INTERPRETATION OF THE OPTICAL FLARE IN CYG-X-1 ON MAY 14, 1997

In May 1997, an optical flare was observed in the X-ray binary system Cyg X-1/V1357 Cyg, the strongest flare in the 26 years of its study. F or 2.5 hours of observations on May 14/15, the system's brightness exc eeded the level of regular ellipsoidal variability by about 15% in U, 12% in B, and a mere 2% in V. The color properties of the hare corresp ond to the emission of a hydrogen plasma. The Balmer lines and the Bal mer continuum fair within the U and B bands, but do not affect the V b and. Estimates show that the observed characteristics correspond to a rapidly expanding cloud of gas (v similar to 1500 km s(-1) ) with a te mperature of approximate to 20000 K, which is observed when the optica l depths in the HP-HE lines are of the order of unity. The size of the emitting region is similar to 3 x 10(12) cm, the gas number density i s similar to 3 x 10(11) cm(-3), and the expansion time is similar to 6 h.