TIME VARIATIONS IN THE X-RAYS FROM CYGNUS-X-3 OBSERVED WITH GINGA

We present the results of time-variation analyses in the X-rays from Cyg X-3 observed with the large-area counters on board Ginga. No significant modulation near 12 ms was detected. The upper limit of the amplitude in the range 10-256 Hz, assuming sinusoidal modulation and taking account of the orbital motion of the X-ray star, is 0.78% of the mean intensity. Aperiodic variation shorter than about 1 Hz is also not detected, but variation longer than 0.1 Hz is observed. Cross spectral analysis shows frequency-independent time lags of the soft X-ray variation. These facts indicate that the X-ray-emitting region is surrounded by cold scattering matter with R-tau(tau > 1) or R(tau < 1) = (3-30) x 10(11) cm, where R and tau are the radius and the Thomson scattering optical depth, respectively. We also get R < 4.8 x 10(11) cm from a diplike structure observed in the light curve. We also found that the time variation during the X-ray-intense phase was greater than that during the X-ray-faint phase. This fact, and the time derivative of the 4.8 hr modulation of the order of 10(-9) s s-1, strongly support the stellar wind model to explain the almost sinusoidal X-ray light curve.