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Precessing black hole-neutron star binaries produce a complicated and particularly informative gravitational wave signal, encoding information in two complementary channels. On the one hand, the orbit shrinks via gravitational radiation; on the other, the orbital plane wobbles, because the orbital angular momentum (and spins) precess through spin-orbit coupling.

The gravitational wave signal reflects these two complementary physical processes in two largely decoupled scales: a high-frequency, slowly-changing “carrier” signal, reflecting the slowly-shrinking orbit; and a low-frequency modulation in amplitude, phase, and polarization.

Gravitational wave detectors can constrain both processes independently, letting us measure the orbital plane and three-dimensional BH spin. Moreover, using simple estimates of how many orbits the binary makes and how much the orbit precesses, we can understand how well the binary’s spins and masses can be measured.

For experts:

• Our analysis suggests simple Fisher-matrix-like studies can estimate how well detailed parameter-estimation calculations perform, even for precessing binaries.

• We found an example demonstrating that even for precessing binaries, higher harmonics can break degeneracies and rule out a significant subset of parameters.

• Our collaborators developed the currently-used-in-LIGO MCMC parameter estimation strategies for precessing binaries. Our analysis and companion studies were part of the calibration process, to gain confidence that we understood our simulations’ results and hence how well parameters could be estimated.

For more information,

• ROS et al., Parameter Estimation of Gravitational Waves from Precessing BH-NS Inspirals with higher harmonics, arxiv.org:1403.0554

• Recent work by Vitale et al (1403.0554) uses the same lalinference codebase

• Recent work by the Montana State group (1402.3581) uses a different code and waveform model to assess parameter estimation of low-mass binaries, including double neutron stars.