In 2022, the Event Horizon Telescope Collaboration (EHTC) published the first image of the black hole in the Milky Way Galactic Center, Sagittarius A* (Sgr A*; 1). The image shows a ring-like structure with a central depression, consistent with the predictions of Einstein’s theory of general relativity. The image was the result of multiple analyses, including the traditional hybrid-imaging approach with the CLEAN algorithm as well as several other algorithms, each with independent assumptions and associated systematics. These methods were extensively verified through simulations that mimicked the raw data in detail. The results were found to be consistent over two separate days of observations.
The EHTC welcomes critical, independent analysis and interpretation of our published results. We publish detailed descriptions of our methods as well as raw data, data products, and analysis scripts to facilitate transparency, rigor, and reproducibility. A recently published re-analysis (2; referred to as M24 here) claims to identify flaws in the EHTC analysis and provides a new image that differs from the EHTC result. EHTC experts find numerous errors in the M24 analysis, including:
- M24 does not address the intrinsic variability of Sgr A* in their imaging process.
- M24 presents one single image as the end result of their analysis, disregarding the full ensemble of suitable images.
- M24 biases their results towards a point-source image through self-calibration.
- M24 misstates the biases in their own methodology as demonstrations of biases in the EHTC methodologies.
Additionally, EHTC experts find numerous mischaracterizations of EHTC methodologies and results, including:
- that the EHTC methodology does not account for the point-spread-function (PSF);
- that the EHTC methodology is untested or unverifiable;
- that the EHTC selected the final image through a biased method, biased its analyses based on dependencies on theoretical simulations, and tunes the image field of view “to improve image clarity”;
- that the EHTC did not release uncalibrated raw data.
The EHTC methods used for imaging Sgr A* are based on the same tools used for the first black hole image of M87 published in 2019 (3), and then subsequently confirmed with new data published in 2024 (4). Analyses by independent researchers supported the EHTC M87 result (5). Additionally, imaging of blazars (6, 7, 8) and radio galaxies (9) have revealed entirely different morphologies such as core-jet structures.
The EHTC stands behind its results. We look forward to publishing rigorous and thorough analyses of new observations with improved image fidelity that will further test the physics and astrophysics of black holes.
Here we discuss some of the primary M24 flaws in greater depth:
- M24 does not attempt to handle the intrinsic variability of Sgr A* in their imaging process. Although the image is constructed from approximately 12 hours of data, Sgr A* is variable on time scales as short as tens of minutes. Variations in the image morphology on timescales shorter than the full duration violate the fundamental theorem of interferometric imaging; addressing this issue requires additional steps in the processing. The EHTC uses a variety of techniques including normalization to the light curve, data-weighting, snapshot imaging, and time-dependent evolution, all of which reveal an image consistent with the published EHTC result. In contrast, the authors do not include the effects of the Sgr A*’s variability in any way without a justification, and thus incorrectly overfit to the data and significantly underestimate the uncertainty in their images.
- M24 presents one single image as the end result of their analysis, disregarding the full ensemble of suitable images. In contrast, the EHTC produced a full ensemble of suitable images, each of which matches the data while using different ways of forming the image. None of the EHTC image formation methods specifically favor a ring shape. This ensemble strongly supports the ring-image hypothesis, but as the EHTC emphasized in its papers and press release, alternative morphologies are not completely ruled out (only 2% of images did not contain a ring).
- M24 biases their results towards a point-source model through self-calibration on a point source model. The procedure will enforce the derived calibration parameters to be maximally consistent with the assumed point source. The authors do not test that the authors’ approach is not biased toward a point-dominated morphology. The EHTC, on the other hand, utilizes a variety of techniques, including approaches that do not require calibration to a specific morphology. These methods are extensively tested on synthetic data derived from both simple geometric models (including a point-like source similar to the authors’ image) and astrophysical simulations.
