LIGO's First Observing Run: Gravitational-Wave Astronomy on the Rise

Chris Pankow (CIERA / Northwestern University) on behalf of the LIGO Scientific Collaboration and Virgo Collaboration

LIGO-G1602321Miami2016, December 19th 2016

GW Available in this Talk

O1 BBH Events2

“Chirps” in the time domain (monotonically increasing in frequency vs time)

Lower mass → Higher frequency content / longer “in band”

Phys. Rev. X 6 041015

Compact Binary Coalescence: Waveforms

Effects of redshiftfrequency stretching

3

“Orbital Hang-up” Effects1,z = s2,z = 0.99

s1,z = 0.99, s2,z = -0.99s1,z = s2,z = -0.99

Effects of Precessionm1 = 5 m2 = 1.4s1,x = s1,y = 0.5

modulation envelope

Bayesian Parameter Estimation4

Parameter Posteriors: Form the posterior on a given parameter set

μ from Bayes’ Law

Bayes Factor: Often overlooked (posterior distributions normalized

manually) but encodes the Bayesian signal vs. noise

comparison

PE Method 1: 15+ dimensional space explored by Markov-Chain Monte Carlo. Parallel tempering: ΛT added for more efficient convergence time. Determine “convergence” by number

of effective, uncorrelated samples drawn using the autocorrelation length

Bayesian Parameter Estimation5

Parameter Posteriors: Form the posterior on a given parameter set

μ from Bayes’ Law

Bayes Factor: Often overlooked (posterior distributions normalized

manually) but encodes the Bayesian signal vs. noise

comparison

PE Method 2: ”Nested Sampling”; swarm of points exploring the likelihood space and estimating the integrand. Allows better estimation of the evidence integral p(d) and thus for

comparisons of signal vs. Gaussian noise hypothesis

BBH Masses and Spins6

Parameter Degeneracies: Primarily sensitive to the chirp mass

— leaves large degeneracies along contours of chirp mass

(GW151226 approaching m2 < 3 region)

Frequency content (and thus “length in band” affected by both effective spin and mass

ratio at same order in expansion of radiation

amplitude/phase

equal mass

equa

l mas

s

Phys. Rev. X 6 041015

BBH Masses and Spins7

equal mass

equa

l mas

s

Cosmological Effects:All events at distances with

significant redshifts — all masses need to be adjusted to account for

frequency “stretching” from GW traversing expanding universe

Precession?GW151226 has posterior on “in plane spin” component bounded

away from zero — spin-orbit coupling causes plane of binary to

precess around total angular momentum. Also, at least one BH must be a Kerr black hole (as are

the final remnants in all cases)Phys. Rev. Lett. 116/241103

BBH Detection8

Towards Measuring Mass Distributions:

Posterior distribution for exponent of m1 inferred from three astrophysically

distinguished events — note peak very close to α = 2.35 (black vertical line)

Signal and Background (GW):Different parameterization, using a likelihood ranking statistic modeling

background with the expected volumetric (ρ-4) distribution superimposed

Phys. Rev. X 6 041015

Dealing with Multiple Event Categories:Being unsure of the intrinsic source populations and origins, we calculate the event rates for

all three events and take the union to derive the overall event rate of BBH coalescence. Also test distributions of events according to uniform in the logarithm of component mass

and according to the stellar initial mass function: p(m1) ∝ m12.35

BBH Event Rates9

Phys. Rev. X 6 041015

Dealing with Multiple Event Categories:Being unsure of the intrinsic source populations and origins, we calculate the event rates for

all three events and take the union to derive the overall event rate of BBH coalescence. Also test distributions of events according to uniform in the logarithm of component mass

and according to the stellar initial mass function: p(m1) ∝ m12.35

BBH Event Rates10

Phys. Rev. X 6 041015

Astrophysics Implications11

Ap. JL. L22 2016

Metallicity Requirements:Solar metallicity environments would not be able to create GW150914{a,b}. Need

half solar metallicity or below — the heavier (and more plentiful) they are the more pristine the environment needs to

be.

Confusion Background by 2020?:If the rates are on the higher side, and

under certain conditions, could be limited by a confusion background of incoherent

BBH at higher redshift

Z☉ = 0.0134

fraction of non H/He “metals” in star

Astrophysics Implications12

Ap. JL. L22 2016

Metallicity Requirements:Solar metallicity environments would not be able to create GW150914{a,b}. Need

half solar metallicity or below --- the heavier (and more plentiful) they are the more pristine the environment needs to

be.

Confusion Background by 2020?:If the rates are on the higher side, and

under certain conditions, could be limited by a confusion background of incoherent

BBH at higher redshift

Formation Channels13

Clusters Scenario:Dense massive stellar environment, three body interactions and possible hierarchical merger

trees produce heavy black hole binaries, hardened by the interaction with the heavier

objects and ejecting the lighter objects

Ap. JL. 824 (1)

GW

1512

26

Ap. JL. L22 2016

Formation Channels14

Field Population Scenario:Complex interplay of isolated galactic

field binaries. Large main sequence stars go through mass transfer episodes,

producing heavy black holes, and if they survive supernova kicks, can form tight

binaries

Nature 534 512-515

Ap. J. 759 (1)

Astrophysics Implications15

Field Population Scenario:Complex interplay of isolated galactic

field binaries. Large main sequence stars go through mass transfer episodes,

producing heavy black holes, and if they survive supernova kicks, can form tight

binaries

5 10 15 20 25 30 35 40 45 50Chirp Mass ( )

