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We present results from quasi-simultaneous multiwavelength observations ofthe Galactic black hole X-ray transient MAXI J1820$+$070 during the decay ofthe 2018 outburst and its entire subsequent mini-outburst in March 2019. We fitthe X-ray spectra with phenomenological and Comptonizaton models and discussthe X-ray spectral evolution comparing with the multiwavelength behaviour ofthe system. The system showed a rebrightening in UV/Optical/NIR bands 7-daysafter the soft-to-hard transition during the main outburst decay while it wasfading in X-rays and radio. In contrast, the mini-outburst occurred 165-daysafter the hard state transition of the initial outburst decay and was detectedin all wavelengths. For both events, the measured timescales are consistentwith those observed in other black hole systems. Contemporaneous hardX-ray/soft $\gamma$-ray observations indicate a non-thermal electron energydistribution at the beginning of the UV/Optical/NIR rebrightening, whereas athermal distribution can fit the data during the hard mini-outburst activity.The broadband spectral energy distributions until the rebrightening areconsistent with the irradiated outer accretion disc model. However, both theSEDs produced for the peak of rebrightening and close to the peak ofmini-outburst provided good fits only with an additional power-law component inthe UV/Optical/NIR frequency ranges which is often interpreted with a jetorigin.

Multiwavelength observations of MAXI J1820+070 during its outburst decay and subsequent mini-outburst