Astrophysical motivation for directed searches for a stochastic
  gravitational wave background
  The nearby universe is expected to create an anisotropic stochastic
gravitational wave background (SGWB). Different algorithms have been developed
and implemented to search for isotropic and anisotropic SGWB. The aim of this
paper is to quantify the advantage of an optimal anisotropic search,
specifically comparing a point source with an isotropic background. Clusters of
galaxies appear as point sources to a network of ground based laser
interferometric detectors. The optimal search strategy for these sources is a
"directed radiometer search". We show that the flux of SGWB created by the
millisecond pulsars in the Virgo cluster produces a significantly stronger
signal than the nearly isotropic background of unresolved sources of the same
kind. We compute their strain power spectra for different cosmologies and
distribution of population over redshifts. We conclude that a localised source,
like the Virgo cluster, can be resolved from the isotropic background with very
high significance using the directed search algorithm. For backgrounds
dominated by nearby sources, up to redshift of about 3, we show that the
directed search for a localised source can have signal to noise ratio more than
that for the all sky integrated isotropic search.
