Pre-Phase A Report - Lisa - Nasa

30 Chapter 1 Scientific Objectives
If sudden collapses to MBHs do occur for gas clouds large enough to give roughly 10 5 to
10 7 M⊙ MBHs, an important question is how much angular momentum will be left. If the
cloud hasn’t lost its angular momentum rapidly enough, a bar instability may occur and
cause considerable gravitational radiation in the main LISA frequency band. Thus looking
for pulses lasting only a few cycles will be important for LISA, as it is for supernova pulse
searches with ground-based detectors.
MBH-MBH binary coalescence. Figure 1.4 shows the expected signal strength of
coalescing MBH binary events in LISA against the LISA noise curve. The signal strengths
Gravitational Wave Amplitude
10 −17
Log h
-17.5
10 −18
-18.5
10 −19
-19.5
10 −20
-20.5
10 −21
-21.5
10 −22
-22.5
10 −23
-23.5
10 6 −10 6 M •O
10 6 /10 6 M ⁄
10 5 −10 5 M •O
LISA Instrumental Threshold
LISA Instrumental 1 yr, S/N=5 Threshold
1 yr, S/N = 5
10 5 /10 5 M ⁄
10 4 /10 4 M ⁄
Binary Confusion
Binary Confusion
Noise Threshold Estimate; Estimate
1 yr, yr, S/N=5 S/N = 5
MBH-MBH Binaries at z=1
500−500 500/500 M M⁄ •O
-5.0 -4.5 -4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0
10
Log Frequency (Hz)
−5
10 −4
10 −3
10 −2
10 −1
Frequency (Hz)
10 4 −10 4 M •O
Figure 1.4 Strain amplitude during the last year before MBH-MBH coalescence.
and frequencies are displayed as a function of time for some possible MBH-MBH coalescence
events at a redshift of 1. The Hubble constant H0 is assumed to be 75 km s−1Mpc−1 .
The straight lines sloping up to the right show the values of the gravitational wave signal
strength h as a function of time during the last year before coalescence for different pairs
of MBH masses. The first 5 symbols from the left correspond to times of 1.0, 0.8, 0.6, 0.4,
and 0.2 yr before coalescence, while the 6th symbol is for 0.5 week before coalescence. The
final symbol for the two highest mass pairs is at the approximate coalescence frequency.
(Note that, at cosmological distances, the observed chirp mass of a binary is 1+z times its
true chirp mass, its radiation is redshifted by the same factor, and its Euclidian distance
is replaced by its luminosity distance. These factors are taken into account in Figure 1.4 .)
The case of 500 M⊙ MBHs is included to correspond to possible coalescences of seed MBHs,
rather than to currently plausible events resulting from galaxy mergers.
The LISA instrumental threshold curve for 1 year of observations and S/N = 5 is included
in Figure 1.4, along with the corresponding binary confusion noise estimate. The instrumental
threshold curve has been arbitrarily and optimistically extended from 0.1 mHz to
0.03 mHz with constant slope, even though there is no recommended error budget yet
3-3-1999 9:33 Corrected version 2.08