Event Horizon Telescope Imaging of Sagittarius A* - Geoff Bower - 05/18/2022

Event Horizon Telescope (EHT) Results for Sagittarius A* (Sgr A*)
📌 The EHT released the first image of Sgr A* (the Milky Way's central black hole) last week, which followed the release of the M87* image.
🔬 Sgr A* appears as a ring with a diameter of 52 micro arc seconds, containing a black hole mass of about 4 million solar masses.
📏 The ring diameter of Sgr A* is about 1600 times smaller than that of M87*, corresponding to an orbital radius similar to that of Mercury.
🔭 Achieving this image required over two and a half years of data processing and testing on data collected from the 2017 observation run involving over 300 people globally.

Technical Achievements and Challenges in Imaging
📡 The EHT achieved the necessary angular resolution of about 20 micro arc seconds using a global millimeter VLBI array operating at a 1 millimeter wavelength.
💾 A major innovation was increasing data recording bandwidth to 64 gigabits per second (up from historic rates of less than 1 Mbit/s in the late 1960s) to improve sensitivity.
💧 Observations are heavily constrained by atmospheric water vapor absorption, necessitating high-altitude sites like Mauna Kea to observe at the 230 GHz window.
🔭 Future upgrades, like adding the 10-meter telescope at an observatory in 2024, aim to fill in short baselines to improve dynamic range and connect the ring to the jet structure.

Astrophysical Interpretations and Modeling
📉 Caltech contributors focused on geometric modeling using Deep Probabilistic Inference (DPI) and developing synthetic data to test imaging methods, including models mimicking variability.
⚙️ Analysis involved complex methods to account for significant flux density oscillation (up to 10% variation between consecutive days) and structural changes in Sgr A*.
🖼️ Imaging across multiple pipelines consistently showed ring-like structures with a diameter of 52 ± 2 micro arc seconds, though details varied, limiting definitive interpretation of specific knots.
❌ Astrophysical models exploring inclination, spin, and magnetic field strengths were tested; notably, most models were too variable relative to the observed data, and high electron temperature models were strongly rejected.

Tests of General Relativity and Future Outlook
⚫ The EHT results help constrain whether an event horizon is present, rejecting a hard surface or highly reflecting surface based on emission properties and morphology.
⚖️ The measurement places constraints on deviations from Kerr geometry, characterized by a $\delta$ parameter, resulting in a constraint comparable to that found for M87*.
🛰️ Future observations will target higher frequencies (345 GHz observations planned for next year) for higher angular resolution to resolve successive nested photon rings, enabling stricter tests of spin and gravity models.
🔭 The data already collected (from 2018, 2021, 2022) will be used for time-domain analysis, potentially revealing dynamics on time scales of tens of minutes or hours.

Key Points & Insights
➡️ The successful imaging of Sgr A* confirms that black holes power active galactic nuclei, linking observations across three orders of magnitude in black hole mass compared to LIGO's gravitational wave tests.
➡️ Significant astrophysical uncertainty (estimated at 15% to 25%) remains on the mass measurement, dominated by a lack of constraint on the models describing the surrounding gas.
➡️ Due to Sgr A*'s rapid dynamics, the collaboration avoided claiming a single, unique true image (unlike the M87* release), favoring an ensemble of consistent ring-like images derived from multiple analysis pipelines.
➡️ Future high-resolution observations are expected to move tests beyond the current astrophysical regime and into a more quantitative regime for measuring parameters like black hole spin.

📸 Video summarized with SummaryTube.com on Dec 02, 2025, 17:53 UTC