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Upper limit map of a background of gravitational waves

Abbott, B. and Abbott, R. and Adhikari, R. and Agresti, J. and Ajith, P. and Allen, B. and Amin, R. and Lockerbie, N.A. (2007) Upper limit map of a background of gravitational waves. Physical Review D: Particles and Fields, 76 (8). ISSN 0556-2821

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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 1.2×10-48 Hz-1 (100 Hz/f)3 and 1.2×10-47 Hz-1 (100 Hz/f)3, depending on the position in the sky. Similarly, in the case of constant strain power spectrum, the upper limits vary between 8.5×10-49 Hz-1 and 6.1×10-48 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.