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

Abstract

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 Var ∼10 per cent FVar∼10 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 +1.6 −1.2 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.

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ORCID for Poshak Gandhi: orcid.org/0000-0003-3105-2615

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