100%

10%

1%

0.1%

GW150914GW151226

Fraction of Systems with

Iso. Fb.Iso. Prop.Iso. FullPolar Fb.Polar Prop.Polar FullClusters

Spin Orientation Consequences

Measuring anti-aligned (χeff < 0) spins, under mild assumptions about evolution would give high

odds towards cluster based formation GW150914 has χeff < 0

at ≳ 75% probability

16

Ap. JL. 832 (1)Phys. Rev. Lett.116 241102

Other Avenues

• Delay times: prevailing opinion — p(τ) ∝ 1/τ

• Long delay times disfavored, but metallicity at higher redshifts (and hence tau) is smaller (e.g. star formation rate thought to peak at z~2) Tension?

• A few more events could allow us to distinguish R(z), at least R(z) ~ uniform versus other hypotheses

• Eccentricity: order of magnitude arguments constrain in-band eccentricity to less than 0.1

• Sources expected to circularize via GW radiation before entering the LIGO band

• Only beginning to develop template families to deal with this — first line of defense nominally handled by generic transient searches

17

Stellar Mass Black Holes in Context18

https://stellarcollapse.org/bhmasses

LIGO-G1601165

Filling in the black hole catalog

Image credit: LIGO

Stellar Mass Black Holes In Context19

GW150914aGW150914bGW150914GW151226aGW151226bGW151226LVT151012aLVT151012bLVT151012

6035 40 45 50 ~

Higher Mass (GW)spins constrained mostly below

maximum

https://stellarcollapse.org/bhmasses

LIGO-G1601165

Filling in the black hole catalog

Image credit: LIGO

Stellar Mass Black Holes In Context20

Lower Mass (X-ray Binaries)

Some near maximal spin (> 0.9)

GW150914aGW150914bGW150914GW151226aGW151226bGW151226LVT151012aLVT151012bLVT151012

6035 40 45 50 ~

Higher Mass (GW)spins constrained mostly below

maximum

https://stellarcollapse.org/bhmasses

GW150914 in 2018: Events like GW150914 will have

SNR 40 / detector with total SNR <=100 (three

detectors)

O2 and Beyond21

Future Detections: Currently at the lower end of

O2 sensitivity projections, but tens of BBH in O3?

O2 and Beyond22

GW150914 in 2018: Events like GW150914 could have SNR 40 / detector with total SNR ≲ 100 (three detectors)

BNS / NSBH Upper Limits23

Compact Sources: Only BBH detections so far, NSBH and BNS remain elusive, but

expected to constrain models in the next few years

arxiv:1607.07456201520162017 201520162017

O2 and Beyond (Summary)

• O(5-10) BBH detections in O2?

• Lower masses very possible — NSBH space!

• Extensive EM facility coordination — multimessenger astronomy will soon be a reality

• Host identification can allow for independent measurement of H0

• In 2018 GW150914 triples in SNR, possibly a SNR of 100 with four detectors

• Detailed catalog of sources parlay into source formation models and clearer understanding of compact object formation, low metallicity environments, and XRB dynamics

• Exquisitely precise tests of general relativity

• 2018: BBH / day? plus likely detections of NSBH and BNS (several / mo or more)

24

Finally...25

A. Simonnet (Sonoma State)

Come visit us and see our GW data releases: https://losc.ligo.org/

about/

~50 “visible” universes!!!

Testing GR26

The single-parameter analysis corresponds to minimally extended models, that can capture deviations from GR that predominantly, but not

only, occur at a specific PN order

Graviton Mass: Confined Compton wavelength > 1013 km (best dynamical bound), by testing dispersion in the expected

frequency content of the waveform

Expand Einstein’s equations in velocity (PN order) and

introduce deviations — use MCMC to test for phase

evolution deviations from GR

arxiv:1606.04856

Testing GR (cont.): Final mass, spin

divide waveform into portion below and above 173 Hz

(red line)

90% confidence contours on probability distributions for

pre and post inspiral portions

post/pre difference distribution

pre-merger

post-merger

27

Back and Forth: Great agreement from ringdown vs. analytical inspiral in masses / spin measured up to cutoff

Inferred Rates / Probability of Astrophysical Origin28

Likelihood of obtaining ensemble of ranking statistics xi with two categories of events: background (terrestrial) and foreground (astrophysical)

Λfg,bg ~ expected counts from each categorypfg, pbg - modeled or measured, for astrophysical distribution of binaries pfg ~ ρ-4

Methods using LR ranking can divide out pbg and use likelihood statistic directly

Obtain posterior on \Lambda which scales with the rate by

the sensitive space-time volume by marginalization

over the xi, applying a Jeffrey’s prior on the rates

arxiv:1302.5341

Sky Localization29

Sky Localization: Two detectors with coherence still restricted to partial annulus on

sky

Observation Biases: Optimal location for single detector:

directly overhead — preference to detect over North America

and Indian Ocean

https://arxiv.org/abs/1602.08492

Sky Localization30

Sky Localization Timeline: Sky position information

released within two days — huge follow up program

followed thereafter

Towards Joint Astronomy: If a binary with a neutron star

component is detected, possibility of multi-wavelength

electromagnetic emission as well!

prompt

hours/days

weeks

possible EM emissionduration

https://arxiv.org/abs/1602.08492