Volume 643, November 2020
|Number of page(s)||13|
|Published online||03 November 2020|
Dynamically important magnetic fields near the event horizon of Sgr A*⋆
Max Planck Institute for Extraterrestrial Physics, Giessenbachstraße 1, 85748 Garching, Germany
2 LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université de Paris, 5 place Jules Janssen, 92195 Meudon, France
3 Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
4 1st Institute of Physics, University of Cologne, Zülpicher Straße 77, 50937 Cologne, Germany
5 Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
6 Universidade de Lisboa - Faculdade de Ciências, Campo Grande, 1749-016 Lisboa, Portugal
7 Faculdade de Engenharia, Universidade do Porto, rua Dr. Roberto Frias, 4200-465 Porto, Portugal
8 European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748 Garching, Germany
9 European Southern Observatory, Casilla, 19001 Santiago 19, Chile
10 Max Planck Institute for Radio Astronomy, Auf dem Hügel 69, 53121 Bonn, Germany
11 Sterrewacht Leiden, Leiden University, Postbus 9513, 2300 Leiden, RA, The Netherlands
12 Departments of Physics and Astronomy, Le Conte Hall, University of California, Berkeley, CA 94720, USA
13 CENTRA – Centro de Astrofísica e Gravitação, IST, Universidade de Lisboa, 1049-001 Lisboa, Portugal
14 Department of Astrophysical & Planetary Sciences, JILA, Duane Physics Bldg., University of Colorado, 2000 Colorado Ave, Boulder, CO 80309, USA
15 Department of Particle Physics & Astrophysics, Weizmann Institute of Science, Rehovot 76100, Israel
16 Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, UK
Accepted: 17 August 2020
We study the time-variable linear polarisation of Sgr A* during a bright near-infrared flare observed with the GRAVITY instrument on July 28, 2018. Motivated by the time evolution of both the observed astrometric and polarimetric signatures, we interpret the data in terms of the polarised emission of a compact region (“hotspot”) orbiting a black hole in a fixed, background magnetic field geometry. We calculated a grid of general relativistic ray-tracing models, created mock observations by simulating the instrumental response, and compared predicted polarimetric quantities directly to the measurements. We take into account an improved instrument calibration that now includes the instrument’s response as a function of time, and we explore a variety of idealised magnetic field configurations. We find that the linear polarisation angle rotates during the flare, which is consistent with previous results. The hotspot model can explain the observed evolution of the linear polarisation. In order to match the astrometric period of this flare, the near horizon magnetic field is required to have a significant poloidal component, which is associated with strong and dynamically important fields. The observed linear polarisation fraction of ≃30% is smaller than the one predicted by our model (≃50%). The emission is likely beam depolarised, indicating that the flaring emission region resolves the magnetic field structure close to the black hole.
Key words: Galaxy: center / black hole physics / polarization / relativistic processes
GRAVITY is developed in a collaboration between the Max Planck Institute for extraterrestrial Physics, LESIA of Observatoire de Paris/Université PSL/CNRS/Sorbonne Université/Université de Paris and IPAG of Université Grenoble Alpes/CNRS, the Max Planck Institute for Astronomy, the University of Cologne, the CENTRA – Centro de Astrofisica e Gravitação, and the European Southern Observatory.
© GRAVITY Collaboration 2020
Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Open Access funding provided by Max Planck Society.
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