A new approach to electromagnetic wave tails on a curved spacetime

TL;DR

This paper introduces a new method for solving electromagnetic wave equations on curved spacetime, revealing universal tail effects caused by gravitational backscattering, with implications for energy estimates in astrophysical systems.

Contribution

It develops a higher-order Green's function approach for electromagnetic waves on curved spacetime, highlighting universal tail behavior independent of multipole order.

Findings

Wave tails occur after primary signals due to gravitational backscattering.

Tail effects are independent of the multipole structure of the source.

Tail energy can be a significant fraction of the primary pulse energy in binary systems.

Abstract

We present an alternative method for constructing the exact and approximate solutions of electromagnetic wave equations whose source terms are arbitrary order multipoles on a curved spacetime. The developed method is based on the higher-order Green's functions for wave equations which are defined as distributions that satisfy wave equations with the corresponding order covariant derivatives of the Dirac delta function as the source terms. The constructed solution is applied to the study of various geometric effects on the generation and propagation of electromagnetic wave tails to first order in the Riemann tensor. Generally the received radiation tail occurs after a time delay which represents geometrical backscattering by the central gravitational source. It is shown that the truly nonlocal wave-propagation correction (the tail term) takes a universal form which is independent of multipole order. In a particular case, if the radiation pulse is generated by the source during a finite time interval, the tail term after the primary pulse is entirely determined by the energy-momentum vector of the gravitational field source: the form of the tail term is independent of the multipole structure of the gravitational source. We apply the results to a compact binary system and conclude that under certain conditions the tail energy can be a noticeable fraction of the primary pulse energy. We argue that the wave tails should be carefully considered in energy calculations of such systems.