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Synthetic gravitational lens image of the Sagittarius A${}^*$ black hole with a thin disk model

Ezequiel F. Boero, Osvaldo M. Moreschi

Abstract

The images of Sagittarius A${}^*$ published by the Event Horizon Telescope (ETH) Collaboration in 2022 present features that were associated with an emission ring consistent with what is expected from an accretion disc surrounding the supermassive black hole at the center of our Galaxy. Here, we generate images of Sgr~A${}^*$ across different configurations of a simple accretion disc model that became successful, in our previous work, in reproducing the main features observed in M87*. Their best image, here reproduced in Fig. 1, suggests a geometric configuration of an inclined disk with three bright regions; which we have considered as our first configuration. Since we were not convinced with the results of this first configuration, we also explore in detail the case of nearly edge-on orientations which are a priori the expected geometry for a relaxed disc, as seen from the plane of the galaxy. We have produced simulated images using an efficient ray tracing and geodesic deviation methodology that allows to account for deformation, relativistic and magnification effects. We compare our synthetic images with the EHT images reconstructed with data from April 6 and 7 of 2017. We found that, although the EHT Collaboration seems to discard the image from April 6, our best suggested image resembles the output from the {\sc Themis} pipeline for April 6; which for us gives support for the edge-on configuration.

Synthetic gravitational lens image of the Sagittarius A${}^*$ black hole with a thin disk model

Abstract

The images of Sagittarius A published by the Event Horizon Telescope (ETH) Collaboration in 2022 present features that were associated with an emission ring consistent with what is expected from an accretion disc surrounding the supermassive black hole at the center of our Galaxy. Here, we generate images of Sgr~A across different configurations of a simple accretion disc model that became successful, in our previous work, in reproducing the main features observed in M87*. Their best image, here reproduced in Fig. 1, suggests a geometric configuration of an inclined disk with three bright regions; which we have considered as our first configuration. Since we were not convinced with the results of this first configuration, we also explore in detail the case of nearly edge-on orientations which are a priori the expected geometry for a relaxed disc, as seen from the plane of the galaxy. We have produced simulated images using an efficient ray tracing and geodesic deviation methodology that allows to account for deformation, relativistic and magnification effects. We compare our synthetic images with the EHT images reconstructed with data from April 6 and 7 of 2017. We found that, although the EHT Collaboration seems to discard the image from April 6, our best suggested image resembles the output from the {\sc Themis} pipeline for April 6; which for us gives support for the edge-on configuration.
Paper Structure (17 sections, 58 equations, 25 figures, 1 table)

This paper contains 17 sections, 58 equations, 25 figures, 1 table.

Table of Contents

  1. Introduction
  2. Exact gravitational lens optical scalars
  3. The basic equations
  4. Optical scalars
  5. Ray tracing and geodesic deviation equations
  6. Null geodesics and the null geodesic deviation equation in Kerr spacetime
  7. Simple accretion disc model in Kerr
  8. Simulated images with gravitational lens magnification and red/blueshift effects
  9. Basic model
  10. Image from the EHT Collaboration of Sagittarius A${}^*$
  11. Graphs with angular momentum aligned perpendicularly to the elliptical shape of the EHT image
  12. Graphs with angular momentum aligned with the angular momentum of the galaxy
  13. End point of null geodesics in Kerr-Schild coordinates
  14. The possible bar structure on the disk
  15. Numerical comparison of the images
  16. ...and 2 more sections

Figures (25)

  • Figure 1: Image from the EHT Collaboration paper III EventHorizonTelescope:2022wok of Sgr A${}^*$; where they show an average of reconstructed images for April 7, in their Figure 13.
  • Figure 2: $\iota=-0.1745=-10^\circ$, and from left to right with Kerr parameter: $a=0.00$, $a=0.25$, $a=0.50$, $a=0.75$ and $a=0.98$.
  • Figure 3: $\iota=-0.3490=-20^\circ$, and from left to right with Kerr parameter: $a=0.00$, $a=0.25$, $a=0.50$, $a=0.75$ and $a=0.98$.
  • Figure 4: $\iota=-0.5235=-30^\circ$, and from left to right with Kerr parameter: $a=0.00$, $a=0.25$, $a=0.50$, $a=0.75$ and $a=0.98$.
  • Figure 5: $\iota=-0.6980=-40^\circ$, and from left to right with Kerr parameter: $a=0.00$, $a=0.25$, $a=0.50$, $a=0.75$ and $a=0.98$.
  • ...and 20 more figures