Observing Capabilities

This section summarizes the observing capabilities of the global gravitational-wave detector network as of early 2019. This as a quick reference to the anticipated commissioning and observing schedule, sensitivity to gravitational-wave transients, and sky localization accuracy, as described in the following external documents:

• White Paper [1] on gravitational-wave data analysis and astrophysics

• Living Review [2] (see preprint of revised version [3]) on prospects for observing and localizing gravitational-wave transients with Advanced LIGO, Advanced Virgo, and KAGRA

• Current O3 Schedule [4]

Timeline

The gravitational-wave observing schedule is divided into Observing Runs or epochs of months to years of operation at fixed sensitivity, down time for construction and commissioning, and transitional Engineering Runs between commissioning and observing runs. The long-term observing schedule is shown below. Since BNS mergers are a well-studied class of gravitational-wave signals, this figure gives the BNS range for each observing run.

Engineering Run 14 (ER14) started on 2019-03-04. The transition into Observing Run 3 (O3) occurred on 2019-04-01. O3 had been scheduled to end on 2020-04-30, but was suspended early on March 27, 2020, due to the COVID-19 pandemic [5]. When it becomes safe to resume normal activities, there may be a short period of additional O3 observations before an extended shutdown to prepare for the next observing run (O4). Updates will be posted in this document as they become available.

During O3, we expect that three facilities (LHO, LLO, and Virgo) will observe for one year. It is possible that the Japanese KAGRA detector may come online and become part of the international gravitational-wave network at some point during O3. The near-term observing schedule is shown below, reproduced from [4].

Live Status

There are a handful of public web pages that report live status of the LIGO/Virgo detectors and alert infrastructure.

• Detector Status Portal: Daily summary of detector performance.

  https://www.gw-openscience.org/detector_status/
• GWIStat: Real-time detector up/down status.

  https://ldas-jobs.ligo.caltech.edu/~gwistat/gwistat/gwistat.html
• LIGO Data Grid Status: Live dashboard showing up/down status of the detectors and online analyses. Status of the LIGO/Virgo alert pipeline is indicated by the “EMFollow” box.

  https://monitor.ligo.org/gwstatus

Sensitivity

The following O3 projections are adapted from the preprint version of the Living Review [3] on prospects for observing and localizing gravitational-wave transients with Advanced LIGO, Advanced Virgo, and KAGRA. The table below gives the range of each individual detector for BNS, NSBH, and BBH mergers, and unmodeled bursts.

Detector	Range (Mpc)
	BNS	NSBH	BBH	Burst
LIGO	110–130	190-240	990-1200	80–90
Virgo	50	90	500	35
KAGRA	8–25	15-45	80-260	5-25

These ranges are given for the following fiducial signals:

BNS

A merger of two \(1.4 M_\odot\) NSs.

NSBH

A merger of a \(10 M_\odot\) BH and a \(1.4 M_\odot\) NS.

BBH

A merger of two \(30 M_\odot\) BHs.

Burst

A monochromatic signal at a frequency of 140 Hz carrying an energy of \(E_\mathrm{GW}=10^{-2} M_\odot c^2\).

Note

The range is defined in relation to the sensitive volume, or the surveyed space-time volume per unit detector time. The range is neither a luminosity distance nor a comoving distance.

Detection Rate and Localization Accuracy

Here we provide predicted detection rates, distances, and localization uncertainties for BNS, NSBH, and BBH mergers in O3 and O4, based on a Monte Carlo simulation of detection and localization of events in O3 and O4. Details of the simulation are described in [3].

Sky localization FITS files from these simulations are provided at https://git.ligo.org/emfollow/obs-scenarios-2019-fits-files.

Summary Statistics

The table below summarizes the predicted detection rate and sky localization accuracy in O3 and O4. All values are given as a 5% to 95% confidence intervals.

