Optical and J, K-photometry of the quiescent black hole X-ray nova A0620−00 in the passive and active states

In 2015-2016, the X-ray nova A0620-00 in quiescence was observed in the passive state, according to the terminology of Cantrell et al., and in less than 230 d it transited into the active state. The system mean luminosity increased by 0.20 mag in the optical and by 0.25 and 0.30 mag in J and K infrared bands, respectively, while the orbital light curves changed drastically, and the flickering amplitude more than doubled, The mean light-curve analysis performed in the context of two models argued that the interaction region where the gas stream reaches the disc is responsible for these effects. The growth of this region's luminosity in the active state implies the increase of the mass transfer rate via the L1 point, likely due to non-stationary processes in the donor star atmosphere. The non-stellar spectrum F-lambda in the observed range lambda lambda 6400-22 000 angstrom obeys a power law of lambda(alpha) where alpha = -(2.13 +/- 0.1) in the passive stale and alpha = -(1.85 +/- 0.1) in the active, while the absolute mean square flickering amplitude behaves like Delta F-fl(lambda) similar to lambda(-2.36) in both states of activity.