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

TL;DR

This study analyzes NuSTAR observations of MAXI J1820+070 during a rare hard-to-soft state transition, revealing flaring behavior, long soft lags, and disc re-filling processes that shed light on accretion dynamics.

Contribution

It provides detailed timing and spectral analysis of flaring and lag phenomena during the state transition, highlighting the role of disc re-filling and low viscosity in the accretion process.

Findings

Detected a 20-second soft lag during flaring

Observed variability strongest at high energies

Identified disc re-filling as a possible cause of flares

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 $F_{\rm Var}\sim10\%$) 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 day before. The timescales of these phenomena imply a low viscosity parameter, $\alpha\sim10^{-3}$, for the inner region of the disc.