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Critical Frequencies and Gravitational Wave Detector Halfwidth for Extragalactic Observations

To get rid of the extragalactic GWB produced by binaries, one can use the above considerations to find the optimal detector halfwidth and frequency for observations. Again, for simple estimates we can suppose the stars in the Universe to be distributed homogeneously. Assuming an average galaxy to consist of tex2html_wrap_inline8911 stars and the total number of galaxies in the Universe to be tex2html_wrap_inline12241 , we get the average number of stars per decade falling into the detector's beam to be


for the flat red end of the GWB, and


for the blue end of the GWB formed by the coalescing binaries. Equating tex2html_wrap_inline12247 to unity, one finds the critical detector halfwidth


One immediately notes that observations at low frequencies ( tex2html_wrap_inline12249 ) are always contaminated by the stochastic binary GWB. In contrast, by observing the sky with a GW detector with a narrow enough halfwidth tex2html_wrap_inline12251 , one will not ``see'' the extragalactic GWB at frequencies tex2html_wrap_inline12253 .

Alternatively, if one uses a GW-detector with a fixed halfwidth of the beam, equation (15.8.1) gives the answer to the question at what frequency one should make observations so that they do not include any binary-produced stochastic noise. In fact, the transparency of the extragalactic GWB  occurs at frequencies above 1 Hz, that is in the part of the spectrum produced by coalescing binary neutron stars. Taking for their rate tex2html_wrap_inline12255 , one finds that the extragalactic GWB begins being transparent above the frequency


as shown in Figure 41.

Mike E. Prokhorov
Sat Feb 22 18:38:13 MSK 1997