Double Compact Objects: Detection Expectations Vicky Kalogera 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 …

Propagation: at the speed of light h  = A  exp(ik  x  ),   =c  Spherical wave solution of Einstein's equation for a small-amplitude, time-dependent perturbation of the metric Source: time-dependent mass quadrupole moment Effect: produce a strain on test particles in the plane perpendicular to the propagation direction  '' x ''  '' = ½ h r 

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 :

Detection requires very accurate measurements of distances Laser interferometry can do the trick ! Changes in the positions of the test masses are encoded in the interferometer fringes h ~ L ~ 4 km  l ~ 0.01 fm

LIGO 2002 GEO 2002 Virgo 2003 TAMA present AIGO Network of GW Interferometers Washington Louisiana LIGO : two 4km and one 2km interferometers  detection confidence  source localization

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 Four other NS -NS detected in our Galaxy What kind of signal ? inspiral chirp GW emission causes orbital shrinkage leading to higher GW frequency and amplitude orbital decay PSR B Taylor & Weisberg 01

Compact Object Inspiral Signals What if gravitational waves were sound waves ? courtesy Benoit Mours NS- NS inspiral : without noise with noise in Andromeda BH- BH inspiral : without noise with noise in Andromeda with noise in Virgo cluster Signal relative to Noise ?

Sensitivity to coalescing binaries future What is the expected detection rate out to D max ? Scaling up from the Galactic rate strength ~ 1/r det. rate ~ r 3 D max for each signal sets limits on the possible detection rate

Galactic Merger Rates required for LIGO II (LIGO I) detection rate of one event per year NS-NS BH-NS BH-BH D max (Mpc) (20) (40) (100) R MW (Myr -1 ) (3000) (400) (25) Maximum distances have been calculated for a S/N=8, for 1.4M o NS and 10M o BH, and for H o =65 km/s/Mpc,  M =0.4,   =0.6

Merger 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

Theoretical Rate Estimates Lipunov et al. 97 Portegies et al. 98 Brown & Bethe 98 Fryer et al. 99 Belczynski et al. 02 Most important model parameters: asymmetric supernova kicks ~ 100 radii of evolved stars ~ 50 wind mass loss from massive stars~ characteristics of mass exchange ~ mass limit for BH formation (BH-BH)~ 10 Events per Myr: NS-NS BH-NS BH-BH ~ uncertainty

Detected NS -NS binaries: binary pulsars One of the two NS emits radio pulses Prototype NS -NS: Hulse -Taylor pulsar PSR B 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 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: ~ 1-500

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%

Radio Pulsars in NS-NS binaries NS-NS Merger Rate Estimates (Kim, VK, Lorimer '02) Recent Results: possible to assign statistical significance to NS-NS rate estimates with Monte Carlo simulations Choose PSR space & luminosity distribution Populate Galaxy with N tot `` like'' pulsars Simulate PSR survey detection and produce observed samples Distribution of N obs for a given N tot : Poisson Calculate P ( 1; N tot ) for best-fitting Poisson distribution

Radio Pulsars in NS-NS binaries NS-NS Merger Rate Estimates Detection Rate (yr -1 ) Galactic Rate (Myr -1 ) LIGO I LIGO II Most likely "Standard" Model Systematics (Kim, VK, Lorimer '02) Likelyhood of total number of " like" PSRs for different models of space and luminosity distributions

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 MW (1/Myr) R det (1/yr) (3x ) ( 3x ) (4x )

Q: What will the detection of compact object inspiral events tell us ? compact object masses discovery of the first BH -NS or BH -BH binaries relative ratios of binary types distance measurements localization (~ few degrees) Discovery of EM counterparts (GRB?) origin of Gamma-Ray Bursts association of BH formation and SNe cosmology (GW: luminosity distances EM: redshift ) Gravitational-Wave Astronomy

Compact Object Mass Distributions: Model Diagnostics first-born CO second-born CO standardmodel reducedstellarwinds inefficientcommon-envelopeejection BH mass measurements can be used to > exclude theoretical models > provide insight to open issues in astronomy Belczynski, Kalogera, Bulik 2002

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 Kalogera 2000

Non-precessing templates: detection rate decreases R det decrease depends on mass ratio spin tilt angle cos( )) ) Grandclement, VK, Vecchio '02

In the near and distant future... Two-year LIGO I science run starts BH-BH inspiral best bet, although cannot count on LIGO II comes on line likely 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