of 39
Living Rev. Relativity
,
19
, (2016), 1
DOI 10.1007/lrr-2016-1
,)6).' 2%6)%73
INRELATIVITY
Prospects for Observing and Localizing Gravitational-Wave
Transients with Advanced LIGO and Advanced Virgo
Abbott, B. P. et al.
The LIGO Scientific Collaboration and the Virgo Collaboration
(The full author list and affiliations are given at the end of paper.)
email: lsc-spokesperson@ligo.org, virgo-spokesperson@ego-gw.it
Accepted: 22 January 2016
Published: 8 February 2016
Abstract
We present a possible observing scenario for the Advanced LIGO and Advanced Virgo
gravitational-wave detectors over the next decade, with the intention of providing infor-
mation to the astronomy community to facilitate planning for multi-messenger astronomy
with gravitational waves. We determine the expected 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 considered
the most promising 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% credible 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
deg
2
to 20
deg
2
will require at least three detectors of sensitivity
within a factor of
2 of each other and with a broad frequency bandwidth. Should the third
LIGO detector be relocated to India as expected, a significant fraction of gravitational-wave
signals will be localized to a few square degrees by gravitational-wave observations alone.
Keywords:
Gravitational waves, Gravitational-wave detectors, Electromagnetic counterparts,
Data analysis
c
The Author(s). This article is distributed under a
Creative Commons Attribution 4.0 International License.
http://creativecommons.org/licenses/by/4.0/
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Living Reviews in Relativity
is a peer-reviewed open access journal published by the Springer
International Publishing AG, Gewerbestrasse 11, 6330 Cham, Switzerland. ISSN 1433-8351.
This article is distributed under the terms of the Creative Commons Attribution 4.0 International
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consent of the original copyright holders.
Abbott, B. P. et al.,
“Prospects for Observing and Localizing Gravitational-Wave Transients with Advanced LIGO
and Advanced Virgo”,
Living Rev. Relativity
,
19
, (2016), 1.
DOI 10.1007/lrr-2016-1.
Article Revisions
Living Reviews
supports two ways of keeping its articles up-to-date:
Fast-track revision.
A fast-track revision provides the author with the opportunity to add short
notices of current research results, trends and developments, or important publications to the
article. A fast-track revision is refereed by the responsible subject editor. If an article has
undergone a fast-track revision, a summary of changes will be listed here.
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A major update will include substantial changes and additions and is subject to
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For detailed documentation of an article’s evolution, please refer to the history document of the
article’s online version at
http://dx.doi.org/10.1007/lrr-2016-1
.
Contents
1 Introduction
5
2 Commissioning and Observing Phases
6
2.1 Commissioning and observing roadmap . . . . . . . . . . . . . . . . . . . . . . . .
6
2.2 Envisioned observing schedule . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8
3 Searches for Gravitational-Wave Transients
9
3.1 Detection and false alarm rates . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10
3.2 Sky localization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
3.2.1
Localization of binary neutron stars . . . . . . . . . . . . . . . . . . . . . .
13
3.2.2
Localization of bursts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
14
4 Observing Scenario
18
4.1 2015 – 2016 run (O1): aLIGO 40 – 80 Mpc . . . . . . . . . . . . . . . . . . . . . . .
19
4.2 2016 – 2017 run (O2): aLIGO 80 – 120 Mpc, AdV 20 – 60 Mpc . . . . . . . . . . . .
19
4.3 2017 – 2018 run (O3): aLIGO 120 – 170 Mpc, AdV 60 – 85 Mpc . . . . . . . . . . .
20
4.4 2019+ runs: aLIGO 200 Mpc, AdV 65 – 130 Mpc . . . . . . . . . . . . . . . . . . .
20
4.5 2022+ runs: aLIGO (including India) 200 Mpc, AdV 130 Mpc . . . . . . . . . . .
20
5 Conclusions
23
A Changes Between Versions
25
A.1 Updates to detector commissioning . . . . . . . . . . . . . . . . . . . . . . . . . . .
25
A.2 Updates to sky localization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
25
References
26
List of Tables
1
Summary of a plausible observing schedule, expected sensitivities, and source local-
ization with the advanced LIGO and Virgo detectors. . . . . . . . . . . . . . . . .
22
Prospects for Observing and Localizing GW Transients with aLIGO and AdV
5
1 Introduction
Advanced LIGO (aLIGO) [
61
,
9
] and Advanced Virgo (AdV) [
24
,
23
,
25
] are kilometer-scale
gravitational-wave (GW) detectors that are expected to yield direct observations of GWs. In this
article we describe the currently projected schedule, sensitivity, and sky-localization accuracy for
the GW-detector network. We discuss the proposed 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 the upcoming 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.
