
Opening (2018– 19, 3 – 8 Mpc)
Early (2019 – 20, 8 – 25 Mpc)
Mid (2020– 21, 25 – 40 Mpc)
Late (2021– 22, 40– 140 Mpc)
Design (2022, 140 Mpc)
10
1
10
2
10
3
10
−
24
10
−
23
10
−
22
10
−
21
Fig. 1
Regions of aLIGO (
top left
), AdV (
top right
) and KAGRA (
bottom
) target strain sensitivities as a
function of frequency. The binary neutron star (BNS) range, the average distance to which these signals
could be detected, is given in megaparsec. Current notions of the progression of sensitivity are given for
early, mid and late commissioning phases, as well as the design sensitivity target and the BNS-optimized
sensitivity. While both dates and sensitivity curves are subject to change, the overall progression represents
our best current estimates
as the distance for which
(
4
π/
3
)
R
3
=
V
z
. In Table
1
we present values of
R
for
different detector networks and binary sources. For further insight into the range,
and a discussion of additional quantities such as the median and average distances to
sources, please see Chen et al. (
2017
). The BNS ranges, assuming two 1
.
4
M
neutron
stars, for the various stages of the expected evolution are provided in Fig.
1
, and the
BNS and BBH ranges are quoted in Table
1
.
There are currently two operational aLIGO detectors (Aasi et al.
2015a
). The orig-
inal plan called for three identical 4-km interferometers, two at Hanford (H1 and H2)
and one at Livingston (L1). In 2011, the LIGO Lab and IndIGO consortium in India
proposed installing one of the aLIGO Hanford detectors (H2) at a new observatory in
India (LIGO-India; Iyer et al.
2011
). In early 2015, LIGO Laboratory placed the H2
interferometer in long-term storage for use in India. The Government of India granted
in-principle approval to LIGO-India in February 2016.
The first observations with aLIGO have been made. O1 formally began 18 Septem-
ber 2015 and ended 12 January 2016; however, data from the surrounding engineering
periods were of sufficient quality to be included in the analysis, and hence the first
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Abbott, B. P. et al.
Table 1
Plausible target detector sensitivities
LIGO
Virgo
KAGRA
BNS
BBH
BNS
BBH
BNS
BBH
range/Mpc
range/Mpc
range/Mpc
range/Mpc
range/Mpc
range/Mpc
Early
40–80
415–775
20–65
220–615
8–25
8–250
Mid
80–120
775–1110
65–85
615–790
25–40
250–405
Late
120–170
1110–1490
65–115
610–1030
40–140
405–1270
Design
190
1640
125
1130
140
1270
The different phases match those in Fig.
1
. We quote the range, the average distance to which a signal could
be detected, for a 1
.
4
M
+1
.
4
M
binary neutron star (BNS) system and a 30
M
+30
M
binary black
hole (BBH) system
observations span 12 September 2015 to 19 January 2016. The run involved the H1 and
L1 detectors; the detectors were not at full design sensitivity (Abbott et al.
2016f
). We
aimed for a BNS range of 40–80 Mpc for both instruments (see Fig.
1
), and achieved
a 60–80 Mpc range. Subsequent observing runs have increasing duration and sensi-
tivity. O2 began 30 November 2016, transitioning from the preceding engineering run
which began at the end of October, and ended 25 August 2017. The achieved sen-
sitivity across the run was typically in the range 60–100 Mpc (Abbott et al.
2017f
).
Several improvements to the aLIGO detectors will be performed between O2 and O3,
including further mitigation of technical noises, increase of laser power delivered to
the interferometer, and installation of a squeezed vacuum source. Assuming that no
unexpected obstacles are encountered, the aLIGO detectors are expected to achieve a
190 Mpc BNS range by 2020. After the first observing runs, it might be desirable to
optimize the detector sensitivity for a specific class of astrophysical signals, such as
BNSs. The BNS range may then become 210 Mpc. The sensitivity for each of these
stages is shown in Fig.
1
.
The H2 detector will be installed in India once the LIGO-India Observatory is
completed, and will be configured to be identical to the H1 and L1 detectors. We refer
to the detector in this state as I1 (rather than H2). Operation at the same level as the
H1 and L1 detectors is anticipated for no earlier than 2024.
