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Astrophysics, Instrumentation and Methods for Astrophysics, astro-ph.IM
Abstract:
We present an analysis of the LISA Pathfinder differential acceleration
performance over the entire mission . We show that the Brownian noise level,
detected for frequencies $f\gtrsim \SI{1}{mHz}$, has been evolving consistently
with the outgassing of a single gaseous species, with an activation temperature
of $(7.0\pm 0.2)\,\text{kK}$. In excess to the Brownian noise, the acceleration
amplitude spectral density (ASD) always shows a sub-mHz tail which is
reasonably well fit, between $f=\SI{36}{\micro\hertz}$ and
$\SI{1}{\milli\hertz}$, to $\widetilde{S}_{\Delta g}^{1/2}(1\, \text{mHz}/f)$.
A Bayesian estimate of $\widetilde{S}_{\Delta g}^{1/2}$ on a partition of the
entire set of measurements in 27 data stretches, each 2.75\,d long, gives
$\widetilde{S}_{\Delta
g}^{1/2}=(1.1\pm0.3)\,\si{\femto\meter\,\second^{-2}/\rtHz}$, with no
particular time pattern over the course of the mission. The width the posterior
contains, in excess of the statistical uncertainty, a true physical fluctuation
of $\widetilde{S}_{\Delta g}^{1/2}$ from run to run, of about
$\SI{0.2}{\femto\meter\,\second^{-2}/\rtHz}$, with no correlation with specific
operating conditions. At the lowest considered frequency of
$f=\SI{18}{\micro\hertz}$, 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 outgassing . We
also present a projection of acceleration noise on the sources for which we had
either a correlation measurement, or an estimate from dedicated
experiments.These sources account for about 40\% of the noise power the $1/f$
tail. We discuss the possible sources of the unaccounted-for fraction, present
a series of analyses that rule many of them out, and identify the possible
measures that may be taken to keep the remaining ones under control in LISA.
