Upper limits to surface-force disturbances on LISA proof masses and the possibility of observing galactic binaries

We have measured surface-force noise on a hollow replica of a LISA proof mass surrounded by its capacitive motion sensor. Forces are detected through the torque exerted on the proof mass by means of a torsion pendulum in the 0.1–30 mHz range. The sensor and electronics have the same design as for the flight hardware, including 4 mm gaps around the proof mass. The measured upper limit for forces would allow detection of a number of galactic binaries signals with signal-to-noise ratio up to $\approx 40$ for 1 yr integration. We also discuss how LISA Pathfinder will substantially improve this limit, approaching the LISA performance.

Figure 1

Left: the electrode housing of the GRS. The electrodes for the $x$-direction motion readout are the pair of shiny vertical plates visible inside the sensor. On the adjacent $y$-face, there is a $y$-sensing electrode, the upper horizontal plate, and one of the electrodes used to polarize the proof mass for capacitive readout. Right: the hollow proof mass used for the torque measurements.

Figure 2

Total torque noise. Data points: square root of torque noise PSD estimated from autocollimator and capacitive sensor cross correlation after time domain subtraction of the effect of PM motion along $y$. Values and error bars are estimated as the mean and its standard deviation of PSD data falling within the corresponding frequency bins. Bins have equal logarithmic width. Dashed lines delimit the error band for the estimate of thermal noise. The continuous smooth line is a linear least square fit to data as described in the text.

Figure 3

Real part of cross coherence between autocollimator and GRS data.

Figure 4

Dashed line: upper limit to pendulum torque noise with GRS converted into equivalent acceleration on one of LISA proof masses. Dots: acceleration level required to detect with LISA, with signal-to-noise ratio of 1, the signal from the corresponding galactic binary system over 1 yr integration. Dotted line: projected upper limit for nonmodeled overall force disturbances from the LPF mission. Solid line: LISA requirements.