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 similar to 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 similar to 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.

Automated summary

This study presents a comprehensive analysis of the spectral and temporal properties of the black hole X-ray transient MAXI J1820+070 during its outbursts in 2018. The study reveals softening of the spectrum during the first outburst and dominance of thermal disc emission during the second outburst. The source exhibits strong reflection signatures and quasi-periodic oscillations throughout the entire outburst.