Abstract

European Space Agency recently adopted the Laser interferometry space antenna (LISA) to launch it in the 2030s to observe Gravitational waves (GWs) in the millihz frequency band. LISA ’s primary sources are massive black hole binaries (MBHBs) of 10^4-108 solar masses due to the merger of galaxies, stellar-mass BHs coalescence into their galactic central MBH , and numerous white dwarf binaries that exist in the Milky Way. I focus on MBH Bs that usually require an environment consisting mainly of gas and stars to merge within the Universe’s lifetime. I primarily consider the effects of a gas accretion disc for a sub-pc separation as it can significantly torque the binary to affect the initial binary parameters in the LISA band, especially orbital eccentricity. While gas perturbations become sub-dominant to GWs a few years before the merger of MBH Bs, they can still cause detectable phase shifts and excite observable eccentricities for reasonable disc properties that hint toward their history before reaching the millihz band. I will use analytical and numerical techniques to show the minimum measurable eccentricity of an MBHB that LISA will measure in a vacuum and gas. To be robust, I consider a high-order post-Newtonian gravitational waveform model, LISA ’s motion in its orbit around the Sun, and the time delay interferometery to enhance astrophysical signal over the dominant laser noise. I also explore if a weak gas effect can be mimicked by a small eccentricity and vice versa to motivate synergies between LISA and electromagnetic observations to unlock mysteries of the MBHB evolution.