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GW170817: Observation of Gravitational Waves
from a Binary Neutron Star Inspiral
Kipp Cannon

Presented at The University of Tokyo, December 19, 2017

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Technical Introduction

I

Gravitational-wave antennas are like radio antennas.

I

Observatory provides digitized record of projection of field strength
onto detector.

INo (real) detector has infinite bandwidth: something sets a
low-frequency cut-off and something sets a high-frequency cut-off.

IFor ground-based detectors: seismic noise and shot noise (laser fieldquantization).

Noise floor set by radiation-pressure (Brownian motion of mirror).
I Compact object merger signals are frequency-swept sinuoids;IIstart at low frequency, move to high.

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Technical Introduction

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Technical Introduction
Strain Noise Spectral Density for 47.9 M –36.6 M

Merger Candidate at 1126259462.43 GPS

10−37
H1 (2241 Mpc Horizon)
SNR = 20.1
L1 (1838 Mpc Horizon)
SNR = 13.5

10−38

Spectral Density (strain2 /Hz)

10−39
10−40
10−41
10−42
10−43
10−44
10−45
10−4610−47101102103

Frequency (Hz)LSC

Technical Introduction
IRaw data must be transformed to gravitational strain —“calibration”.

Processed, to identify potential signals — using a “detectionpipeline”.
I Triggers a swarm of automated “follow-up” activities, including:I

low-latency sky location and distance posterior PDF — computed by”Bayestar”;
I high-latency MCMC-based Bayesian posteriors on intrinsic andextrinsic parameters — “parameter estimation” / “PE”.

II

Bayestar’s approximation: pipeline has measured the correct intrinsic(masses, etc.) and extrinsic (SNR, etc.) signal parameters.

IAllows sky location posterior PDF to be derived exclusively frompipeline’s complex signal-to-noise ratio time series — no waveformgeneration required.

LSCDetection SystemI“gstlal” detection
system uses consumer
multimedia processing
software called“gstreamer”.

I< 20 s of latency fromphase centre to alert
database.

I

Latency dominated bydata calibration anddistribution: ∼ 14 s

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DetectionI

On August 17, 2017, at
about 21:50 JST I wasjust leaving a yakitori
shop in Sendagaya.

I

My phone’s alarm wastriggered.

I

What I saw was this →IA compact object
merger,Iwith masses of 1.5 and1.2 × our sun,I

and a false-alarm rate(FAR) of about 1 in
10,000 years.

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Detection
At the GraceDB web site (gravitational-wave candidate event database)
I A trigger from the Fermi gamma-ray burst monitor following thegravitational-wave candidate by 2 seconds.
I 2 seconds ← GW-EM delay = First Discovery! (actually severaldiscoveries)

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Detection
At the GraceDB web site (gravitational-wave candidate event database)

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Detection
At the GraceDB web site (gravitational-wave candidate event database)

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Detection
At the GraceDB web site (gravitational-wave candidate event database)

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Detection
At the GraceDB web site (gravitational-wave candidate event database)LSCDetection
At the GraceDB web site (gravitational-wave candidate event database)

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Detection
At the GraceDB web site (gravitational-wave candidate event database)

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Detection
At the GraceDB web site (gravitational-wave candidate event database)

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D. A. Coulter, et al., “Swope Supernova Survey 2017a (SSS17a), theoptical counterpart to a gravitational wave source” Science, October(2017). Photo taken August 18, 08:33 JST, reported 10:05:23 JST.

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LSCDetection SystemI“gstlal” detection
system uses consumer
multimedia processing
software called“gstreamer”.

I< 20 s of latency fromphase centre to alert
database.

I

Latency dominated bydata calibration anddistribution: ∼ 14 s

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GlitchI

Blinded online detection
system to signal in L1.

I

Glitch blinding causedby known bug we hadchosen to not fix.

I

Did not expect to be so(un)lucky.

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GlitchI

Already had fix in thenext version of the code(for O3), but not in thetagged version runningonline.

I

False-alarm rate boundof < 1/(106 year) isobtained with the fix
applied, to allow L1
signal to be identified.

I

Extremely significant
without any analysis
changes: detection
claim does not rely on
post facto analysis
changes.

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GlitchI

Parameter estimation
used a Bayesian glitch
structure inference
system to estimate thetime structure andremove.

I

Simulated signals added
to data with other,similar, glitches,I

glitch subtraction foundto introduce biases
smaller than intrinsic
parameter uncertainties.

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I... For the parametersthat have been
published.

I

Tests of GR still inprogress, and glitch
subtraction still being
validated.

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Gravitational Wave Signal

I

Unlike electromagnetic sources, as GW sources compact object
mergers have very few degrees of freedom.