- M24 misstates the biases in their own methodology as demonstrations of biases in the EHTC methodologies. In the procedure they run on test data they first pre-calibrate the data to a ring morphology before imaging. They show that their imaging approach incorrectly produces a ring artifact when this is done. This result effectively demonstrates flaws in their own imaging methods – their approach is heavily biased to the morphology they initially assumed. As this is significantly different from the EHTC methods, their tests do not directly address the EHTC methods or resultant images.
Below we discuss some of the most prominent M24 misstatements:
- M24 erroneously claims that the EHTC methodology does not account for the point-spread-function (PSF) of the observation. In fact, the EHTC hybrid imaging approach explicitly corrects for the PSF through the CLEAN algorithm. Other algorithms implicitly incorporate PSF effects into the reconstruction algorithms. Furthermore, non-imaging (e.g., model fitting) methods confirm the ring-like structure is the most likely simple model that fits the data. Note that the EHTC methods recover non-ring structures in both synthetic data tests as well as real data that has the same PSF, for example when imaging the non-ring-like polarization of the black holes M87* and Sgr A* (10).
- M24 erroneously claims that the EHTC methodology is untested or unverifiable. In fact, the methods used for imaging Sgr A* are based on the same tools used for the first black hole image of M87 published in 2019 (3), and then subsequently confirmed with new data published in 2024 that have different characteristics including a different PSF (4). In the cases of Sgr A* and M87, the methods were extensively tested on synthetic data based on simple geometric models and astrophysical simulations. As presented on the EHT website, independent analyses supported the EHTC M87 result (5).
- M24 erroneously claims that the EHTC selected the final image through a biased method, by prioritizing the appearance rate of a similar structure from a large imaging parameter space over data consistency. In fact, all the images were initially selected based on data consistency and therefore are consistent with data within its uncertainties.
- M24 erroneously claims that the EHTC biased its analyses by building in dependencies from theoretical simulations, i.e., deriving data weights to account for the effects from time variations seen in the Sgr A* structure by utilizing general relativistic magnetohydrodynamic (GRMHD) simulations. In fact, the EHTC estimated those effects from EHT data alone. These methods were, however, extensively tested on synthetic data from both simple geometric models and astrophysical GRMHD simulations.
- M24 erroneously claims that the EHTC did not release uncalibrated raw data. In fact, all of 2017 EHTC raw and calibrated data have been publicly available since May 2022 (11).
- The authors erroneously claim the EHTC tunes the image field of view to improve image clarity. In fact, as described in the EHTC paper that the authors cite (12), the field of view was determined from a source size constraint derived from EHT data itself.
References
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EHTC et al ApJL 930, 12, 2022: First Sagittarius A* Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole in the Center of the Milky Way
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Miyoshi et al MNRAS 534, 3237, 2024: An independent hybrid imaging of Sgr A* from the data in EHT 2017 observations
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EHTC et al ApJL 875, 1, 2019: First M87 Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole
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EHTC et al A&A 681, 79, 2024: The persistent shadow of the supermassive black hole of M 87. I. Observations, calibration, imaging, and analysis
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https://eventhorizontelescope.org/blog/imaging-reanalyses-eht-data
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Kim et al A&A 640, 69, 2020: Event Horizon Telescope imaging of the archetypal blazar 3C 279 at an extreme 20 microarcsecond resolution
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Issaoun et al ApJ 934, 145, 2022: Resolving the Inner Parsec of the Blazar J1924-2914 with the Event Horizon Telescope
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Jorstad et al. ApJ 943, 170, 2023: The Event Horizon Telescope Image of the Quasar NRAO 530
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Janssen et al NatAs 5, 1017, 2021: Event Horizon Telescope observations of the jet launching and collimation in Centaurus A
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EHTC et al ApJL 964, 25, 2024: First Sagittarius A* Event Horizon Telescope Results. VII. Polarization of the Ring
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EHTC et al ApJL, 875, 4, 2019: First M87 Event Horizon Telescope Results. IV. Imaging the Central Supermassive Black Hole