Double Compact Objects: Double Compact Objects: Detection ExpectationsDetection Expectations

Vicky KalogeraPhysics & Astronomy DeptNorthwestern University

with

Chunglee Kim (NU)Duncan Lorimer (Manchester)Philippe Grandclement (NU)

Mia Ihm (NU)

In this talkIn this talk : :

• Gravitational Waves, Gravitational Waves,

Ground Based Interferometers and Ground Based Interferometers and

Astrophysical SourcesAstrophysical Sources

• Double Compact ObjectsDouble Compact Objects

NS-NS, BH-NS, BH-BHNS-NS, BH-NS, BH-BH o Event RatesEvent Rates : : newly discovered NS-NS !newly discovered NS-NS !

o Physical PropertiesPhysical Properties : : what will we learn ?what will we learn ?o Data AnalysisData Analysis : : challenges … challenges …

The strongest sources of gravitational waves are massive compact objects moving at relativistic speeds

GW amplitude: GW amplitude: h ~h ~

Still their effect is very weak :10 M10 M

oo BH at the Galactic center: BH at the Galactic center: h ~ 10 h ~ 10 -17-17

10 M10 Moo BH at the Virgo cluster: BH at the Virgo cluster: h ~ 10 h ~ 10 -20-20

1r

GM

c2

äc

2

h ~ 10 -20

L ~ 4 km l ~ 0.01 fm

LIGO :

'

~~ '

IFO Noise Level and Astrophysical Sources

Seismic at low freq. Thermal at intermediate freq. Laser shot noise at high freq.

Double Compact Objects Inspiral and Coalescence

Compact Object FormationCore collapse-Supernovae

Spinning Compact Objects Asymmetries-Instabilities

Early UniverseFluctuations-Phase Transitions

Binary Compact Object Inspiral

Do they exist ? YES!Prototype NS -NS: binary radio pulsar PSR B1913+16

What kind of signal ?

inspiral chirp

GW emission causes orbital shrinkage leading to higher GW frequency and amplitude

orbitaldecay

PSR B1913+16

Weisberg &Taylor 03

Sensitivity to coalescing binaries

What is the expecteddetection rate out to

Dmax ? Scaling up from

the Galactic rate

strength ~ 1/r

detection rate ~ r3

Dmax for each signal sets limits on the possible detection rate

Inspiral Rates for the Milky Way

Theoretical Estimates

Based on models of binary evolution until binary compact objects form.

for NS -NS, BH -NS, and BH -BH

Empirical Estimates

Based on radio pulsar evolution and survey selection effects.

for NS -NS only

Population synthesis models: follow evolution of primordial binaries until

double compact objects form involve: physical properties of primordial binaries mass exchange between binary components mass and angular momentum loss from binary asymmetric core collapse events

Theoretical Rate Estimates

Large number of possible evolutionary phases result in a large number of formation channels with different relative efficiencies and different physical properties for double compact objects

Rate predictions: sensitive to model assumptions estimates are uncertain by 3-4 orders of magnitude !

Detected NS -NS binaries: binary pulsars

One of the two NS One of the two NS emits radio pulsesemits radio pulses

Prototype NS -NS: Prototype NS -NS: Hulse -Taylor pulsarHulse -Taylor pulsarPSR B1913+16PSR B1913+16

Also: Also: B1534+12 and B1534+12 and J0737-3039 J0737-3039 (Burgay et al. 2003)(Burgay et al. 2003)

pulsar as a`lighthouse'

Radio pulsar surveys have strong selection effects and most of the pulsars remain undetected

QuickTime™ and aGIF decompressorare needed to see this picture.

Radio Pulsarsin

NS-NS binaries

NS-NSMerger

Rate Estimates

Use of observed sample and models for PSR survey selection effects: estimates of total NS- NS number combined with lifetime estimates

(Narayan et al. '91; Phinney '91)

Dominant sources of rate estimate uncertainties identified: (VK, Narayan, Spergel, Taylor

'01)

small - number observed sample (2 NS - NS in Galactic field)

PSR population dominated by faint objects

Robust lower limit for the MW: 10-6 per yr

Upward correction factor for faint PSRs: ~ 1 - 500

X3

small-N sample is: > assumed to be representative of the Galactic population > dominated by bright pulsars, detectable to large distances total pulsar number is underestimated

pulsar luminosity function:

~ L-2

i.e., dominated by faint, hard-to-detect pulsars

NG

Nest

median

25%

(VK, Narayan, Spergel, Taylor '01)

Radio Pulsarsin

NS-NS binaries

NS-NSMerger

Rate Estimates

(Kim, VK, Lorimer '02)

It is possible to assign statistical significance

to NS-NS rate estimates with Monte Carlo simulationsBayesian analysis used to derive the

probability density of NS-NS inspiral rate

Probability Distribution of NS-NS Inspiral Rate

Choose PSR space & luminosity distribution

power-law constrained from radio pulsar obs.

