Letter to the Editor

Polarimetry and astrometry of NIR flares as event horizon scale, dynamical probes for the mass of Sgr A*^⋆

¹ 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 Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
⁴ European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748 Garching, Germany
⁵ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
⁶ 1st Institute of Physics, University of Cologne, Zülpicher Straße 77, 50937 Cologne, Germany
⁷ CENTRA – Centro de Astrofísica e Gravitação, IST, Universidade de Lisboa, 1049-001 Lisboa, Portugal
⁸ Universidade de Lisboa – Faculdade de Ciências, Campo Grande, 1749-016 Lisboa, Portugal
⁹ European Southern Observatory, Casilla, 19001 Santiago 19, Chile
¹⁰ Universidade do Porto, Faculdade de Engenharia, Rua Dr. Roberto, Frias, 4200-465 Porto, Portugal
¹¹ Department of Astrophysical & Planetary Sciences, JILA, Duane Physics Bldg., 2000 Colorado Ave, University of Colorado, Boulder, CO 80309, USA
¹² Faculdade de Engenharia, Universidade do Porto, rua Dr. Roberto Frias, 4200-465 Porto, Portugal
¹³ Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
¹⁴ Departments of Physics & Astronomy, Le Conte Hall, University of California, Berkeley, CA 94720, USA
¹⁵ Max Planck Institute for Astrophysics, Karl-Schwarzschild-Straße 1, 85748 Garching, Germany
¹⁶ Department of Physics, University of Illinois, 1110 West Green Street, Urbana, IL 61801, USA
¹⁷ Max Planck Institute for Radio Astronomy, auf dem Hügel 69, 53121 Bonn, Germany
¹⁸ Institute of Multidisciplinary Mathematics, Universitat Politècnica de València, València, Spain
¹⁹ Kavli Institute for Astronomy and Astrophysics, Beijing, PR China
²⁰ Advanced Concepts Team, ESA, TEC-SF, ESTEC, Keplerlaan 1, 2201 AZ, Noordwijk, The Netherlands
²¹ Niels Bohr International Academy, Niels Bohr Institute, Blegdamsvej 17, 2100 Copenhagen, Denmark
²² ORIGINS Excellence Cluster, Boltzmannstraße 2, 85748 Garching, Germany
²³ Department of Physics, Technical University of Munich, 85748 Garching, Germany
²⁴ Leiden University, 2311EZ Leiden, The Netherlands
²⁵ Higgs Centre for Theoretical Physics, Edinburgh, UK
²⁶ Department of Physics, Jadwin Hall, Washington Road, Princeton, New Jersey 08544, USA

Received: 10 July 2023
Accepted: 2 August 2023

Abstract

We present new astrometric and polarimetric observations of flares from Sgr A* obtained with GRAVITY, the near-infrared interferometer at ESO’s Very Large Telescope Interferometer (VLTI), bringing the total sample of well-covered astrometric flares to four and polarimetric flares to six. Of all flares, two are well covered in both domains. All astrometric flares show clockwise motion in the plane of the sky with a period of around an hour, and the polarization vector rotates by one full loop in the same time. Given the apparent similarities of the flares, we present a common fit, taking into account the absence of strong Doppler boosting peaks in the light curves and the EHT-measured geometry. Our results are consistent with and significantly strengthen our model from 2018. First, we find that the combination of polarization period and measured flare radius of around nine gravitational radii (9R_g ≈ 1.5R_ISCO, innermost stable circular orbit) is consistent with Keplerian orbital motion of hot spots in the innermost accretion zone. The mass inside the flares’ radius is consistent with the 4.297 × 10⁶ M_⊙ measured from stellar orbits at several thousand R_g. This finding and the diameter of the millimeter shadow of Sgr A* thus support a single black hole model. Second, the magnetic field configuration is predominantly poloidal (vertical), and the flares’ orbital plane has a moderate inclination with respect to the plane of the sky, as shown by the non-detection of Doppler-boosting and the fact that we observe one polarization loop per astrometric loop. Finally, both the position angle on the sky and the required magnetic field strength suggest that the accretion flow is fueled and controlled by the winds of the massive young stars of the clockwise stellar disk 1–5″ from Sgr A*, in agreement with recent simulations.