The quadrupole moment of compact binaries to the fourth post-Newtonian order: relating the harmonic and radiative metrics

Motivated by the completion of the fourth post-Newtonian (4PN) gravitational-wave generation from compact binary systems, we analyse and contrast different constructions of the metric outside an isolated system, using post-Minkowskian (PM) expansions. The metric in "harmonic' coordinates has been investigated previously, in particular to compute tails and memory effects. However, it is plagued by powers of the logarithm of the radial distance r when r -> infinity (with t - r/c = const). As a result, the tedious computation of the "tail-of-memory' effect, which enters the gravitational-wave flux at 4PN order, is more efficiently performed in the so-called "radiative' coordinates, which admit a (Bondi-type) expansion at infinity in simple powers of r(-1), without any logarithms. Here we consider a particular construction, performed order by order in the PM expansion, which directly yields a metric in radiative coordinates. We relate both constructions, and prove that they are physically equivalent as soon as a relation between the "canonical' moments which parametrize the radiative metric, and those parametrizing the harmonic metric, is verified. We provide the appropriate relation for the mass quadrupole moment at 4PN order, which will be crucial when deriving the "tail-of-memory' contribution to the gravitational flux.