Accretion scenario of MAXI J1820+070 during 2018 outbursts with multimission observations

We present a comprehensive spectral and temporal study of the black hole X-ray transient MAXI J1820+070 during its outbursts in 2018 using Swift/XRT, NICER, NuSTAR, and AstroSat observations. The Swift/XRT and NICER spectral study shows a plateau in the light curve with spectral softening (hardness changes from ~ 2.5 to 2) followed by a gradual decline without spectral softening during the first outburst. Also, spectral modelling suggests that the first outburst is in the low/hard state throughout with a truncated disc, whereas the thermal disc emission dominates during the second outburst. During the entire outburst, strong reflection signature (reflection fraction varies in the range ~ 0.38-3.8) is observed in the simultaneous wideband (NICER-NuSTAR, XRT-NuSTAR, AstroSat) data due to the presence of a dynamically evolving corona. The NICER timing analysis shows quasi-periodic oscillation signatures and the characteristic frequency increases (decreases) in the plateau (decline) phase with time during the first outburst. We understand that the reduction of the electron cooling time-scale in the corona due to spectral softening and the resonance oscillation with the local dynamical time-scale may explain the above behaviour of the source during the outburst. Also, we propose a possible scenario of outburst triggering and the associated accretion geometry of the source.