Dirty waveforms: multiband harmonic content of gas-embedded
  gravitational wave sources
  We analyse the effect of stochastic torque fluctuations on the orbital
evolution and the gravitational wave (GW)emission of gas-embedded sources with
intermediate and extreme mass ratios. We show that gas-driven fluctuations
imprint additional harmonic content in the GWs of the binary system, which we
dub dirty waveforms(DWs). We find three interesting observational prospects for
DWs, provided that torque fluctuations do indeed persist beyond the resolution
limit of current hydrodynamical simulations. Firstly, DWs can produce a
significant stochastic GW background, comparable to other GW noise sources.
Secondly, the energy flux implied by the additional harmonics can cause a
detectable secular phase shift in Laser Interferometer Space Antenna (LISA)
sources, even if the net torque fluctuations vanish when averaged over orbital
time-scales. Lastly, the DWs of moderate-redshift nHz supermassive binaries
detectable by pulsar timing arrays (PTAs) could be detectable in the mHz range,
producing a new type of PTA-LISA multiband gravitational source. Our results
suggest that searching for DWs and their effects can potentially be a novel way
to probe the heaviest of black holes and the physics of the accretion discs
surrounding them. We find these results to be a further confirmation of the
many exciting prospects of actively searching for environmental effects within
the data stream of future GW detectors.
