Gravitational-wave tails and memory effect for mergers in astrophysical environments

Abstract

Gravitational waves from the coalescence of compact objects carry information about their dynamics and the spacetime in regions where they are evolving. In particular, late-time tails and memory effects after the merger are two low-frequency phenomena, not detectable by current instruments, but which can be observed by future detectors. Their low-frequency nature could, in principle, make them more sensitive to larger-scale structures at galactic length scales. We show that indeed there are transient features, such as amplitude changes, in both tails and (linear) memory when the merger occurs while immersed in an astrophysical environment. For realistic galaxies, the environment's compactness is small enough that the effect is strongly suppressed, but these effects could become relevant for mergers occurring in regions with matter overdensities, like the ones recently observed numerically for wave dark matter. On the other hand, the memory (the difference between the amplitude asymptotically early and late) and asymptotically late decay are independent on the properties of the environment.