Precession During Merger

The merger of two precessing black holes produces a complicated gravitational wave signal which in principle fully encodes its highly nonlinear dynamics. Intuition gained from analytic studies prior to merger provide a simple picture: the two black holes spiral inward, with their spin and orbital angular momenta precessing around one another. Analytic studies after merger show the isolated black hole loses its hair", radiating away energy and angular momentum in quasinormal modes,” determined by eigenfunctions of the final black hole. How are these two epochs bridged for generic sources? How can observations probe those dynamics?

        In 2011, I identifed a robust [``corotating frame"](http://adsabs.harvard.edu/abs/2011PhRvD..84l4002O), a way of choosing time-dependent angular coordinates at large
        distances using only the radiated  signal.   In this frame the binary's orbital plane is not precessing prior to merger.  Prior to and during merger,
        the corotating-frame waveform signal is easy to understand and similar to previously-simulated nonprecessing results.  We suspect  differences can be modeled with perturbation theory.

In our recent paper, we re-expressed the gravitational wave signal from simulated binary black hole mergers in this frame.

• our preferred frame seems to be physical, and connected to the line of sight polarization.

Intuitively, the binary radiates cicularly polarized gravitational waves perpendicular to the instantaneous orbital plane. As the orbital plane precesses, radiation along the line of sight receives distinctive amplitude and phase modulations.

• the corotating-frame signal does not (and cannot) be identical to the signal from nonprecessing binaries. For example, corotating-frame modes generically have modulations on a precession timescale.

• the merger of precessing binaries coherently excites quasinormal modes, which “precess” around the total angular momentum direction. We speculate the details of this process provide a signature of strong-field gravity.

For more information, see

Our paper: ROS et al 2013 PRD 87 4038

My talk at the 2012 KITP meeting, “Rattle and Shine: Gravitational Wave and Electromagnetic Studies of Compact Binary Mergers”