In-depth analysis of LISA Pathfinder performance results: Time evolution, noise projection, physical models, and implications for LISA

TL;DR

This paper provides a comprehensive analysis of LISA Pathfinder's performance, focusing on noise evolution, physical models, and implications for future gravitational wave observatories, with detailed insights into acceleration noise sources over 500 days.

Contribution

It offers a detailed characterization of the acceleration noise and its sources, including the effects of outgassing and temperature fluctuations, informing improvements for LISA.

Findings

Brownian noise decay linked to outgassing pressure changes

Identification of a 1/f noise tail with 20% fluctuations

Temperature fluctuations modulate pressure gradients affecting low-frequency noise

Abstract

We present an in-depth analysis of the LISA Pathfinder differential acceleration performance over the entire course of its science operations, spanning approximately 500 days. We find that: 1) the evolution of the Brownian noise that dominates the acceleration amplitude spectral density (ASD), for frequencies $f\gtrsim 1\,\text{mHz}$, is consistent with the decaying pressure due to the outgassing of a single gaseous species. 2) between $f=36\,\mu\text{Hz}$ and $1\,\text{mHz}$, the acceleration ASD shows a $1/f$ tail in excess of the Brownian noise of almost constant amplitude, with $\simeq 20\%$ fluctuations over a period of a few days, with no particular time pattern over the course of the mission; 3) at the lowest considered frequency of $f=18\,\mu\text{Hz}$, the ASD significantly deviates from the $1/f$ behavior, because of temperature fluctuations that appear to modulate a quasi-static pressure gradient, sustained by the asymmetries of the outgassing pattern. We also present the results of a projection of the observed acceleration noise on the potential sources for which we had either a direct correlation measurement, or a quantitative estimate from dedicated experiments. These sources account for approximately $40\%$ of the noise power in the $1/f$ tail. Finally, we analyze the possible sources of the remaining unexplained fraction, and identify the possible measures that may be taken to keep those under control in LISA.