Observing white dwarfs orbiting massive black holes in the gravitational wave and electro-magnetic window

TL;DR

This paper explores a new class of gravitational wave sources involving white dwarfs merging with massive black holes, which can be detected through both gravitational waves and electromagnetic signals, offering diverse scientific insights.

Contribution

It models the gravitational and electromagnetic signals from white dwarf-black hole mergers, estimating detection rates and highlighting their potential for multi-messenger astronomy.

Findings

Detectable in both gravitational and electromagnetic signals up to 200 Mpc.

Estimated detection rate ranges from 0.01 to 100 per year.

High likelihood of observing several events with LISA during its mission.

Abstract

We consider a potentially new class of gravitational wave sources consisting of a white dwarf coalescing into a massive black hole in the mass range ~10^4-10^5\msun. These sources are of particular interest because the gravitational wave signal produced during the inspiral phase can be detected by the Laser Interferometer Space Antenna (LISA) and is promptly followed, in an extended portion of the black hole and white dwarf mass parameter space, by an electro-magnetic signal generated by the tidal disruption of the star, detectable with X-ray, optical and UV telescopes. This class of sources could therefore yield a considerable number of scientific payoffs, that include precise cosmography at low redshift, demographics of black holes in the mass range ~10^4-10^5\Msun, insights into dynamical interactions and populations of white dwarfs in the cores of dwarf galaxies, as well as a new probe into the structure and equation of state of white dwarfs. By modelling the gravitational and electromagnetic radiation produced by these events, we find them detectable in both observational windows at a distance ~200 Mpc, and possibly beyond for selected regions of the parameter space. We also estimate the detection rate for a number of model assumptions about black hole and white dwarf mass functions and dynamical interactions: the rate is (not surprisingly) highly uncertain, ranging from ~0.01 yr^-1 to ~100 yr^-1. This is due to the current limited theoretical understanding and minimal observational constraints for these objects and processes. However, capture rate scaling arguments favor the high end of the above range, making likely the detection of several events during the LISA lifetime.