=== GRG relativistic astrophysics (Monday) ===

Pablo Laguna - Numerical relativity at Penn State

- Infrastructure: BSSN, moving punctures (Ansorg for initial punctures), Newman-Penrose Psi_4 extraction, 4th-order finite differences, Kranc for code generation, Cactus for parallelization, Carpet for mesh refinement, MoveIt for mesh refinement adaptation

- Superkicks for opposite spins aligned (or perpendicular?) with L
- Kidder's formula: fit for parameters, but need to determine "entrance angles"
- Formula reproduces results closely, explore different geometries
- Parametrized formula not complete yet

- Comparison between numerical and 3.5PN; matches quite well

- NRwaves (?) collaboration to compare and validate waveforms

- Question about asymmetry in waveforms with large kicks: asymmetry is there all the time, but it's average out except for the very final part

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Frans Pretorius - Interesting results in NR BH mergers

- Two interesting results that seem bizarre and unusual
	- Unstable quasi-circular orbits
	- Some kicks
- Black-hole scattering problem; depending on impact parameter:
	- One BH after a collision
	- Two isolated BHs after deflection
	- There must be a threshold behavior
- Geodesic behavior (tuning to a few orbits before deflection) is also expected for equal masses
	- Scaling exponents (from Lyapunov analysis of geodesic equation) are very similar
	- Estimates of radiated energies suggest 15 orbits
- Superkicks for opposite spins in the orbital plane
	- Kick normal to orbital plane
	- Frame dragging explains it (think of one BH moving in the background of the other); systems oscillates above and below the initial orbital plane
- Surprising that non-nonlinear reasoning can explain these phenomena
	- Rich geodesic orbital motion
	- Instability to GW emission
	- End state of two-body problem as perturbed Kerr BH
	- Any surprises to come? Probably not in generic scenarios