Spectral and timing analysis of Cyg X-1 with the ASCA archive data Original Research Article

Rare Soft State of Cyg X-1 was observed by ASCA in 1996, and the data were compared with those obtained in 1993 in Hard State. We studied the difference of the X-ray emission from optically thick accretion disk. The average energy spectra in both Hard and Soft States can be well reproduced by the multi-color disk model plus power law modified by the cold matter absorption. We compared parameters of multi-color disk model and found that the mass accretion rate may be 3 times larger in Hard State than in Soft State. Light curves in Hard State showed intensity variation of about 20% in time scale of ∼1 hour. Spectral analysis shows that the disk and the power law component vary hand in hand. Time variations similar to shot noise were observed in time scale of ∼ 1 sec with amplitude up to 30%. Significant variations are recognized only in the power law component.