The gravitational wave background from star-massive black hole fly-bys

TL;DR

This paper estimates the gravitational wave background from star-MBH fly-bys, finding it is weaker than LISA's noise and unlikely to be detected, especially from the Galactic centre.

Contribution

It models the gravitational wave background from stellar fly-bys near MBHs, incorporating stellar dynamics and astrophysical processes, and compares it to LISA's sensitivity.

Findings

The EMBB is mainly from stellar black holes.

The background noise is at least 10 times below LISA's instrumental noise.

LISA is unlikely to detect relativistic bursts from the Galactic centre.

Abstract

Stars on eccentric orbits around a massive black hole (MBH) emit bursts of gravitational waves (GWs) at periapse. Such events may be directly resolvable in the Galactic centre. However, if the star does not spiral in, the emitted GWs are not resolvable for extra-galactic MBHs, but constitute a source of background noise. We estimate the power spectrum of this extreme mass ratio burst background (EMBB) and compare it to the anticipated instrumental noise of the Laser Interferometer Space Antenna (LISA). To this end, we model the regions close to a MBH, accounting for mass-segregation, and for processes that limit the presence of stars close to the MBH, such as GW inspiral and hydrodynamical collisions between stars. We find that the EMBB is dominated by GW bursts from stellar mass black holes, and the magnitude of the noise spectrum (f S_GW)^{1/2} is at least a factor ~10 smaller than the instrumental noise. As an additional result of our analysis, we show that LISA is unlikely to detect relativistic bursts in the Galactic centre.