MAXI J1820+070 with NuSTAR - II. Flaring during the hard to soft state transition with a long soft lag

We continue the analysis of NuSTAR data from the recent discovery outburst of MAXI J1820+070 (optical counterpart ASASSN-18ey), focussing on an observation including unusual flaring behaviour during the hard to soft state transition, which is a short phase of outbursts and so comparatively rarely observed. Two plateaus in flux are separated by a variable interval lasting ~10 ks, which shows dipping then flaring stages. The variability is strongest (with fractional variability up to $F_{\rm Var}\sim 10{{\ \rm per\ cent}}$) at high energies and reduces as the contribution from disc emission becomes stronger. Flux-resolved spectra show that the variability is primarily due to the power-law flux changing. We also find a long soft lag of the thermal behind the power-law emission, which is $20_{-1.2}^{+1.6}$ s during the flaring phase. The lag during the dipping stage has a different lag-energy spectrum, which may be due to a wave passing outwards through the disc. Time-resolved spectral fitting suggests that the lag during the flaring stage may be due to the disc re-filling after being disrupted to produce the power-law flare, perhaps related to the system settling after the jet ejection which occurred around 1 d before. The time-scales of these phenomena imply a low viscosity parameter, α ~ 10^-3, for the inner region of the disc.