The AdV interferometer (V1; Accadia et al.
2012
) officially joined O2 on 1 August
2017. We aimed for an early step with sensitivity corresponding to a BNS range of
20–65 Mpc; however, AdV used steel wires to suspend the test masses, instead of fused
silica fibers. This limited the highest possible BNS range in O2 to 40–60 Mpc; the BNS
range achieved was 25–30 Mpc. Fused silica fibers will be reinstalled between O2 and
O3. Other improvements such as reduction of technical noises, laser power increase
and installation of a squeezed vacuum source will also be performed, bringing the
AdV BNS range to 65–85 Mpc in 2018–2019. A configuration upgrade at this point
will allow the range to increase to approximately 65–115 Mpc in 2020. The design
sensitivity, with a BNS range of 125 Mpc, is anticipated circa 2021. The corresponding
BNS-optimized range would be 140 Mpc. The sensitivity curves for the various AdV
configurations are shown in Fig.
1
.
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Prospects for observing and localizing GW tansients...
Page 7 of 57 3
The KAGRA detector (K1; Somiya
2012
; Aso et al.
2013
) is located at the Kamioka
underground site. The first operation of a detector in an initial configuration with a
simple Michelson interferometer occurred in March 2016 (Akutsu et al.
2018
). The
detector is now being upgraded to its baseline design configuration. Initial operation at
room temperature is expected in 2018. Subsequently, the detector will be cryogenically
cooled to reduce thermal noise. Early cryogenic observations may come in 2019–2020
with a range of 8–25 Mpc. Since sensitivity will lag behind that of aLIGO and AdV,
observing runs are planned to be short to allow commissioning to proceed as quickly as
possible; longer observing runs may begin when the detector nears design sensitivity
of 140 Mpc. The exact timing of observations has yet to be decided, but it is currently
intended to have a three-month observing run in early 2020, and a six-month run at the
start of 2021. The sensitivity curves for the various KAGRA commissioning stages
are shown in Fig.
1
.
GEO 600 (Lück et al.
2010
; Dooley et al.
2016
) will continue to operate as a GW
detector beyond O3 as techniques for improving the sensitivity at high frequency are
investigated (Affeldt et al.
2014
). At its current sensitivity, it is unlikely to contribute
to detections, but with a deliberate focus on high frequency narrow band sensitivity at
a few kilohertz, GEO 600 may contribute to the understanding of BNS merger physics,
as well as sky localization for such systems, by around 2021. In the meantime, it will
continue observing with frequent commissioning and instrument science investiga-
tions related to detuned signal recycling and novel applications of squeezed light, as
well as increasing the circulating power and levels of applied squeezing (Abadie et al.
2011
; Grote et al.
2013
;Aasietal.
2013a
; Brown et al.
2017
).
Finally, further upgrades to the LIGO and Virgo detectors, within their existing
facilities (e.g., Hild et al.
2012
; Miller et al.
2015
;Aasietal.
2015c
)aswellas
future third-generation observatories, for example, the Einstein Telescope (Punturo
et al.
2010
; Hild et al.
2011
; Sathyaprakash et al.
2012
) or Cosmic Explorer (Abbott
et al.
2017d
), are envisioned in the future. It is also possible that for some sources,
there could be multiband gravitational-wave observations, where detection with a
space-borne detector like the
Laser Interferometer Space Antenna
(
LISA
; Amaro-
Seoane et al.
2012
,
2013
) could provide early warning and sky localization (Sesana
2016
), as well as additional information on system parameters (Vitale
2016
), forma-
tion mechanisms (Nishizawa et al.
2016a
,
b
; Breivik et al.
2016
) and tests of general
relativity (Barausse et al.
2016
). These potential future developments are beyond the
scope of this paper.
2.2 Past and envisioned observing schedule
Keeping in mind the important caveats about commissioning affecting the scheduling
and length of observing runs, the following are plausible scenarios for the operation
of the ground-based GW detector network over the next decade:
2015–2016 (O1) A four-month run (12 September 2015–19 January 2016) with
the two-detector H1L1 network at early aLIGO sensitivity (60–80 Mpc BNS
range). This is now complete.
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