The full science of aLIGO and AdV is broad [
8
], and is not covered in this article. We concentrate
solely on candidate GW transient signals. We place particular emphasis on the coalescence of
binary neutron-star (BNS) systems, which are the GW source for which electromagnetic follow-up
seems most promising. For more general introductory articles on GW generation, detection and
astrophysics, we point readers to [33, 87, 94].
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
LIGO and Virgo detector sensitivity evolution and observing times given here represent our best
estimates as of January 2016. They 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 second version of the article, written to coincide with the first observing
run (O1) of the advanced-detector era. Changes with respect to the first version [
4
] are given in
Appendix A. Progress has been made in the commissioning of the detectors, and the plausible
observing scenarios are largely the same; the predicted sky-localization accuracies have been updated
following improvements in parameter estimation.
Living Reviews in Relativity
DOI 10.1007/lrr-2016-1
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Abbott, B. P. et al. (The LIGO Scientific Collaboration and the Virgo Collaboration)
2 Commissioning and Observing Phases
We divide the development of the aLIGO and AdV observatories into three components:
Construction
includes the installation and testing of the detectors. This phase ends with
accep-
tance
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 (on time and
on budget) in March 2015. The acceptance of AdV is expected in the first part of 2016.
Commissioning
takes the detectors from their configuration at acceptance through progressively
better sensitivity to the design advanced-generation detector 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. Rather than proceeding directly to design sensitivity
before making astrophysical observations, commissioning is interleaved with
observing runs
of
progressively better sensitivity.
Observing
runs begin when the detectors have reached (and can stably maintain) a significantly
improved sensitivity compared with previous operation. It is expected that observing runs
will produce astrophysical results, including upper limits on the rate of sources and possibly
the first detections of GWs. During this phase, exchange of GW candidates with partners
outside the LIGO Scientific Collaboration (LSC) and the Virgo Collaboration will be governed
by memoranda of understanding (MOUs) [
17
,
2
]. After the first four detections, we expect
free exchange of GW event candidates with the astronomical community and the maturation
of GW astronomy.
The progress in sensitivity as a function of time will affect the duration of the runs that we plan
at any stage, as we strive to minimize the time to successful GW observations. 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. As the detectors begin to be commissioned, information on the cost in time and benefit in
sensitivity will become more apparent and drive the schedule of runs. More information on event
rates, including the first detection, could also change the schedule and duration of runs.
In Section 2.1 we present the commissioning plans for the aLIGO and AdV detectors. A
summary of expected observing runs is in Section 2.2.
2.1 Commissioning and observing roadmap
The anticipated strain sensitivity evolution for aLIGO and AdV is shown in Figure 1. A standard
figure of merit for the sensitivity of an interferometer is the BNS
range
BNS
: the volume- and
orientation-averaged distance at which a compact binary coalescence consisting of two 1
.
4
neutron stars gives a matched filter signal-to-noise ratio (SNR) of 8 in a single detector [
58
].
1
The
1
Another often quoted number is the BNS
horizon
– the distance at which an optimally oriented and located
BNS system would be observed with an SNR of 8. The horizon is a factor of 2
.
26 larger than the range [
58
,
13
,
20
].
Living Reviews in Relativity
DOI 10.1007/lrr-2016-1
Prospects for Observing and Localizing GW Transients with aLIGO and AdV
7
BNS ranges for the various stages of aLIGO and AdV expected evolution are also provided in
Figure 1.
Frequency/Hz
Strain noise amplitude/Hz
1
/
2
Advanced LIGO
Early (2015 – 16, 40 – 80 Mpc)
Mid (2016 – 17, 80 – 120 Mpc)
Late (2017 – 18, 120 – 170 Mpc)
Design (2019, 200 Mpc)
BNS-optimized (215 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 (2016–17, 20 – 60 Mpc)
Mid (2017–18, 60 – 85 Mpc)
Late (2018–20, 65 – 115 Mpc)
Design (2021, 130 Mpc)
BNS-optimized (145 Mpc)
10
1
10
2
10
3
10
24
10
23
10
22
10
21
Figure 1:
aLIGO (
left
) and AdV (
right
) target strain sensitivity 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 final 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.
The commissioning of aLIGO is well under way. The original 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) [
64
]. As of early 2015, LIGO Laboratory has placed
the H2 interferometer in long-term storage for possible use in India. Funding for the Indian portion
of LIGO-India is in the final stages of consideration by the Indian government.
