1 Merging Black Holes and Gravitational Waves Joan Centrella Chief, Gravitational Astrophysics Laboratory NASA Goddard Space Flight Center Observational Signatures of Black Hole Mergers STScI March 30, 2009

2 It takes a team…. and a community… Groups & colleagues at: AEI Caltech Cornell FAU… GA Tech/Penn State Jena LSU Princeton RIT/UTB UMCP UT Austin Goddard Numerical Relativity Group JC, Sean McWilliams, Bernard Kelly, Jim van Meter, Darian Boggs, John Baker

3 Final coalescence is driven by GW emission… Strong-field merger is “brightest” GW source –Luminosity ~ L SUN  releases more energy than all the stars in visible universe combined Must solve Einstein’s equations numerically…. Very difficult problem…unsolved for > 40 years (graphic courtesy of Kip Thorne)

4 Black Hole Merger Simulations…. Recent Breakthroughs: 2004: 1 st full orbit 2005: 1 st orbits and mergers 2006: unequal masses, spins, kicks, 1 st long waveforms 2007: “super kicks” 2008: larger mass ratios, more generic spins (Visualization by Chris Henze, NASA/Ames) Units: set c = G = 1  1 M ~ 5 x (M/M Sun ) sec ~ 1.5 (M/M Sun ) km

5 Gravitational waveforms…….circa 2006 Compare GWs from equal mass, nonspinning case 3 different, independently-written codes Baker, Campanelli, Pretorius, Zlochower, Class. Quantum Grav. 24 (2007) S25-S31 (gr-qc/ )

6 Gravitational waveforms………circa 2008 Compare GWs from equal mass, nonspinning case 5 different, independently-written codes Hannam, et al. (ariXiv: [gr-qc])

7 GWs from unequal mass, nonspinning BHs… Sum over modes up to l = 3 at θ=0, φ = 0 Scale by η = (m 1 + m 2 )/(m 1 + m 2 ) 2 Baker, et al., Phys. Rev. D 78 (2008) (arXiv: ) Mass ratio 10:1  lm (t) for l = m modes Gonzales, Sperhake, & Bruegmann, (arXiv:0811:3952 [gr-qc] )

8 GWs from equal mass BHs with spin… Equal up-up and down-down spins Equal masses, each BH has a = 0.75 m Initially MΩ = 0.05  T orbital ~ 125M Campanelli, et al., Phys.Rev. D74 (2006) (gr-qc/ ) Anti/aligned  attractive/repulsive Final spins: - a=0.9M (aligned) - a=0.44M (anti)

9 GWs from precessing unequal mass BHs… m 1 /m 2 ~ 0.8, a 1 /m 1 ~ 0.6, a 2 /m 2 ~ 0.4 Spins initially at arbitrary orientations Completes ~ 9 orbits before merger Campanelli, et al. (arXiv: [gr-qc] ) Trajectory difference r = x 1 – x 2

10 Detecting MBH mergers… Equal mass, nonspinning black holes Contours of SNR for detection using LISA sensitivity curve Baker, et al., PRD 75 (2007) (gr-qc/ )

11 What can we learn from MBH mergers? Gravitational waves: –Very strong GW sources –LISA will observe with large signal-to-noise at high redshift –Get masses, spins to very high precision –Also get distances to good precision  trace merger history of BHs in universe

12 Recoil kicks from BH mergers... For binaries with asymmetric spins and/or unequal masses: –the GW emission is asymmetric –the GWs are “beamed” in some direction… Since the GWs carry momentum, the final BH that forms suffers a recoil ‘kick’ in the opposite direction If this kick velocity is large enough, the final BH that forms could be ejected from its host structure Also may see “displaced” MBHs in galactic nuclei Simulations show kicks < 200 km/s for aligned binaries, but up to ~ 4000 km/s for BHs with spins in orbital plane

13 Can we “see” what LISA will “hear”? Does the merger produce an EM signal? –Merging MBHs could be surrounded by gas, accretion disk, magnetic fields… –Will there be any effects of the merger that produce EM radiation? –Can we see effects of ejected or dislodged central MBHs? Many possibilities…active area of research: –Inspiraling binary may cause “pulses” in the disk –~ 4% of mass emitted in GWs  disk may react to this change in the gravitational potential –Gas flows and accretion onto the merging BHs themselves….

14 Modeling flows around BH mergers… Model the behavior of gas and magnetic fields in the dynamical spacetime around the merging BHs First step: map the flow of test particles around the merging BHs –Set up initial distribution of particles around BH binary –Evolve the BH binary using numerical relativity –Trace the motion of the particles along the geodesics as the binary evolves Estimate energetics of the flow from “collisions” –For each particle, calculate minimum distance between it and another particle –If this minimum distance is < 0.05M  “collision” –Each particle can have only 1 collision…

15 Merger of Schwarzschild BHs, m 1 =m 2 Initial state: 25,000 particles, uniformly distributed - extent: 8M < r < 25M, -5M < z < +5M - thermal velocities: Keplerian at initial radius, with random directions BHs complete ~ 2 orbits & form common horizon at ~ 126 M

16 Particles move at relatively high radial velocities Outer regions show high radial outflow velocities Merger of Schwarzschild BHs, m 1 =m 2

17 Summary and outlook... Impressive progress on a broad front: many research groups, different codes, different methods… Equal mass, nonspinning BBHs: –Excellent agreement on simple waveform shape –total GW energy emitted in last few cycles ΔE ~ 0.04M –final BH has spin a ~ 0.7M Run for many orbits  long wavetrains… Comparisons with post-Newtonian analysis…. Applications to GW data analysis are beginning… Explosion of work on nonequal mass and spinning BH mergers and the resulting kicks –Higher mass ratios, more generic precessing spins… –Important astrophysical applications… Triggering new work on EM counterparts…. Modeling flows around MBH mergers –Preliminary results show high velocity flows  may produce high energy emissions

18 The emerging picture….

19 Stay Tuned!