  We have developed a new method of data processing for radio telescope
observation data to measure time-dependent temporal coherence, and we named it
cross-correlation spectrometry (XCS). XCS is an autocorrelation procedure that
expands time lags over the integration time and is applied to data obtained
from a single-dish observation. The temporal coherence property of received
signals is enhanced by XCS. We tested the XCS technique using the data of
strong H2O masers in W3 (H2O), W49N and W75N. We obtained the temporal coherent
lengths of the maser emission to be 17.95 $\pm$ 0.33 {\mu}s, 26.89 $\pm$ 0.49
{\mu}s and 15.95 $\pm$ 0.46 {\mu}s for W3 (H2O), W49N and W75N, respectively.
These results may indicate the existence of a coherent astrophysical maser.
