MAXI J1820+070 with NuSTAR – II. Flaring during the hard to soft state transition with a long soft lag. Issue 3 (12th November 2020)

• Record Type: Journal Article
• Authors: Buisson, D J K; Fabian, A C; Gandhi, P; Kara, E; Parker, M L; Shaw, A W; Tomsick, J A; Walton, D J; Wang, J
• Is Part Of: Monthly notices of the Royal Astronomical Society. Volume 500:Issue 3(2021)
• Page Start: 3976
• Page End: 3986
• Publication Date: 2020-11-12
• Subjects: accretion, accretion discs -- black hole physics -- X-rays: binaries; Astronomy -- Periodicals; Periodicals; 520.5
• DOI: 10.1093/mnras/staa3510
• Languages: English
• ISSNs: 0035-8711

Abstract

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 $F_{\rm Var}\sim 10{{\ \rm per\ cent}}$ ) 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.