2024MNRAS.534.3237M

We propose that the ring structure found by the Event Horizon Telescope Collaboration (EHTC) as the black hole shadow of Sgr A* is an artefact caused by the bumpy point spread function (PSF) of the EHT 2017 data. The imaging using sparse u-v data requires detailed scrutiny of the PSF. The estimated shadow diameter ($\mathrm{48.7\pm 7~\mu \rm as}$) is equal to the spacing between the main beam and the first sidelobe of the PSF ($\mathrm{49.09~\mu \rm as}$), which immediately suggests a potential problem in the deconvolution of the PSF. We show that the ring image can be derived from non-ring simulated data sets (noise only; point source) with a narrow field-of-view (FOV) and an assumed self-calibration, suggesting that the EHT 2017 u-v coverage is insufficient for reliable imaging. The EHTC analysis, based on calibrations with assumptions about the source's size and properties, selected the final image by prioritizing the appearance rate of a similar structure from a large imaging parameter space over data consistency. Our independent analysis with conventional hybrid mapping reveals an elongated east-west structure, consistent with previous observations. We believe it to be more reliable than the EHTC image, owing to half the residuals in normalized visibility amplitude. The eastern half is brighter, possibly due to a Doppler boost from the rapidly rotating disc. We hypothesize that our image shows a portion of the accretion disc from about 2 to a few R_S (where R_S is the Schwarzschild radius) away from the black hole, rotating with nearly $\mathrm{60~{{\ \rm per\ cent}}}$ of the speed of light and viewed from an angle of 40°-45°.