LISALISA - iucaa

1.2 Low-frequency sources of gravitational radiation
would have shortened rapidly due to gravitational radiation until it reached a minimum of about
600 s when the secondary mass was reduced to roughly 0.2M ⊙ . Later evolution due mainly to
mass transfer would be to longer periods, and the rate of period change would become slower
as the secondary mass decreased.
Hils [42] has estimated the HeCV signal strength under the above scenario. Because of the rapid
evolution of these binaries before they reach the AM CVn stage with low secondary masses, and
the weakness of the signals from then on, the resulting contributions to the rms binary signal
strength as a function of frequency are fairly small. However, the estimated number of such
sources in the frequency range of interest is large, so they fill many of the frequency bins that
otherwise would be open between roughly 1 and 3 mHz.
Another estimate for the HeCV space density based on a different assumption about the nature
of their progenitors has been given by Tutukov and Yungelson [43]. It considers the helium
star secondary to already be degenerate or semi-degenerate at the time of Roche lobe contact.
The resulting estimated space density of AM CVn binaries is much higher than the estimate of
Warner [40]. Hils and Bender [44] have recently made a new estimate based on the Tutukov-and-
Yugelson scenario, but with 10 % of their calculated space density. The results are quite similar
to those from [42]. Until the likelyhood of the different assumptions is better understood, the
uncertainty in the contribution of HeCVs to the confusion noise should be remembered. However,
they will not contribute much at frequencies above about 3mHz in any of the assumed scenarios.
A curve for the confusion noise including the HeCV contribution [42] as well as those from CWDBs
and other binaries is given in Figure 1.3 and in later figures.
Normal detached binaries, contact binaries, and cataclysmic variable binaries.
These three types of binaries have been discussed in some detail [16]. By normal detached
binaries we mean binaries consisting of normal, “unevolved” stars whose Roche lobes are not
in contact. “Unevolved”, as used here, means that the stars have not yet reached a giant
phase or started helium burning. Contact binaries are the WUMa binaries studied first by
Mironowskii [13], which are two unevolved stars with their Roche lobes in contact. A cataclysmic
variable binary consist of a white dwarf which accretes mass spilling over from a low mass
hydrogen-burning secondary.
Some individual binaries of each of these types will be close enough and at high enough frequency,
so their signals will be resolvable. This includes the normal detached binary ι Boo and the
cataclysmic variable WZSge, which are the two lowest-frequency circles shown in Figure 1.3 .
The expected confusion limits for the W UMa and cataclysmic variables are comparable with
the LISA noise budget level over the frequency range from 0.1 to 0.4 mHz. Thus, if the spectral
amplitudes for the CWDBs and helium cataclysmics should turn out to be low enough, the
abundance of these other types of binaries could be determined.
1.2.2 Massive black holes in distant galaxies
It is clear from the preceding sections that LISA will provide valuable information concerning
the populations of various types of binaries in different parts of our galaxy. However, the most
exciting scientific objectives for LISA involve the search for and detailed study of signals from
sources that contain massive black holes (MBHs).
The most spectacular event involving MBHs will be the coalescence of MBH-MBH binaries.
Because the signal has the unique signature of a “chirp” and can be followed over many months,
and because it is intrinsically very strong, LISA can recognise MBH coalescence events in its
frequency band almost anywhere in the Universe they occur. If LISA sees even one such event,
Corrected version 1.04 25 13-9-2000 11:47