The Electromagnetic Christodoulou Memory Effect in Neutron Star Binary Mergers

TL;DR

This paper investigates the electromagnetic contribution to the nonlinear gravitational wave memory effect in neutron star mergers, showing it enhances the signal detected by interferometers and aids in understanding the source energy.

Contribution

It extends previous work by computing the electromagnetic Christodoulou memory effect specifically for neutron star mergers, linking electromagnetic fields to gravitational wave observations.

Findings

Electromagnetic fields significantly contribute to the nonlinear memory effect.

The electromagnetic contribution can enlarge the gravitational wave signal in detectors.

Results help estimate energy radiated in neutron star mergers.

Abstract

Gravitational waves are predicted by the general theory of relativity. In [6] D. Christodoulou showed that gravitational waves have a nonlinear memory. We proved in [3] that the electromagnetic field contributes at highest order to the nonlinear memory effect of gravitational waves. In the present paper, we study this electromagnetic Christodoulou memory effect and compute it for binary neutron star mergers. These are typical sources of gravitational radiation. During these processes, not only mass and momenta are radiated away in form of gravitational waves, but also very strong magnetic fields are produced and radiated away. Thus the observed effect on test masses of a laser interferometer gravitational wave detector will be enlarged by the contribution of the electromagnetic field. Therefore, the present results are important for the planned experiments. Looking at the null asymptotics of spacetimes, which are solutions of the Einstein-Maxwell (EM) equations, we derived in [3] the electromagnetic Christodoulou memory effect. Moreover, our results allow to answer astro- physical questions, as the knowledge about the amount of energy radiated away in a neutron star binary merger enables us to gain information about the source of the gravitational waves.