I

Waveform predictable from first-principles

Allows many properties of the system to be inferred directly fromthe GW signal.
I The precise location, obtained from the optical counterpart,
constrains the model: very important.
II

Allows distance, orbit inclination, and tidal deformability of thecomponent masses to be extracted

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Gravitational Wave Signal
Strain Noise Spectral Density for 1.53 M –1.24 M

Merger Candidate at 1187008882.45 GPS

10−35
10−36

H1 (103 Mpc Horizon)
SNR = 14.5
L1 (Off, Last Seen With 210.8 Mpc Horizon)

10−37

V1 (Off, Last Seen With 55.34 Mpc Horizon)

Spectral Density (strain2 /Hz)

10−38
10−39
10−40
10−41
10−42
10−43
10−44
10−45
10−46
10−47
10−48

101

102

Frequency (Hz)

103

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Details

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Details

IDistance: 85 × 106 light year–160 × 106 light year

IPrimary mass: 1.36 M –2.26 M

I

Secondary mass: 0.86 M –1.36 M

I

Radius of a 1.4 M

I

Spin of stars: ??? cannot tell.

I−3 −1Merger rate: 1540+3200
a−1230 Gpc

Ineutron star: ≤ 14 km

Implies there should exist a detectable stochastic background ofgravitational waves from distant neutron star collisions.

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I

Neutron star tidal deformability constraints. Left panel imposes noconstraint on spins, right panel requires spins to be small.

I

Bottom-left corner is the black-hole limit.

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GRBs are neutron star collisions.

I

After travelling together for 130 million years, the GWs and theγ-rays arrived within 2 s of each other ...

I... so gravity travels at the speed of light to 1 part in 1015 -ish.

I

The delay from NS merger to γ-ray flash is 2 s.

I

Light emission consistent with the signature of heavy element
formation from the debris.

I→ about 1/2 of the heavy elements in the universe are from NScollisions.

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Prospects for future
observations have beenchanging rapidly.

IThe “Observing Scenarios
Document”, arXiv:1304.0670v4.

I

Last updated late this summer,I

accepted for publication, but
still not in print.

I

Discusses the possibility of a
BNS observation as a future
hypothetical possibility,I

Was osbolete before it was evenposted to arXiv on Sept 8th.

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O1 Results
Strain Noise Spectral Density for 47.9 M –36.6 M

Merger Candidate at 1126259462.43 GPS

10−37
H1 (2241 Mpc Horizon)
SNR = 20.1
L1 (1838 Mpc Horizon)
SNR = 13.5

10−38

Spectral Density (strain2 /Hz)

10−39
10−40
10−41
10−4210−4310−4410−45
10−4610−47101102103

Frequency (Hz)LSC

O1 Results

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O1 Results

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Future Science RunsLSC

Future Science Runs

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What Does Observing Scenarios Document Say?
BBH detection rate prediction.

from arXiv:1606.04856 (not Observing scenarios, still best version of thisplot)

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What Does Observing Scenarios Document Say?

BNS detection rate prediction.
III

I

Today, most up-to-date informatoin is in Phys. Rev. Lett.,
119:161101 (2017) (report of GW170817 discovery)−3 −1
BNS merger rate 1500+3200−1220 Gpc a .

≈ what we have been calling the “plausible” rate estimate prior tothe detection. (1000 Gpc−3 a−1 )Translates to 30 detections above SNR 8 in two detectors per year inlate advanced detector era.

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Early Warning

det ect ions yr − 1103
102
101
10010-1
103

102 101 100 10-1 10-2t ime before coalescence (s)LSCPressing Questions: What we can doIIs GW170817 the tip of the iceberg? Is there a previously
undiscovered population of nearby GRBs?

IWas GW170817 in fact a “GRB”?

INS-BH detections?

I

Lots more 30+30 Msun black hole collisions: what is spin
distribution, can we say if they are from stars or maybe primordial?

I

Are there compact objects below 1 Msun?

I

Are there compact objects between 3 Msun and 5 Msun?

I

Stochastic GW flux from compact object mergers: how long ago did
the mergers begin?

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Pressing Questions: What we cannot do (yet)I

Tests of GR: need predictions from alternative theories of gravity.

IE.g., evidence of extra dimensions in BH merger waveforms?

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Pressing Questions: What we cannot do (yet)IHigh performance waveform models: PE now computationallylimited, e.g., could constrain tidal deformability better if PE’s
MCMC samplers could compute overlaps starting at 10 Hz insteadof 24 Hz.

IGW170817’s tidal deformability parameters could be better
constrained with integrations starting at lower frequency.

But PE runs would not complete in a practical amount of time.
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m1 m2I GW170817’s Mchirp = (m1 + m2 )uncertainty
2(m1 +m2 )dominated by waveform model systematics.
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Pressing Questions: What we cannot do (yet)Correct waveform systematic errors: there is a gap between end ofvalidity of post-Newtonian expansion and start of validity of NR
simulations.
I Hybridization is intrinsically biased until that gap can be closed.
I

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Pressing Questions: What we cannot do (yet)See, e.g, MacDonald, et al., “Suitability of hybrid gravitational
waveforms for unequal-mass binaries”, arXiv:1210.3007.
I 33 orbit simulation, PN expansion up to (v /c)6 already invalid atstart.

I

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Pressing Questions: What we cannot do (yet)I

They estimated that PN expansion up to (v /c)8 would still not begood enough

IThat expansion only became available earlier this year.