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Evolving optical polarisation of the black hole X-ray binary MAXI J1820+070
KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Univ Turku, Dept Phys & Astron, Turku 20014, Finland.;Stockholm Univ, Roslagstullsbacken 23, S-10691 Stockholm, Sweden.;Russian Acad Sci, Space Res Inst, Profsoyuznaya Str 84-32, Moscow 117997, Russia..
Univ Turku, Dept Phys & Astron, Turku 20014, Finland..
Univ Turku, Dept Phys & Astron, Turku 20014, Finland.;St Petersburg State Univ, Dept Astrophys, Univ Skiy Pr 28, St Petersburg 198504, Russia..
Univ Turku, Finnish Ctr Astron ESO FINCA, Turku 20014, Finland..
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2019 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 623, article id A75Article in journal (Refereed) Published
Abstract [en]

Aims. The optical emission of black hole transients increases by several magnitudes during the X-ray outbursts. Whether the extra light arises from the X-ray heated outer disc, from the inner hot accretion flow, or from the jet is currently debated. Optical polarisation measurements are able to distinguish the relative contributions of these components. Methods. We present the results of BVR polarisation measurements of the black hole X-ray binary MAXI J1820+070 during the period of March-April 2018. Results. We detect small, similar to 0.7%, but statistically significant polarisation, part of which is of interstellar origin. Depending on the interstellar polarisation estimate, the intrinsic polarisation degree of the source is between similar to 0.3% and 0.7%, and the polarisation position angle is between similar to 10 degrees-30 degrees. We show that the polarisation increases after MJD 58222 (2018 April 14). The change is of the order of 0.1% and is most pronounced in the R band. The change of the source Stokes parameters occurs simultaneously with the drop of the observed V-band flux and a slow softening of the X-ray spectrum. The Stokes vectors of intrinsic polarisation before and after the drop are parallel, at least in the V and R filters. Conclusions. We suggest that the increased polarisation is due to the decreasing contribution of the non-polarized component, which we associate with the the hot flow or jet emission. The low polarisation can result from the tangled geometry of the magnetic field or from the Faraday rotation in the dense, ionised, and magnetised medium close to the black hole. The polarized optical emission is likely produced by the irradiated disc or by scattering of its radiation in the optically thin outflow.

Place, publisher, year, edition, pages
EDP SCIENCES S A , 2019. Vol. 623, article id A75
Keywords [en]
polarization, stars: black holes, stars: individual: MAXI J1820+070, X-rays: binaries
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Physical Sciences
Identifiers
URN: urn:nbn:se:kth:diva-247813DOI: 10.1051/0004-6361/201834140ISI: 000460371400002Scopus ID: 2-s2.0-85062874589OAI: oai:DiVA.org:kth-247813DiVA, id: diva2:1300897
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QC 20190401

Available from: 2019-04-01 Created: 2019-04-01 Last updated: 2019-04-01Bibliographically approved

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Veledina, AlexandraPoutanen, Juri

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