Charactering instrumental noises and stochastic gravitational wave signals from combined time-delay interferometry

Abstract

LISA will detect gravitational wave (GW) in the milli-Hz frequency band in space. Time-delay interferometry (TDI) is developed to suppress laser frequency noise beneath the acceleration noise and optical metrology noise. To identify stochastic GW signals, it would be required to characterize these noise components entangled in TDI data streams. In this work, we demonstrate noises characterization by combining the first-generation TDI channels from Michelson and Relay configurations. By assuming stationary Gaussian noise in three-year observation, the combination of Michelson and Relay could effectively break the degeneracy between the different optical path noises on three spacecraft. Based on the characterized noises, we further reconstruct the power spectrum density of noise in the selected TDI channel. Two cases are performed to characterize the spectrum shape of a stochastic GW signal. For a modeled signal, its parameter(s) could be directly estimated from the TDI data, and its spectrum could be recovered from the inferred values. And for an unexpected signal, its spectrum may be recognized and retrieved from noise-subtracted residual in which its power spectrum density surpasses the noise level.

On the effectiveness of null TDI channels as instrument noise monitors in LISA

“…We want to reinforce that the upper and lower bounds we compute here are fully agnostic to the actual instrument performance and don't rely on any model of the individual noise spectral shapes or stationarities (with the aforementioned caveat that we still rely on specific assumptions on noise correlations). This is in contrast to some other results in the literature (see for instance [14][15][16]31]), which showed it is possible to put significantly more stringent bounds on the noise assuming stationarity over the whole mission duration and a fixed (and known) noise shape which only depends on a single amplitude parameter. If indeed such a priori knowledge of the noise level and shape were possible, it would be possible to resolve SGWB even below the threshold of the instrument noise.…”

“…On the contrary, we will show that TM acceleration noise is effectively suppressed in the ζ channel, such that it is dominated by OMS noise at all frequencies. This behaviour of the null channels has already been pointed out in [14,15] for the null channel T that is built out of X and the two channels Y and Z (obtained from X by cyclic satellite permutations), considering the case of LISA with equal arm-lengths. However, as already shown in [16] and [4], T as a null channel is strongly compromised when the arm lengths are not exactly equal, especially at low frequencies.…”