Observing run	Network	Source class
		BNS	NSBH	BBH
Merger rate per unit comoving volume per unit proper time (Gpc-3 year-1, log-normal uncertainty)
		110-3840	0.60-1000	25-109
Sensitive volume: detection rate / merger rate (Gpc3, Monte Carlo uncertainty)
O3	HLV	\(0.0033 ^{+0.00028} _{-0.00026}\)	\(0.02 ^{+0.0016} _{-0.0015}\)	\(0.34 ^{+0.026} _{-0.025}\)
O3	HLVK	\(0.0034 ^{+0.00028} _{-0.00027}\)	\(0.020 ^{+0.0016} _{-0.0015}\)	\(0.35 ^{+0.026} _{-0.025}\)
O4	HLVK	\(0.016 ^{+0.0014} _{-0.0013}\)	\(0.092 ^{+0.0077} _{-0.0072}\)	\(1.5 ^{+0.10} _{-0.096}\)
Annual number of detections (log-normal merger rate uncertainty \(\times\) Poisson counting uncertainty)
O3	HLV	\(1 ^{+12} _{-1}\)	\(0 ^{+19} _{-0}\)	\(17 ^{+22} _{-11}\)
O3	HLVK	\(1 ^{+12} _{-1}\)	\(0 ^{+19} _{-0}\)	\(18 ^{+22} _{-12}\)
O4	HLVK	\(10 ^{+52} _{-10}\)	\(1 ^{+91} _{-1}\)	\(79 ^{+89} _{-44}\)
Median luminosity distance (Mpc, Monte Carlo uncertainty)
O3	HLV	\(110 ^{+3.7} _{-4.6}\)	\(210 ^{+6.6} _{-8.2}\)	\(640 ^{+29} _{-19}\)
O3	HLVK	\(110 ^{+3.8} _{-4.2}\)	\(210 ^{+7.7} _{-6.9}\)	\(630 ^{+25} _{-23}\)
O4	HLVK	\(170 ^{+6.3} _{-4.8}\)	\(330 ^{+7.0} _{-13}\)	\(990 ^{+35} _{-29}\)
Median 90% credible area (deg2, Monte Carlo uncertainty)
O3	HLV	\(270 ^{+34} _{-20}\)	\(330 ^{+24} _{-31}\)	\(280 ^{+30} _{-23}\)
O3	HLVK	\(190 ^{+36} _{-30}\)	\(240 ^{+37} _{-44}\)	\(220 ^{+33} _{-24}\)
O4	HLVK	\(33 ^{+4.9} _{-5.3}\)	\(50 ^{+8.0} _{-8.4}\)	\(41 ^{+7.2} _{-5.7}\)
Median 90% credible comoving volume (103 Mpc3, Monte Carlo uncertainty)
O3	HLV	\(120 ^{+19} _{-24}\)	\(860 ^{+150} _{-150}\)	\(16000 ^{+2200} _{-2500}\)
O3	HLVK	\(79 ^{+27} _{-19}\)	\(560 ^{+190} _{-160}\)	\(11000 ^{+2300} _{-2300}\)
O4	HLVK	\(52 ^{+9.9} _{-9.1}\)	\(430 ^{+100} _{-78}\)	\(7700 ^{+1500} _{-920}\)

Merger rate per unit comoving volume per unit proper time is the astrophysical rate of mergers in the reference frame that is comoving with the Hubble flow. It is averaged over a distribution of masses and spins that is assumed to be non-evolving.

Caution

The merger rate per comoving volume should not be confused with the binary formation rate, due to the time delay between formation and merger.

It should also not be confused with the merger rate per unit comoving volume per unit observer time. If the number density per unit comoving volume is \(n = dN / dV_C\), and the merger rate per unit proper time \(\tau\) is \(R = dn/d\tau\), then the merger rate per unit observer time is \(R / (1 + z)\), with the factor of \(1 + z\) accounting for time dilation.

See [9] for further discussion of cosmological distance measures as they relate to sensitivity figures of merit for gravitational-wave detectors.

