Time delay interferometry combinations as instrument noise monitors for LISA

The LISA mission will likely be a signal dominated detector, such that one challenge is the separation of the different astrophysical sources, and to distinguish between them and the instrumental noise. One of the goals of LISA is to probe the early Universe by detecting stochastic gravitational wave (GW) backgrounds. As correlation with other detectors is uncertain for LISA, discriminating such a GW background from the instrumental noise requires a good estimate of the latter. To this purpose, we have revisited time delay interferometry (TDI) to look for new TDI signal combinations that fulfill the laser frequency noise suppression requirements. We illustrate that it is possible to do a linear combination of these TDI channels to find special null combinations that suppress GWs and mainly carry information about instrumental noise. We find that there exist many null combinations that show different sensitivities to GWs, some of which seem more suitable than the traditional T combination for estimating test-mass (TM) acceleration noise. In an idealized LISA configuration, they are all sensitive to a particular linear combination of the six TMs acceleration, similar to a rigid rotation of the LISA triangle. In the following article, we illustrate what are the noise properties that can be extracted by monitoring these interferometry signals and discuss the implication of these findings for the detection of stochastic GW backgrounds.