THE 1996 SOFT STATE TRANSITION OF CYGNUS-X-1

We report continuous monitoring of Cygnus X-1 in the 1.3-200 keV band using Ah-Sky Monitor/Rossi X-Ray Timing Explorer and BATSE/Compton Gam ma Ray Observatory for about 200 days from 1996 February 21 to early S eptember. During this period, Cygnus X-1 experienced a hard-to-soft an d then a soft-to-hard state transition. The low-energy X-ray (1.3-12 k eV) and high-energy X-ray (20-200 keV) fluxes are strongly anticorrela ted during this period. During the state transitions, flux variations of about a factor of 5 and 15 were seen in the 1.3-3.0 keV and 100-200 keV bands, respectively, while the average 4.8-12 keV flux remains al most unchanged. The net effect of this pivoting is that the total 1.3- 200 keV luminosity remained unchanged to within similar to 15%. The bo lometric luminosity in the soft state may be as high as 50%-70% above the hard state luminosity, after color corrections for the luminosity below 1.3 keV. The blackbody component flux and temperature increase i n the soft state are probably caused by a combination of the optically thick disk mass accretion rate increase and a decrease of the inner d isk radius.