Hot spots around Sagittarius A*

Joint fits to astrometry and polarimetry

¹ Department of Astrophysics/IMAPP, Radboud University, PO Box 9010 6500 GL Nijmegen, The Netherlands
² Instituto de Astrofísica de Andalucía-CSIC, Glorieta de la Astronomía s/n, E-18008 Granada, Spain
³ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany

^⋆ Corresponding authors; a.yfantis@astro.ru.nl, maciek@wielgus.info

Received: 13 August 2024
Accepted: 10 October 2024

Abstract

Context. Observations of Sagittarius A* (Sgr A*) in the near-infrared (NIR) show irregular flaring activity. Flares coincide with the astrometric rotation of the brightness centroid and with looping patterns in fractional linear polarization. These signatures can be explained with a model of a bright hot spot, transiently orbiting the black hole.

Aims. We extend the capabilities of the existing algorithms to perform parameter estimation and model comparison in the Bayesian framework using NIR observations from the GRAVITY instrument, and simultaneously fitting the astrometric and polarimetric data.

Methods. Using the numerical radiative transfer code ipole, we defined several geometric models describing a hot spot orbiting Sgr A*, threaded with a magnetic field, and emitting synchrotron radiation. We then explored the posterior space of our models with a nested sampling code dynesty. We used Bayesian evidence to make comparisons between the models.

Results. We have used 11 models to sharpen our understanding of the importance of various aspects of the orbital model, such as non-Keplerian motion, hot-spot size, and off-equatorial orbit. All considered models converge to realizations that fit the data well, but the equatorial super-Keplerian model is favoured by the currently available NIR dataset.

Conclusions. We have inferred an inclination of ∼155 deg, which corroborates previous estimates, a preferred period of ∼70 minutes, and an orbital radius of ∼12 gravitational radii with the orbital velocity of ∼1.3 times the Keplerian value. A hot spot with a diameter smaller than 5 gravitational radii is favoured. Black hole spin is not constrained well.