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Galactic Gravitational Wave Background

Let the sky be observed with a detector having a pencil beam diagram tex2html_wrap_inline12147 across. The number of stars per frequency decade that is swept by the detector is a function of celestial coordinates (say, galactic coordinates l,b):


For a homogeneous distribution of stars over the sky this number is coordinate independent:


Here tex2html_wrap_inline12063 is defined as


All sky ( tex2html_wrap_inline12159 steradian) corresponds to tex2html_wrap_inline8925 , and for small tex2html_wrap_inline12063 this is just the halfwidth of the beam.

The transparency of the background begins once tex2html_wrap_inline12165 ; this inequality provides us with the critical angular resolution tex2html_wrap_inline12167 the detector should have in order not to ``notice'' the background. To obtain rough estimates we can use a ``homogeneous'' galaxy to find


Factor tex2html_wrap_inline12169 denotes


(below the dependence () will be omitted). Obviously, the red part is of no interest (unless one observes a region of the sky very poor in stars), and for the blue part one finds a critical frequency, tex2html_wrap_inline8915 , above which no stars occur in the region of the sky subtended by the detector's beam. The function tex2html_wrap_inline12177 is shown in Figure 41.


Figure 41: The regions of transparency of the GWB from different objects in the angular resolution of the detector - critical frequency tex2html_wrap_inline8915 diagram. The GWB from a homogeneous model of our Galaxy (the total number of stars is tex2html_wrap_inline8911 ) is transparent below the line ``Our Galaxy''. The line marked ``sample galaxy'' shows the transparency boundary for a Milky Way-type galaxy with observed angular size tex2html_wrap_inline8919 tex2html_wrap_inline8921 . The break at 0.05 Hz is caused by the continuity limit of coalescing binary WD at a rate of 1 per 100 year. The bottom curve shows the transparency boundary for the GWB from tex2html_wrap_inline8911 external galaxies. The upper horizontal line corresponds to the all-sky detector ( tex2html_wrap_inline8925 ). The hatched region corresponds to ``absolutely hopeless'' (Lipunov et al., 1995a). 

It is clear from the figure that for an all-sky detector the Galaxy becomes transparent above the frequency tex2html_wrap_inline12191  Hz. For a realistic LIGO detector network angular resolution of about tex2html_wrap_inline12065 this frequency reduces to tex2html_wrap_inline89452 tex2html_wrap_inline8845 tex2html_wrap_inline12199  Hz.

The isolines of the critical frequency on the (l,b) sky map are presented in Figure 42 for the realistic galactic model described above and 10 percent of the mass of the corona consisting of stars, seen by the tex2html_wrap_inline12065-detector.


Figure 42: Isolines of the critical frequency on the (l,b)-sky map for the model of the Galaxy described in the text (Lipunov et al., 1995a). 

next up previous contents index
Next: Extragalactic Gravitational Wave Background Previous: Transparency of the Stochastic

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