Letter to the Editor

Orbital motion near Sagittarius A^*

Constraints from polarimetric ALMA observations

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
e-mail: maciek.wielgus@gmail.com
² Nicolaus Copernicus Astronomical Centre, Polish Academy of Sciences, Bartycka 18, 00-716 Warsaw, Poland
³ Black Hole Initiative at Harvard University, 20 Garden Street, Cambridge, MA 02138, USA
⁴ Department of Astrophysics/IMAPP, Radboud University, PO Box 9010, 6500 GL Nijmegen, The Netherlands
⁵ Center for Astrophysics – Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
⁶ Departament d’Astronomia i Astrofísica, Universitat de València, C. Dr. Moliner 50, 46100 Burjassot, València, Spain
⁷ Observatori Astronómic, Universitat de València, C. Catedrático José Beltrán 2, 46980 Paterna, València, Spain
⁸ Las Cumbres Observatory, 6740 Cortona Drive, Suite 102, Goleta, CA 93117-5575, USA
⁹ Department of Physics, University of California, Santa Barbara, CA 93106-9530, USA
¹⁰ Italian ALMA Regional Centre, INAF-Istituto di Radioastronomia, Via P. Gobetti 101, 40129 Bologna, Italy
¹¹ Dipartimento di Fisica, Università degli Studi di Cagliari, SP Monserrato-Sestu km 0.7, 09042 Monserrato, Italy
¹² Universidade de São Paulo, Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Departamento de Astronomia, São Paulo, SP 05508-090, Brazil
¹³ Joint ALMA Observatory, Alonso de Cordova 3107, Vitacura, 763-0355 Santiago de Chile, Chile

Received: 13 July 2022
Accepted: 6 September 2022

Abstract

We report on the polarized light curves of the Galactic Center supermassive black hole Sagittarius A^*, obtained at millimeter wavelength with the Atacama Large Millimeter/submillimeter Array (ALMA). The observations took place as a part of the Event Horizon Telescope campaign. We compare the observations taken during the low variability source state on 2017 Apr. 6 and 7 with those taken immediately after the X-ray flare on 2017 Apr. 11. For the latter case, we observe rotation of the electric vector position angle with a timescale of ∼70 min. We interpret this rotation as a signature of the equatorial clockwise orbital motion of a hot spot embedded in a magnetic field dominated by a dynamically important vertical component, observed at a low inclination ∼20°. The hot spot radiates strongly polarized synchrotron emission, briefly dominating the linear polarization measured by ALMA in the unresolved source. Our simple emission model captures the overall features of the polarized light curves remarkably well. Assuming a Keplerian orbit, we find the hot spot orbital radius to be ∼5 Schwarzschild radii. We observe hints of a positive black hole spin, that is, a prograde hot spot motion. Accounting for the rapidly varying rotation measure, we estimate the projected on-sky axis of the angular momentum of the hot spot to be ∼60° east of north, with a 180° ambiguity. These results suggest that the accretion structure in Sgr A^* is a magnetically arrested disk rotating clockwise.