Eccentricity estimation from initial data for numerical relativity simulations*

We describe and study an instantaneous definition of eccentricity to be applied at the initial

moment of full numerical simulations of binary black holes. The method consists of evaluating

the eccentricity at the moment of maximum separation of the binary. We estimate it using up

to third post-Newtonian (3PN) order and compare these estimates with the results of evolving

(conservative) 3PN equations of motion for a full orbit and compute the eccentricity er from the

radial turning points, finding excellent agreement. We then include terms with spins up to 3.5PN,

and next compare this initial data eccentricity estimate method with the turning point estimates of the eccentricity erNR during full numerical evolutions of spinning binary black holes,characterized invariantly by a fractional factor 0 ≤ f ≤ 1 of the initial tangential momenta to a quasicircular one.

We find that our initial instantaneous definition is a particularly useful and accurate tool to predict

and characterize even highly eccentric full numerical simulations.