LISALISA - iucaa

Chapter 7 Signal Extraction and Data Analysis
Figure 7.9 As Figure 7.5, but for U.
7.5 Data analysis
The objective on data analysis for a gravitational wave detector is to reconstruct as far as
possible the incoming gravitational wave. From the reconstruction, it is possible to make the
kind of inferences about sources that we have described in Chapter 1 . The parameters that
describe the wave are:
• Its direction on the sky in, say, galactic coordinates (l,b). These are constants that must
be maintained during the observation. Proper motion and parallax are unlikely because
the observations of Galactic objects are unlikely to attain better than a few arcminutes
directional accuracy. (A stochastic background will not have a precise direction, but that
caused by binaries may be anisotropic on the scale of tens of degrees.)
• Its amplitude and polarisation, or alternatively the amplitudes of two independent components
h + and h × , and their relative phase. For most LISA sources, these are constant
in time, or at least very slowly varying. Binary orbital precession will cause an intrinsic
amplitude modulation of the signal. As LISA orbits the Sun, the projection of the wave
on the detector will change, which also causes an apparent amplitude modulation, even if
the intrinsic amplitude and polarisation of the signal remain constant.
• Signal phase Φ(t). Gravitational wave detectors are coherent detectors, because their
operating frequencies are low enough to allow them to track the phase of the signal. The
phase, as a function of time, contains interesting information if it is not regular: binaries
that chirp, or even coalesce, provide important clues to their masses and distances in the
phase function, and the phase function of a black-hole binary allows LISA to track the
orbit to test general relativity.
The extraction of this information from the LISA data will use the same principles that have been
developed for ground-based interferometers. But there are a number of important differences
13-9-2000 11:47 112 Corrected version 1.04