First Sagittarius A* Event Horizon Telescope Results. III: Imaging of the Galactic Center Supermassive Black Hole (2311.09479v1)

Abstract: We present the first event-horizon-scale images and spatiotemporal analysis of Sgr A* taken with the Event Horizon Telescope in 2017 April at a wavelength of 1.3 mm. Imaging of Sgr A* has been conducted through surveys over a wide range of imaging assumptions using the classical CLEAN algorithm, regularized maximum likelihood methods, and a Bayesian posterior sampling method. Different prescriptions have been used to account for scattering effects by the interstellar medium towards the Galactic Center. Mitigation of the rapid intra-day variability that characterizes Sgr A* has been carried out through the addition of a "variability noise budget" in the observed visibilities, facilitating the reconstruction of static full-track images. Our static reconstructions of Sgr A* can be clustered into four representative morphologies that correspond to ring images with three different azimuthal brightness distributions, and a small cluster that contains diverse non-ring morphologies. Based on our extensive analysis of the effects of sparse $(u,v)$-coverage, source variability and interstellar scattering, as well as studies of simulated visibility data, we conclude that the Event Horizon Telescope Sgr A* data show compelling evidence for an image that is dominated by a bright ring of emission with a ring diameter of $\sim$ 50 $\mu$as, consistent with the expected "shadow" of a $4\times10^6 M_\odot$ black hole in the Galactic Center located at a distance of 8 kpc.

• The paper demonstrates breakthrough imaging of Sagittarius A* at the event-horizon scale by effectively mitigating rapid variability and interstellar scattering.
• It employs CLEAN, RML, and Bayesian posterior sampling methods to validate a bright, ring-like morphology of roughly 50 microarcseconds.
• The study confirms a black hole shadow structure and offers new insights into dynamic accretion processes and relativistic effects near the event horizon.

Imaging of the Galactic Center Supermassive Black Hole

The paper "First Sagittarius A* Event Horizon Telescope Results III: Imaging of the Galactic Center Supermassive Black Hole," authored by the Event Horizon Telescope Collaboration et al., presents pioneering results of imaging the supermassive black hole located at the center of our galaxy, Sagittarius A* (Sgr A*), using the Event Horizon Telescope (EHT). This research is crucial as Sgr A* is the closest supermassive black hole to Earth and provides an unparalleled opportunity to explore event horizon scale phenomena.

The primary objective of this work is to achieve imaging at the event-horizon scale of Sgr A* and address the challenges associated with the rapid variability and scattering effects driven by the interstellar medium. The imaging data were gathered during the EHT observations in April 2017 and analyzed through comprehensive imaging techniques.

Key Methodologies

1. Variability Mitigation: Sgr A* exhibits rapid intra-day variability, which complicates conventional static imaging techniques. The paper introduces a "variability noise budget" to incorporate these variations into the noise model, mitigating the static imaging assumption.
2. Scattering Mitigation: The line of sight to Sgr A* is obscured by angular broadening due to interstellar scattering. This effect is countered by applying a scattering kernel to observed visibilities and accounting for refractive noise substructure, ensuring the clarity and integrity of the images.
3. Imaging Approaches:
   • CLEAN and RML Methods: These traditional and regularized maximum likelihood imaging methods are pivotal. Surveys over a range of regularization parameters ensure the robustness of images.
   • Bayesian Posterior Sampling: This method enables an exploration of the imaging parameter space to provide a probabilistic assessment of image accuracy and fidelity.
   • Dynamic Imaging: The approach attempts to unravel time-resolved structures reflecting the intrinsic dynamics of the source.

Findings

• Ring-like Morphology: The most frequently identified morphology was a bright ring-like structure, in line with the predicted shadow of a black hole. This morphology was consistently observed across imaging strategies, highlighting a similar diameter of ∼50 microarcseconds.
• Variability and Azimuthal Asymmetry: Variability in brightness and azimuthal asymmetry was evident, possibly linked to dynamic accretion processes or relativistic effects near the event horizon. Such features, while challenging to resolve conclusively due to limited baseline coverage, provide a basis for future studies of black hole dynamics.
• Theoretical Implications: The ring-like structure corroborates theoretical expectations of a black hole shadow, reinforcing general relativity predictions. The variability characterization offers insights into the complex astrophysical processes governing supermassive blackhole environments.

Implications and Future Directions

The successful imaging of Sgr A* highlights the potential of VLBI technologies like the EHT to probe black hole environments in unprecedented detail. This paper provides a benchmark for future theoretical and observational efforts aimed at understanding the central engine of our galaxy and black holes generally. The methodologies refined herein are pivotal for preparing upcoming EHT campaigns, particularly in handling rapid variability and scattering more effectively.

This work paves the way for further exploration into magnetic field structures, jet emissions, and accretion dynamics around black holes, drawing from simultaneous multi-wavelength observations. Enhancements in the EHT array will further improve resolution and sensitivity, potentially resolving finer details in Sgr A*'s image structure and enabling deeper understanding of relativistic jet physics—key to unveiling the mysteries of galactic centers and fundamental physics under extreme gravity.