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 $F_{\mathrm{Var}}\sim 10\%$) 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, $\alpha \sim 10^{-3}$, for the inner region of the disc.

MNRAS, resubmitted after minor revision. 12 pages, 10 figures, 1 table

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Cited by in corpus (10)

• Disk, Corona, Jet Connection in the Intermediate State of MAXI J1820+070 Revealed by NICER Spectral-Timing Analysis
• Measuring fundamental jet properties with multi-wavelength fast timing of the black hole X-ray binary MAXI J1820+070
• The NICER "Reverberation Machine": A Systematic Study of Time Lags in Black Hole X-Ray Binaries
• The Varying Kinematics of Multiple Ejecta from the Black Hole X-ray Binary MAXI J1820+070
• A timing-based estimate of the spin of the black hole in MAXI J1820+070
• Accretion Scenario of MAXI J1820+070 during 2018 Outbursts with Multi-mission Observations
• Rapidly alternating flux states of GX 339-4 during its 2021 outburst captured by Insight-HXMT
• Hidden biases in flux-resolved X-ray spectroscopy
• MAXI J1535-571 2017 outburst Seen by INTEGRAL/SPI and Investigating the Origin of Its Hard Tail
• Evolution of X-ray and optical rapid variability during the low/hard state in the 2018 outburst of MAXI J1820+070 = ASASSN-18ey