Contribution of Compact Mass Transferring Systems to the Galactic Gravitational Wave Background

Compact object binaries, mostly double with dwarfs, are believed to be a potential source of confusion-limited noise for the Laser Interferometer Space Antenna (LISA). In a specific frequency range, this noise may rise above the instrumental noise sources and hence hinder detection of other types of signals, e.g. extreme mass ratio inspirals of stellar mass objects into supermassive black holes. In most previous studies only detached populations of compact object binaries have been considered. Here, we investigate the influence of Galactic populations of compact mass transferring binaries on the shape and strength of the LISA signal and compare our results with the signal for detached binaries only. Our population synthesis includes all binary systems containing two compact remnants. It is found that 99.5% of these systems are double white dwarfs, and therefore we consider only binaries consisting of two white dwarfs when calculating the LISA signal. We find that the mass transferring binaries contributing to the confusion limited noise dominate the population of LISA systems by number (60%), due to their long lifetimes (few Gyrs). However, due to the very specific physical properties of mass transferring systems, it is found that their contribution to the noise is negligible at low frequencies and detached systems will set the magnitude of the confusion-limited signal in this range. However, the mass transferring systems begin to dominate the signal at higher frequencies (f>6 mHz). The transition frequency at which the confusion limited regime becomes important is found at 4 mHz. The addition of mass transferring systems tends to slightly increase the transition frequency (from 3.5 to 4 mHz).