Journal
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY
Volume 493, Issue 4, Pages 5389-5396Publisher
OXFORD UNIV PRESS
DOI: 10.1093/mnras/staa564
Keywords
accretion, accretion discs; X-rays: binaries; black hole physics
Categories
Funding
- European Research Council Advanced Grant FEEDBACK
- NASAthrough Einstein Postdoctoral Fellowship - Chandra X-ray Center [PF6-170160]
- NASA [NAS8-03060.340442]
- UK Science and Technology Facilities Council (STFC) [ST/R000867/1]
- European Research Council (ERC) under the European Union [834203]
- European Research Council (ERC) [834203] Funding Source: European Research Council (ERC)
- STFC [ST/N004027/1, ST/S000623/1, ST/R000506/1, ST/R000867/1] Funding Source: UKRI
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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(circle dot)). 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.
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