Pre-Phase A Report - Lisa - Nasa

118 Chapter 4 Measurement Sensitivity
thickness shells ranging all the way out to cosmological distances, and thus will be nearly
isotropic. The small anisotropies in the background due to nearby concentrations of stars
such as the LMC, M31, and the Virgo cluster, as discussed by Lipunov et al. [117], will be
difficult to detect.
The only handle for separating the possible cosmic backgrounds from the dominant
isotropic part of the extragalactic binary background is the spectrum. There is enough
uncertainty in the ratios of the numbers of binaries of various kinds in other galaxies to the
numbers in our galaxy so that the strength of the extragalactic binary background cannot
be predicted reliably. However, since the types of binaries contributing most strongly
at the frequencies of interest probably will be evolving mostly by emitting gravitational
radiation, the spectrum may be known quite well. If the spectrum of a cosmic background
were significantly different and the amplitude were large enough, such a background could
be separated and quantified.
Perhaps the most significant question is how well all of the backgrounds can be separated
from instrumental noise. To discuss this question, it is useful to divide the instrumental
noise above roughly 10−4 Hz into three different types. One is stationary noise with steady
amplitude at all frequencies of interest. The second is noise which varies at one and two
cycles/year, in such a way that it mimics the interaction of the galactic background with
the rotating antenna pattern. The third is noise with all other types of time variations.
The first and second types cannot be separated from isotropic and galactic backgrounds,
respectively, except if they are substantially higher than experimental limits which can
be put on instrumental noise. The third type of noise would not be confused with real
background signals.
For measuring the difference in distances between proof masses, the main noise sources
are photon shot noise and phase shifts from fluctuations in laser beam pointing. The
shot noise contribution can be calculated from the received light level and at least partly
subtracted out. For beam pointing fluctuations, it is difficult to say how much of the noise
may be of the first two types, but an estimate of a third or less of the level given in the
error budget seems reasonable. Specific experiments during the mission to characterize the
noise by changing the gain of the beam-pointing servo loops and temporarily defocusing
the beams somewhat should be considered.
For spurious accelerations of the proof masses, there are a number of items of comparable
size in the error budget. A few, like random residual gas impacts on the proof masses,
may be quite stationary, although they also may have variations at annual and six month
periods from spacecraft temperature variations. However, it seems likely that most of
the spurious acceleration sources will not be predominantly stationary. For example,
this would apply to sources such as the interaction of the average charge on the proof
mass with the fluctuations in the solar wind magnetic field. Consideration will be given
to including diagnostic experiments, such as changing the average proof-mass charge or
changing how tightly the spacecraft follow the proof masses, in order to characterize the
spurious acceleration noise sources as well as possible.
Overall, it seems reasonable to estimate that perhaps a third of the total instrumental
noise in the distance measurement and spurious acceleration error budgets would be difficult
to separate from real background signals. The extent to which data from the third
arm of LISA would aid in searching for background signals has not yet been investigated.
3-3-1999 9:33 Corrected version 2.08