The black hole binary V404 Cygni ( = GS 2023+338 ) had an outburst on June 15 , 2015 after 26 years of quiescence .
We report the multi-color optical observation ( g ^ { \prime } , R _ { C } , and I _ { C } ) of this object at the beginning of its outburst performed by the MITSuME 50cm telescope in Akeno , Yamanashi , and the MURIKABUSHI 105 cm Telescope at Ishigakijima Astronomical Observatory .
With time domain analysis of the multi-color light curves , we successfully decomposed optical variations into two components : a highly-variable , and a little-variable component .
The loci of the little-variable component in the color-color diagram are consistent with that of the multi-temperature blackbody radiation , while those of the highly-variable component trace out a power-law spectrum with a spectral index \alpha \sim 0.6–1.0 .
For the little-variable component , we argue that an irradiated disk with the innermost temperature higher than \sim 2.0 \times 10 ^ { 4 } K and the outermost temperature lower than \sim 6.5 \times 10 ^ { 3 } K is the most plausible source .
The gradual rise trend of the light curve during our observation is probably due to the growing of the little-variable component .
The observed spectral energy distribution ( SED ) from the optical to ultraviolet can be expressed by a model consisting of a power-law component and an irradiated disk component .