Double Compact Objects: Detection Expectations Vicky Kalogera Physics & Astronomy Dept Northwestern University with Chunglee Kim (NU) Duncan Lorimer (Manchester) Philippe Grandclement (NU) Mia Ihm (NU)

In this talk : In this talk : Gravitational Waves, Gravitational Waves, Ground Based Interferometers and Ground Based Interferometers and Astrophysical Sources Astrophysical Sources Double Compact Objects Double Compact Objects NS-NS, BH-NS, BH-BH NS-NS, BH-NS, BH-BH o Event Rates : newly discovered NS-NS ! o Physical Properties : what will we learn ? o Data Analysis : challenges …

The strongest sources of gravitational waves are massive compact objects moving at relativistic speeds GW amplitude: h ~ Still their effect is very weak : 10 M o BH at the Galactic center: h ~ M o BH at the Virgo cluster: h ~ h ~ 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 Formation Core collapse-Supernovae Spinning Compact Objects Asymmetries-Instabilities Early Universe Fluctuations-Phase Transitions

Binary Compact Object Inspiral Do they exist ? YES! Prototype NS -NS: binary radio pulsar PSR B What kind of signal ? inspiral chirp GW emission causes orbital shrinkage leading to higher GW frequency and amplitude orbital decay PSR B Weisberg & Taylor 03

Sensitivity to coalescing binaries What is the expected detection rate out to D max ? Scaling up from the Galactic rate strength ~ 1/r detection rate ~ r 3 D max 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 emits radio pulses Prototype NS -NS: Hulse -Taylor pulsar PSR B Also: B and J (Burgay et al. 2003) pulsar as a `lighthouse' Radio pulsar surveys have strong selection effects and most of the pulsars remain undetected

Radio Pulsars in NS-NS binaries NS-NS Merger 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: per yr Upward correction factor for faint PSRs: ~ X 3

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 NGNG N est median 25% (VK, Narayan, Spergel, Taylor '01)

Radio Pulsars in NS-NS binaries NS-NS Merger Rate Estimates (Kim, VK, Lorimer '02) It is possible to assign statistical significance to NS-NS rate estimates with Monte Carlo simulations Bayesian 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 N tot ‘‘ like’’ pulsars same pulsar period, pulse profile, orbital period Simulate PSR survey detection and produce lots of observed samples for a given N tot Distribution of N obs for a given N tot : it is Poisson Calculate P ( 1; N tot ) Use Bayes’ theorem to calculate P(N tot ) --> P(N tot  x f b  N tot  x f b = 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 % opt: peak % Burgay et al VK et al (Nature embargo)

Current expectations for LIGO II (LIGO I) detection rates of inspiral events NS -NS BH -NS BH -BH D max (Mpc) (20) (40) (100) R det ,000 (1/yr) ( ) (3x ) (4x ) less reliable (pop - syn) Use empirical NS-NS rates:constrain pop 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 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 NS SN + NS kick SN + NS kick

Q: What are the expected spin tilt angles ? 10 M o BH 1.4 M o NS Ignoring precession effects in the templates can decrease the detection rate by a significant factor VK 2000

with non-precessing templates: detection rate decreases R det decrease depends on spin magnitude and tilt angle: Grandclement, VK, Vecchio 2002 Grandclement & VK 2003 Grandclement, Ihm, VK, Belczynski 2003 see also Buonanno et al Pan et al cos(spin tilt angle) Maximum BH spin cos(spin tilt angle) For a Mo BH-NS binary templates that can mimic the precession effects can increase the detection rate: Precessing inspiral binaries

3 (BH spin) 10 M o BH 1.4 M o NS Rate drop expected from astrophysical predictions for spin tilts in BH-NS binaries Grandclement, Ihm, VK, Belczynski 2003 rate drop by 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