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

ABSTRACT We consider a potentially new class of gravitational wave sources consisting of a white dwarf (WD) coalescing into a massive black hole (MBH) in the mass range ∼10⁴-10⁵ M_⊙. 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 WD mass parameter space, by an electro-magnetic signal generated by the tidal disruption of the star, detectable with X-ray, optical and ultraviolet (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 ⁴-10⁵ M_⊙, insights into dynamical interactions and populations of WDs in the cores of dwarf galaxies, as well as a new probe into the structure and equation of state of WDs. 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 WD mass functions and dynamical interactions: the rate is (not surprisingly) highly uncertain, ranging from ∼0.01 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 favour the high end of the above range, making likely the detection of several events during the LISA lifetime.