Populate Galaxy with Ntot ‘‘1913+16-like’’ pulsars

same pulsar period,

pulse profile,

orbital period

Simulate PSR survey detection and produce lots of

observed samples for a given Ntot

Distribution of Nobs for a given Ntot : it is Poisson

Calculate P ( 1; Ntot )

Use Bayes’ theorem to calculate P(Ntot) --> P(Ntot x fb

Ntot x fb = rate

Repeat for each of the other two known NS-NS binaries

Current Rate Predictions

3 NS-NS : a factor of 6-7 rate increase

Initial LIGO Adv. LIGO per 1000 yr per yr

ref: peak 75 400

95% 15 - 275 80 - 1500

opt:peak 200 1000

95% 35 - 700 200 - 3700

Burgay et al. 2003VK et al. 2003(Nature embargo)

Current expectations for LIGO II (LIGO I)detection rates of inspiral events

NS -NS BH -NS BH -BHDmax 350 700 1500 (Mpc) (20) (40) (100)

Rdet 5 - 3700 1.5 -1500 15 -10,000(1/yr) (10-3 - 0.7) (3x10-4 -0.3) (4x10-3 -3) less reliable (pop - syn)

Use empirical NS-NS rates:Use empirical NS-NS rates:constrain constrain pop syn models > BH inspiral ratespop syn models > BH inspiral rates

Q: What will the detection of compact object inspiral events tell us ?

discovery of the first BH -NS or BH -BH binaries

compact object mass measurements relative ratios of binary types

more constraints on pop syn models and more constraints on pop syn models and binary evolution binary evolution

distance measurements localization (~ few degrees)

spatial distribution association with EM sources

Gravitational-Wave Astronomy

Challenges in the near future...

Technical: achieve target noise level

Data analysis: optimal methods for signal retrieval detection of inspiral signal requires: template waveforms and matched filtering techniques

Precession and Inspiral Waveforms

Compact object binaries can precess if spins are of significant magnitude and misaligned with

respect to the orbital angular momentum.

Precession can modify inspiral waveforms and decrease the detection efficiency of matched

filtering techniques.

Precession effects are more important for binaries of high mass ratios (BH-NS) and with spin tilt angles

of the massive object in excess of ~30°.(Apostolatos 95)

Q: What is the origin of spin tilt angles in compact object binaries ?

Mass transfer episodes in binaries tend to align spin and orbital angular momentum vectors.

However, asymmetric supernova explosions can tilt the orbital plane relative to the spin of the non-exploding star.

BH

BHNS

SN + NS kick SN + NS kick

Q: What are the expected spin tilt angles ?

10 Mo BH

1.4 Mo NS

Ignoring precession effects in the templatescan decrease the detection rate by a

significant factor

VK 2000

with non-precessing templates: detection rate decreasesR

det decrease depends on

spin magnitude and tilt angle:

Grandclement, VK, Vecchio 2002Grandclement & VK 2003Grandclement, Ihm, VK, Belczynski 2003see also Buonanno et al. 2003 Pan et al. 2003

cos(spin tilt angle)

Maximum BH spin

cos(spin tilt angle)

For a 10-1.4 Mo BH-NS binary

templates that can mimicthe precession effects can increase the detection rate:

Precessing inspiral binariesPrecessing inspiral binaries

3

(BH spin)

10 Mo BH

1.4 Mo NS

Rate drop expected Rate drop expected from from astrophysical predictionsastrophysical predictions for spin tilts in BH-NS binariesfor spin tilts in BH-NS binaries

Grandclement, Ihm, VK, Belczynski 2003

rate dropby 40%

In the near and distant future ... Initial LIGO 3 NS-NS ---> detection possible

BH-BH ---> possible detection too

Advanced LIGO expected to detect compact object inspiral as well as NS or BH birth events, pulsars, stochastic background past experience from EM: there will be surprises!

Laser Interferometry in space: LISA sources at lower frequencies supermassive black holes and background of wide binaries