Time Delay Interferometry combinations as instrument noise monitors for LISA

TL;DR

This paper explores new Time Delay Interferometry combinations for LISA that can effectively monitor instrumental noise, aiding the detection of stochastic gravitational wave backgrounds by distinguishing noise from signals.

Contribution

It introduces novel TDI null-combinations that suppress gravitational waves, providing improved methods for instrumental noise estimation in LISA.

Findings

Existence of multiple null-combinations sensitive to instrumental noise.

Some null-combinations outperform traditional TDI channels in noise estimation.

Null-combinations relate to specific test-mass acceleration patterns.

Abstract

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 GW backgrounds. As correlation with other detectors is not possible for LISA, discrimination of 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 gravitational waves and mainly carry information about instrumental noise. We find that there exist many null-combinations that show different sensitivities to gravitational waves, some of which seem more suitable than the traditional T combination for estimating test-mass acceleration noise. In an idealised LISA configuration, they are all sensitive to a particular linear combination of the six test-masses 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.