LIGO's First Observing Run: Gravitational-Wave …s First Observing Run: Gravitational-Wave...
Transcript of LIGO's First Observing Run: Gravitational-Wave …s First Observing Run: Gravitational-Wave...
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
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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
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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)
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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
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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