Advanced LIGO detectors began taking sensitive data in August 2015 in preparation for the
first observing run. O1 formally began 18 September 2015 and ended 12 January 2016. It involved
the H1 and L1 detectors; the detectors were not at full design sensitivity. We aimed for a BNS
range of 40 – 80
Mpc
for both instruments (see Figure 1), and both instruments were running with a
60 – 80
Mpc
range. Subsequent observing runs will have increasing duration and sensitivity. We aim
for a BNS range of 80 – 170
Mpc
over 2016 – 2018, with observing runs of several months. Assuming
that no unexpected obstacles are encountered, the aLIGO detectors are expected to achieve a
200
Mpc
BNS range circa 2019. After the first observing runs, circa 2020, 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 215 Mpc. The sensitivity for each of these stages is shown in Figure 1.
As a consequence of the planning for the installation of one of the LIGO detectors in India, the
installation of the H2 detector has been deferred. This detector will be reconfigured to be identical
to H1 and L1 and will be installed in India once the LIGO-India Observatory is complete. The final
schedule will be adopted once final funding approvals are granted. If project approval comes soon,
site development could start in 2016, with installation of the detector beginning in 2020. Following
this scenario, the first observing runs could come circa 2022, and design sensitivity at the same
level as the H1 and L1 detectors is anticipated for no earlier than 2024.
The time-line for the AdV interferometer (V1) [
23
] is still being defined, but it is anticipated
that in 2016 AdV will join the aLIGO detectors in their second observing run (O2). Following an
early step with sensitivity corresponding to a BNS range of 20 – 60
Mpc
, commissioning is expected
Living Reviews in Relativity
DOI 10.1007/lrr-2016-1
8
Abbott, B. P. et al. (The LIGO Scientific Collaboration and the Virgo Collaboration)
to bring AdV to a 60 – 85
Mpc
in 2017 – 2018. A configuration upgrade at this point will allow the
range to increase to approximately 65 – 115
Mpc
in 2018 – 2020. The final design sensitivity, with a
BNS range of 130 Mpc, is anticipated circa 2021. The corresponding BNS-optimized range would
be 145 Mpc. The sensitivity curves for the various AdV configurations are shown in Figure 1.
The GEO 600 [
76
] detector will likely be operational in the early to middle phase of the AdV and
aLIGO observing runs, i.e. 2015 – 2017. The sensitivity that potentially can be achieved by GEO
in this time-frame is similar to the AdV sensitivity of the early and mid scenarios at frequencies
around 1
kHz
and above. GEO could therefore contribute to the detection and localization of
high-frequency transients in this period. However, in the
100
Hz
region most important for BNS
signals, GEO will be at least 10 times less sensitive than the early AdV and aLIGO detectors, and
will not contribute significantly.
Japan has begun the construction of an advanced detector, KAGRA [
100
,
28
]. KAGRA is
designed to have a BNS range comparable to AdV at final sensitivity. We do not consider KAGRA
in this article, but the addition of KAGRA to the worldwide GW-detector network will improve
both sky coverage and localization capabilities beyond those envisioned here [96].
Finally, further upgrades to the LIGO and Virgo detectors, within their existing facilities (e.g.,
[
63
,
78
,
11
]) as well as future underground detectors (for example, the Einstein Telescope [
93
]) are
envisioned in the future. These affect both the rates of observed signals as well as the localizations
of these events, but this lies beyond the scope of this paper.
2.2 Envisioned observing schedule
Keeping in mind the important caveats about commissioning affecting the scheduling and length of
observing runs, the following is a plausible scenario for the operation of the LIGO–Virgo network
over the next decade:
2015 – 2016 (O1)
A four-month run (beginning 18 September 2015 and ending 12 January 2016)
with the two-detector H1L1 network at early aLIGO sensitivity (40 – 80 Mpc BNS range).
2016 – 2017 (O2)
A six-month run with H1L1 at 80 – 120 Mpc and V1 at 20 – 60 Mpc.
2017 – 2018 (O3)
A nine-month run with H1L1 at 120 – 170 Mpc and V1 at 60 – 85 Mpc.
2019+
Three-detector network with H1L1 at full sensitivity of 200
Mpc
and V1 at 65 – 115
Mpc
.
2022+
H1L1V1 network at full sensitivity (aLIGO at 200
Mpc
, AdV at 130
Mpc
), with other
detectors potentially joining the network. Including a fourth detector improves sky localiza-
tion [
72
,
109
,
79
,
91
], so as an illustration we consider adding LIGO-India to the network.
2022 is the earliest time we imagine LIGO-India could be operational, and it would take
several more years for it to achieve full sensitivity.
This time-line is summarized in Figure 2. The observational implications of this scenario are
discussed in Section 4.
Living Reviews in Relativity
DOI 10.1007/lrr-2016-1