Improved Eccentricity Reduction in SpEC

Because orbital eccentricity is ill-defined in general relativity, the question of how to simulate a compact binary with a desired eccentricity is an open problem for numerical relativity codes. To simulate quasicircular binaries in particular, SpEC iteratively refines the initial conditions according to the eccentricity measured from a series of short runs. This procedure can fail because of the choice of eccentricity model, the numerical techniques used to fit the model, or inaccuracies in the run data itself. We have improved the reliability of SpEC's eccentricity reduction in a number of ways. The most important of these are the use of variable projection for nonlinear least squares fitting and high-frequency filtering of the run data before fitting. This filtering can remove confounding spin effects from the run so that spin can be omitted entirely in SpEC's eccentricity model. These methods not only improve the accuracy and consistency of SpEC's eccentricity reduction, but also have possible further application in simulating eccentric binaries.