For BNS and BBH, the merger rate is inferred from fitting the observed population of LIGO/Virgo events in O1 and O2 [6][7]. For NSBH, the merger rate is taken from [8]. The quoted confidence interval assumes that the uncertainty in the rate has a log-normal distribution.

Sensitive volume is the quotient of the rate of detected events per unit observer time and the merger rate per unit comoving volume per unit proper time. The definition is given in the glossary entry for sensitive volume. To calculate the detection rate, multiply the merger rate by the sensitive volume.

The quoted confidence interval represents the uncertainty from the Monte Carlo simulation.

Annual number of detections is the number of detections in one calendar year of observation. The quoted confidence interval incorporates both the log-normal distribution of the merger rate and Poisson counting statistics, but does not include the Monte Carlo error (which is negligible compared to the first two sources of uncertainty).

The remaining sections all give median values over the population of detectable events.

Luminosity distance is the luminosity distance in Mpc of detectable events. The quoted confidence interval represents the uncertainty from the Monte Carlo simulation.

90% credible area is the area in deg\(^2\) of the smallest (not necessarily simply connected) region on the sky that has a 90% chance of containing the true location of the source.

90% credible volume is the comoving volume enclosed in the smallest region of space that has a 90% chance of containing the true location of the source.

Cumulative Histograms

Below are cumulative histograms of the 90% credible area, distance, and 90% credible comoving volume of detectable events in O3 and O4.

Distribution of 90% credible areas for simulated compact binary merger events from O3 and O4. The best-localized BNS merger (GW170817) and BBH merger (GW170818) as of this writing are shown at top as black tick marks.

Distribution of luminosity distances for simulated compact binary merger events from O3 and O4. The best-localized BNS merger (GW170817) and BBH merger (GW170818) as of this writing are shown at top as black tick marks.

Distribution of 90% credible comoving volumes for simulated compact binary merger events from O3 and O4. The best-localized BNS merger (GW170817) and BBH merger (GW170818) as of this writing are shown at top as black tick marks.

[1]

LIGO Scientific Collaboration & Virgo Collaboration 2019, The LSC-Virgo White Paper on Gravitational Wave Data Analysis and Astrophysics. LIGO-T1900541-v2

[2]

Abbott, B. P., Abbott, R., Abbott, T. D., et al. 2018, Living Rev. Rel., 21, 3. doi:10.1007/s41114-018-0012-9

[3] (1,2,3)

LIGO Scientific Collaboration & Virgo Collaboration 2019, Prospects for Observing and Localizing Gravitational-Wave Transients with Advanced LIGO, Advanced Virgo and KAGRA. arXiv:1304.0670

[4] (1,2)

LIGO Scientific Collaboration & Virgo Collaboration 2019, Current O3 Schedule. LIGO-G1901531-v3

[5]

LIGO Laboratory 2020, LIGO Ending O3 Observations. LIGO-L2000133-v3

[6]

LIGO Scientific Collaboration & Virgo Collaboration 2019, Binary Black Hole Population Properties Inferred from the First and Second Observing Runs of Advanced LIGO and Advanced Virgo. Astropys. J. Lett., 882, 24. doi:10.3847/2041-8213/ab3800

[7]

LIGO Scientific Collaboration & Virgo Collaboration 2019, GWTC-1: A Gravitational-Wave Transient Catalog of Compact Binary Mergers Observed by LIGO and Virgo during the First and Second Observing Runs. Phys. Rev. X, 9, 031040. doi:10.1103/PhysRevX.9.031040

[8]

LIGO Scientific Collaboration & Virgo Collaboration 2010, Predictions for the rates of compact binary coalescences observable by ground-based gravitational-wave detectors. Class. Quantum Grav., 27, 173001. doi:10.1088/0264-9381/27/17/173001

[9]

Chen, H.-Y., Holz, D. E., et al. 2017, Distance measures in gravitational-wave astrophysics and cosmology. arXiv:1709.08079