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The soft state of the black hole transient source MAXI J1820+070: emission from the edge of the plunge region?

Fabian, A C, Buisson, D J, Kosec, P, Reynolds, C S, Wilkins, D R, Tomsick, J A, Walton, D J, Gandhi, P, Arzoumanian, Z, Cackett, E M, Dyda, S, Garcia, J A, Gendreau, K C, Grefenstette, B W, Homan, J, Kara, E, Ludlam, R M, Miller, J M and Steiner, J F (2020) The soft state of the black hole transient source MAXI J1820+070: emission from the edge of the plunge region? Monthly Notices of the Royal Astronomical Society, 493 (4), 5389-5396.

Record type: Article

Abstract

The Galactic black hole X-ray binary MAXI J1820+070 had a bright outburst in 2018 when it became the second brightest X-ray source in the sky. It was too bright for X-ray CCD instruments such as XMM–Newton and Chandra, but was well observed by photon-counting instruments such as Neutron star Inner Composition Explorer (NICER) and Nuclear Spectroscopic Telescope Array(NuSTAR). We report here on the discovery of an excess-emission component during the soft state. It is best modelled with a blackbody spectrum in addition to the regular disc emission, modelled as either diskbb or kerrbb. Its temperature varies from about 0.9 to 1.1 keV, which is about 30–80 per cent higher than the inner disc temperature of diskbb. Its flux varies between 4 and 12 per cent of the disc flux. Simulations of magnetized accretion discs have predicted the possibility of excess emission associated with a non-zero torque at the innermost stable circular orbit (ISCO) about the black hole, which, from other NuSTAR studies, lies at about 5 gravitational radii or about 60 km (for a black hole, mass is 8M⊙⁠). In this case, the emitting region at the ISCO has a width varying between 1.3 and 4.6 km and would encompass the start of the plunge region where matter begins to fall freely into the black hole.

Text
2002.09691 - Accepted Manuscript

Accepted/In Press date: 21 February 2020
Published date: 27 February 2020

Identifiers

Local EPrints ID: 442308
URI: http://eprints.soton.ac.uk/id/eprint/442308
ISSN: 0035-8711
PURE UUID: 203d861d-ec26-45e7-af76-a9a4b43b8e9e
ORCID for P Gandhi: orcid.org/0000-0003-3105-2615

Catalogue record

Date deposited: 13 Jul 2020 16:30

Contributors

Author: A C Fabian
Author: D J Buisson
Author: P Kosec
Author: C S Reynolds
Author: D R Wilkins
Author: J A Tomsick
Author: D J Walton
Author: P Gandhi
Author: Z Arzoumanian
Author: E M Cackett
Author: S Dyda
Author: J A Garcia
Author: K C Gendreau
Author: B W Grefenstette
Author: J Homan
Author: E Kara
Author: R M Ludlam
Author: J M Miller
Author: J F Steiner