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

## TL;DR

This paper investigates how astrophysical environments influence gravitational-wave tails and memory effects from mergers, revealing that environmental factors can cause transient features, but the long-term memory remains largely unaffected.

## Contribution

It demonstrates that environmental matter overdensities can induce transient features in gravitational-wave tails and memory, expanding understanding of these effects in realistic astrophysical settings.

## Key findings

- Transient amplitude changes occur due to environmental effects.
- Memory effects are largely independent of the environment.
- Environmental influence is significant only in matter overdense regions.

## 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.