  In the light of the history of researches on electromagnetic wave spectrum, a
sharp emission line of gravitational-wave background (GWB) would be an
interesting observational target. Here we study an efficient method to detect a
line GWB by correlating data of multiple ground-based detectors. We find that
the width of frequency bin for coarse graining is a critical parameter, and the
commonly-used value 0.25 Hz is far from optimal, decreasing the signal-to-noise
ratio by up to a factor of seven. By reanalyzing the existing data with a
smaller bin width, we might detect a precious line signal from the early
universe.
