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Optical and J,K-photometry of black hole X-ray nova A0620-00 in passive and active stages of quiescence

Photometric observations of the low-mass X-ray binary system A0620-00=V616 Mon are performed in the optical (unfiltered light, lambda_eff~6400A) and the near-infrared J and K-bands. The mean system flux, the orbital light curve shape and the flickering amplitude dependences on wavelength are examined for two activity stages of the system remaining in quiescence. In 2015-16 A0620-00 was in passive stage (as by Cantrell et al., 2008) exhibiting the regular orbital light curves and low flickering. In less than 230 days in 2016-17 the system switched into active stage: the brightness increased by ~0.2-0.3 mag, the orbital light curve changed while the flickering amplitude increased more than twice. The object regular orbital light curves were fitted by models with "cold" spots on the optical star surface and without those. These models reproduce the observed orbital light curves both in passive and in active stages. The dependence of the mean square flickering amplitude (in fluxes, extinction corrected) on wavelength is computed in the lambda 6400-22000AA range. In active stage, the observed flickering amplitude decreases over the whole studied range and may be represented as Delta F_fl~lambda^{-2} which corresponds to the free-free emission of optically thin high-temperature plasma. In passive stage, flickering obeys Delta F_fl~lambda^{-4} law in the range 6400-12500AA that corresponds to the thermal radiation of optically thick high-temperature plasma. At longer wavelengths the flickering amplitude dependence is flat which may imply existence of a synchrotron component of the relativistic jets emission. These flickering features let us propose that the flickering mechanism includes at least two components: thermal and, apparently, synchrotron, that agrees with the recent discovery of the variable linear polarization of the IR system emission (Russell et al., 2016).