MITSuME observation of V404 Cygni in the 2015 outburst: Two optical variable components with different variability

Abstract

Abstract The black hole binary V404 Cygni (= GS 2023+338) had an outburst on 2015 June 15 after 26 years of its quiescence. We report on the multi-color optical observation (g΄, RC, and IC) of this object at the beginning of its outburst performed by the MITSuME 50 cm telescope in Akeno, Yamanashi, and the MURIKABUSHI 105 cm telescope at Ishigakijima Astronomical Observatory. Through time domain analysis of the multi-color light curves, we successfully decomposed the optical variations into two components: a highly variable and a less-variable component. The loci of the less-variable component in the color–color diagram are consistent with that of multi-temperature blackbody radiation, while those of the highly variable component trace out a power-law spectrum with a spectral index α ∼ 0.6–1.0. For the less-variable component, we argue that an irradiated disk with the innermost temperature higher than ∼2.0 × 104 K and the outermost temperature lower than ∼6.5 × 103 K is the most plausible source. The gradual rising trend of the light curve during our observation is probably due to the growing of the less-variable component. The observed spectral energy distribution from optical to ultraviolet can be expressed by a model consisting of a power-law component and an irradiated disk component.