Extreme-mass-ratio-bursts from extragalactic sources

TL;DR

This paper explores the potential of detecting gravitational wave bursts from stellar objects orbiting massive black holes in other galaxies, assessing their usefulness for understanding black hole properties.

Contribution

It analyzes the detectability of extragalactic EMRBs with space-based interferometers and evaluates their potential to constrain black hole mass and spin.

Findings

Detectable EMRBs from nearby galaxies are possible with LISA-like detectors.

Detection requires periapse radius to be less than about 8 gravitational radii.

EMRBs could constrain black hole mass and spin with approximately 1% accuracy.

Abstract

Extreme-mass-ratio bursts (EMRBs) are a class of potentially interesting gravitational wave signals. They are produced when a compact object passes through periapsis on a highly eccentric orbit about a much more massive object; we consider stellar mass objects orbiting the massive black holes (MBHs) found in galactic centres. Such a system may emit many EMRBs before eventually completing the inspiral. There are several nearby galaxies that could yield detectable bursts. For a space-borne interferometer like the Laser Interferometer Space Antenna, sensitivity is greatest for EMRBs from MBHs of ~10^6-10^7 solar masses, which could be detected out to ~100 Mpc. Considering the examples of M32, NGC 4945 and NGC 4395 we investigate if extragalactic EMRB signals can provide information about their sources. This is possible, but only if the periapse radius of the orbit is small, of the order of r_p < 8 r_g, where r_g = GM/c^2 is the gravitational radius of the MBH. This limits the utility of EMRBs as an astronomical tool. However, if we are lucky, we could place constraints on the mass and spin of nearby MBHs with 1% precision.