Upper limit map of a background of gravitational waves

We searched for an anisotropic background of gravitational waves using data from the LIGO S4 science run and a method that is optimized for point sources. This is appropriate if, for example, the gravitational wave background is dominated by a small number of distinct astrophysical sources. No signal was seen. Upper limit maps were produced assuming two different power laws for the source strain power spectrum. For an $f^{-3}$ power law and using the 50 Hz to 1.8 kHz band the upper limits on the source strain power spectrum vary between $1.2\times {10}^{-48}{\mathrm{Hz}}^{-1}$ $(100\mathrm{Hz}/f{)}^3$ and $1.2\times {10}^{-47}{\mathrm{Hz}}^{-1}$ $(100\mathrm{Hz}/f{)}^3$, depending on the position in the sky. Similarly, in the case of constant strain power spectrum, the upper limits vary between $8.5\times {10}^{-49}{\mathrm{Hz}}^{-1}$ and $6.1\times {10}^{-48}{\mathrm{Hz}}^{-1}$. As a side product a limit on an isotropic background of gravitational waves was also obtained. All limits are at the 90% confidence level. Finally, as an application, we focused on the direction of Sco-X1, the brightest low-mass x-ray binary. We compare the upper limit on strain amplitude obtained by this method to expectations based on the x-ray flux from Sco-X1.