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

MAXI J1820+070 with NuSTAR - II. Flaring during the hard to soft state transition with a long soft lag
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 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.
0035-8711
3976–3986
Buisson, D.J.K.
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Fabian, A.C.
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Gandhi, Poshak
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Kara, E.
72a75c47-f7eb-4fea-8b45-d2c4d0aa1456
Parker, E.L.
00e1fbda-4a71-4cf8-a342-e19b7431df96
Shaw, A.W.
de07f04d-acd5-4eef-8f06-3bbc5faec9bd
Tomsick, J.A.
17479878-1c1a-4de2-98ce-cd7d0ae5581d
Walton, D.J.
0ca7f4d0-934d-4f76-b27a-8623eefe0f40
Wang, J.
df67cbf7-6922-433d-8575-00e47420812a
Buisson, D.J.K.
6285a846-5456-4b0e-a39b-85d66738e9b6
Fabian, A.C.
0c353376-f159-4129-b03e-d3993c0c5406
Gandhi, Poshak
5bc3b5af-42b0-4dd8-8f1f-f74048d4d4a9
Kara, E.
72a75c47-f7eb-4fea-8b45-d2c4d0aa1456
Parker, E.L.
00e1fbda-4a71-4cf8-a342-e19b7431df96
Shaw, A.W.
de07f04d-acd5-4eef-8f06-3bbc5faec9bd
Tomsick, J.A.
17479878-1c1a-4de2-98ce-cd7d0ae5581d
Walton, D.J.
0ca7f4d0-934d-4f76-b27a-8623eefe0f40
Wang, J.
df67cbf7-6922-433d-8575-00e47420812a

Buisson, D.J.K., Fabian, A.C., Gandhi, Poshak, Kara, E., Parker, E.L., Shaw, A.W., Tomsick, J.A., Walton, D.J. and Wang, J. (2020) MAXI J1820+070 with NuSTAR - II. Flaring during the hard to soft state transition with a long soft lag. Monthly Notices of the Royal Astronomical Society, 500 (3), 3976–3986. (doi:10.1093/mnras/staa3510).

Record type: Article

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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MAXI J1820+070 with NuSTAR - II. Flaring during the hard to soft state transition with a long soft lag - Accepted Manuscript
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Accepted/In Press date: 5 November 2020
e-pub ahead of print date: 12 November 2020
Additional Information: ARXIV IS AM

Identifiers

Local EPrints ID: 446230
URI: http://eprints.soton.ac.uk/id/eprint/446230
ISSN: 0035-8711
PURE UUID: 42f17dd9-0fcc-41d7-8978-5e6ca61db245
ORCID for Poshak Gandhi: ORCID iD orcid.org/0000-0003-3105-2615

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Date deposited: 29 Jan 2021 17:32
Last modified: 20 Jul 2022 01:48

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Contributors

Author: D.J.K. Buisson
Author: A.C. Fabian
Author: Poshak Gandhi ORCID iD
Author: E. Kara
Author: E.L. Parker
Author: A.W. Shaw
Author: J.A. Tomsick
Author: D.J. Walton
Author: J. Wang

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